.a .h . r:- . up w 2: .e. .r 1.3.7:}... 5.. xii, (5.1.! e .x. I... .csfiflu... ” dull...“- ,.tv) 2535!:3‘3... . 5.53“.“ nth; .. 3. it... (at! , If}. 0..- ¢ .1 112....)3 l ...I9.(.t...lt (.0 ....,v .uxr fl: .. .. .1! :4 s .tmfii 5? gm 4 L t. . .. . s... E: El! 1.. £703: :11 mys‘ [a LIBRARY 9067‘ Michigan State University This is to certify that the thesis entitled SEDIMENTOLOGY AND STRATIGRAPHY OF TRIASSIC CLASTIC AND CARBONATE UNITS OF THE CHULITNA TERRANE, ALASKA RANGE, SOUTH-CENTRAL ALASKA presented by Jennifer L. DeLoge has been accepted towards fulfillment of the requirements for the Master of degree in Geological Sciences Science “5.41% fl Major Pi‘éfeséror’s Signature H - 2.0 ”93 Date MSU is an Affin'native Action/Equal Opportunity Employer PLACE IN RETURN BOX to remove this Checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 5/08 K:IProj/Acc&Pres/CIRC/DateDuevindd SEDIMENTOLOGY AND STRATIGRAPHY OF UPPER TRIASSIC CARBONATE AND CLASTIC STRATA OF THE CHULITNA TERRANE, ALASKA RANGE, SOUTH-CENTRAL ALASKA By Jennifer L. DeLoge A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Geological Sciences 2009 ABSTRACT SEDIMENTOLOGY AND STRATIGRAPHY OF UPPER TRIASSIC CARBONATE AND CLASTIC STRATA OF THE CHULITNA TERRANE, ALASKA RANGE, SOUTH-CENTRAL ALASKA By Jennifer L. DeLoge The Chulitna terrane of the southern Alaska Range is one of the more enigmatic tectonic terranes in the North American Cordillera and is defined in part by the occurrence of Upper Triassic red bed and carbonate units that occur in association with basalt and serpentinite in the Alaska Range suture zone. New geologic mapping and measured stratigraphic sections of Upper Triassic strata from the Chulitna terrane reveal a continuous record of siliciclastic, volcaniclastic, and carbonate sedimentation. Red bed units consist largely of interbedded successions of poorly-sorted and matrix- supported sandstone, siltstone, and conglomerate. Individual beds are typically 0.25— 2 m thick and exhibit tabular geometries. Upper Triassic units in the Chulitna terrane were originally thought to represent three distinct successions that were stratigraphically unrelated. However, measured sections revel that carbonate and red bed units are interbedded throughout the Chulitna terrane. Clastic and carbonate strata are interpreted to represent deposition in a marginal marine fan-delta complex characterized largely by debris flow processes, fluvial-deltaic and beach-barrier island sedimentation, as well as lagoon and carbonate reef/bank sedimentation. Upper Triassic carbonate and clastic strata of the Chulitna terrane are interpreted to represent sedimentation associated with an isolated period of exhumation and clastic sedimentation, bimodal volcanism, and carbonate precipitation during at the end of the Triassic. ACKNOWLEDGMENTS The author wishes to express sincere appreciation to her guidance committee including Brian Hampton, Julie Libarkin, and Kazuya Fujita for their assistance in the preparation of this manuscript and their input during each phase of this study. In addition, special thanks to the Geological Society of America and to the Department of Geological Sciences and the College of Natural Science at Michigan State University for their generous financial support of this project. Thanks also to Michael Ackerson for his support with the field study and the US Geological Survey, particularly Dwight Bradley for logistical support in Alaska. I also would like to thank my family, friends, and peers for their time, understanding, and patience throughout this process. TABLE OF CONTENTS LIST OF TABLES .................................................................................. vi LIST OF FIGURES ............................................................................... vii CHAPTER I: INTRODUCTION ............................................................... 1 The Alaska Range ............................................................................. 1 The Alaska Range Suture Zone ....................................................... 3 Geologic History ................................................................................ 6 Precambrian—Paleozoic .......................................................... 7 Mesozoic ................................................................................. 7 Cenozoic ................................................................................. 8 The Chulitna Terrane ...................................................................... 14 Previous Work ................................................................................. 17 Devonian Stratigraphy ........................................................... 19 Mississippian—Permian Stratigraphy ..................................... 21 Triassic-Jurassic Stratigraphy .............................................. 23 Purpose of Study ............................................................................. 28 Little Shotgun Creek Study Area ..................................................... 31 Methods and Approach ................................................................... 32 CHAPTER II: SEDIMENTOLOGY,STRATIGRAPHY, AND DEPOSITIONAL SYSTEMS ................................................................. 37 Introduction ...................................................................................... 37 General Facies Methodology .......................................................... 38 Stratigraphic Overview .................................................................... 39 Upsection Stratigraphic Trends ....................................................... 48 Sedimentary Facies ........................................................................ 50 F acies Associations ........................................................................ 62 Facies Association 1 (FA 1) .................................................. 63 Description .................................................................. 63 Interpretation - Debris flow (clastic) ............................. 63 Facies Association 2 (FA 2) .................................................. 64 Description .................................................................. 64 Interpretation - Channels and bars ............................. 65 Facies Association 3 (FA 3) .................................................. 66 Description .................................................................. 66 Interpretation — Floodplain/tidal flat ............................. 66 Facies Association 4 (FA 4) .................................................. 67 Description .................................................................. 67 Interpretation — Beach/barrier island ........................... 67 Facies Association 5 (FA 5) .................................................. 68 Description .................................................................. 68 iv Interpretation — Carbonate reef/bank - lagoon ............ 68 Facies Association 6 (FA 6) .................................................. 69 Description .................................................................. 69 Interpretation —Debris flow (carbonate) ....................... 69 Depositional System ....................................................................... 70 Provenance Overview ..................................................................... 75 Preliminary Provenance Summary ................................................. 87 Discussion ....................................................................................... 88 Preliminary basin model and tectonic framework ................. 89 CHAPTER III: CONCLUSIONS ............................................................ 93 Summary of study ........................................................................... 93 Conclusions ..................................................................................... 96 APPENDIX A: MEASURED SECTIONS .............................................. 99 REFERENCES ................................................................................... 140 LIST OF TABLES Table Number Page Table 1.1. Summary of Triassic stratigraphy from the Alaska Range suture zone ........................................................... 13 Table 1.2. Summary of the age constraint from Triassic stratigraphy in the Chulitna terrane .................................. 28 Table 2.1. Summary of siliciclastic sedimentary facies ...................... 51 Table 2.2. Summary of carbonate sedimentary facies ....................... 52 Table 2.3. Summary of facies associations for Upper Triassic strata of Little Shotgun Creek ............................... 72 Table 2.4. Description of samples from measured section 1 ............. 79 Table 2.5. Description of samples from measured section 2 ............. 81 Table 2.6. Description of samples from measured section 3 ............. 85 vi LIST OF FIGURES Figure Number Page Figure 1.1. Digital elevation model of south-central Alaska" .............. 2 Figure 1.2. Generalized geology of the Alaska Range suture zone" ..................................................................... 5 Figure 1.3. Geologic map of the Chulitna terrane“ ........................... 16 Figure 1.4. Geologic map of the Little Shotgun Creek study area** ............................................................................... 34 Figure 2.1. Generalized Geologic map of study area** ...................... 41 Figure 2.2. Measured stratigraphic sections from the Little Shotgun Creek study area ................................................ 43 Figure 2.3. Exposure of Upper Triassic stratigraphy in study area** ............................................................................... 45 Figure 2.4. Interbedded siliciclastic and carbonate units of measured section 2** ....................................................... 46 Figure 2.5. Upper Triassic stratigraphic relationships from northwestern part of the Chulitna terrane“ ..................... 47 Figure 2.6. Summary of gravel facies“ .............................................. 54 Figure 2.7. Summary of sandstone facies“ ....................................... 57 Figure 2.8. Summary of mudstone/siltstone facies“ ......................... 59 Figure 2.9. Summary of carbonate facies“ ........................................ 61 Figure 2.10. Conceptual model for mixed elastic and carbonate fan-delta depositional system" ..................... 76 Figure 2.11. Conceptual model for backarc basin development to explain exhumation, bimodal volcanism, and carbonate sedimentation" ............................................ 91 “Images in this thesis are presented in color CHAPTER I: INTRODUCTION THE ALASKA RANGE The northernmost extent of the North American Cordillera in southern Alaska is defined by the Talkeetna Mountains, Wrangell-St Elias Mountains, and the Alaska Range (Figure 1.1). The Alaska Range extends >600 km throughout south-central Alaska and parts of the Yukon Territory and contains some of the highest topography in North America (Mount McKinley at an elevation of 20,135 ft.). The northern part of the Alaska Range is dissected by the presently-active Denali fault and Hines Creek fault. The Denali fault is a continent-scale, strike- slip fault that extends >1000 km as an arced lineament though southern Alaska (Csejtey et al., 1982). Right-lateral offset of up to Up to 400 km of dextral displacement has been proposed for the Denali fault since Late Cretaceous time based on geologic studies (Eisbacher, 1976; Nokleberg et al., 1985). The Denali fault can be observed as a discreet north-south trending structure in parts of southeastern Alaska but appears to splay into a more complex series of east- west trending structures in south-central and southwestern Alaska (Figure 1.1). This region of south—central and southwestern Alaska is referred to as the Alaska Range suture zone and contains some of the more complicated and least studied parts of the North American Cordillera. Figure 1.1. Digital elevation model (DEM) of south-central Alaska and location reference map. The Alaska Range is one of the topographically highest orogenic belts in North America and is bisected by the Denali fault, black line in the bottom right- hand corner reference map, in southern Alaska. The major faults in south- central Alaska are noted by dashed red lines. Note that the Denali fault splays into the at the Hines Creek fault which is one of the faults that has been proposed to represent the suture between the Mesozoic continental margin of North America and the accreted terranes. To the south of the Denali fault system the general location of the Chulitna terrane (purple) is marked by a blue polygon to identify the location of Figure 1.3. (Geologic map of the Chulitna terrane). The Chulitna terrane is shown on this DEM in the context of major faults that dissect the Alaska Range and Talkeetna Mountains. The location of Figure 1.2. (Alaska Range suture zone) is indicated by the black square. THE ALASKA RANGE SUTURE ZONE The Alaska Range suture zone (Figure 1.2.) in south-central Alaska consists primarily of exposed Jurassic-Cretaceous sedimentary strata with numerous, isolated occurrence of highly-deformed Triassic and older tectonic terranes. The suture zone is bound and dissected by several large strike-slip faults, each of which has been proposed to represent the suture between the Yukon-Tanana terrane (pericratonic terrane) to the north and the Wrangellia composite terrane (island arc) to the south. The Alaska range suture zone is informally defined to occur across a wide zone (>100 km) between the Talkeetna fault that defined the southern boundary and Hines Creek and Denali faults that mark the northern boundaries. Numerous Triassic and older, fault-bounded tectonic terranes (e.g. Dillinger, Chulitna, McKinley-Windy terranes) are exposed throughout the Alaska Range suture zone (Figure 1.2). Each of these terranes has been studied within the general context of regional mapping projects and, although there are some stratigraphic similarities shared between terranes, there is little consensus on the pre-Jurassic tectonic history of the North American Cordillera in the context of Triassic and older terranes along the inboard margin of the Wrangellia composite terrane in southern Alaska. Figure 1.2. Generalized Geology of the Alaska Range Suture Zone. The Alaska Range suture zone is informally defined to occur between the Talkeetna fault (to the south) and Hines Creek and Denali faults (to the north). Note the occurrence of the Chulitna terrane, marked by a C, in the central part of the Alaska Range suture zone. The major faults within the Alaska Range suture zone are indicated by dashed red lines. Note that the Kahiltna assemblage (Submarine deposits), shown in black, is thought to represent synorogenic sedimentation associated with the accretion of the Wrangellia composite terrane to the North American Cordillera during Jurassic — Cretaceous time. The focus of this study is on Upper Triassic siliciclastic and carbonate strata of the Chulitna terrane that were deposited prior to accretion of the Wrangellia composite terrane. The blue polygon represents the mapped area of the Chulitna terrane by Clautice et al (2001). Note that the Chulitna terrane is one of several Paleozoic — Mesozoic terranes that occur in the Alaska Range suture zone. Map has been modified from Wilson et al (1998). L I... :3 mwmcmm $9.0m mcmcmhéox? E mmcatcu uEaEmzwamocouoS Econ—20:3 uca 0:98 Minn. 8983:3993 In. ..... mcmtg 5.39.an HI fix“ . . . . . . 829 35:5 H- n n _ ocmtfl . t , . _ . . >uc_>>->o_c_v_os_ Ii xf' . . memes 2:35 a kw . . mambo“ ozmoquo . = . A96 “Em—m: EELS Emanou «Eon—.23 Amézaxv 238% Ea occmEnam 389209392. xanoo Emcozfloom FIL 6326 NS. Embm ozmflo NONE fl $.09 38:9 .to»-.§o E ,E:_>:=m EmEmamao I o z m 0 m .— m < s. _ Figure 1.2. GEOLOGIC HISTORY The accretion of the Wrangellia Composite terrane to the western margin of Canada and southern Alaska during Jurassic-Cretaceous time is one of the more significant tectonic events in the history of the North American Cordillera (Dickinson W.R., 2004; Plafker, G., Berg, HQ, 1994; Ridgway, K.D., and others, 2002). However, little is known about the Triassic history of the continental margin and the island arc prior to accretion. Alaska is made up of numerous geologic terranes. All of south-central Alaska originated elsewhere and over time was accreted onto the proto North American craton. The terranes consist of fragments derived from continental, oceanic, and island arc sources. Most of south-central Alaska’s terranes originated as either volcanic island arcs or sediments that were eroded off of these island arcs. The major tectonic events, since the late Paleozoic, leading to addition of material to the North American Plate include; (1) addition of the Yukon-Tanana terrane during the Late Paleozoic-Early Triassic (Dickinson, W.R., 2004); (2) Early to mid-Cretaceous accretion of the Wrangellia island arc composite terrane (Nokleberg and others, 2000); (3) Late Cretaceous to present tectonic migration of terranes along the Denali fault to there present locations in Alaska and continued addition of material to southern Alaska (Nokleberg and others, 2000). The tectonic events leading to various terranes discussed previously, other than the Yukon-Tanana and Wrangellia composite terrane, are not well constrained by current models. Generally, in south-central Alaska, the oldest terranes are found inland of the North American craton, and tend to decrease in age toward the plate margin. The youngest terranes are still in the process of accreting onto Alaska. The edges, sutures, of these terranes are typically major strike slip faults such as the Denali Fault and the Border Ranges Fault. Precambrian-Paleozoic The Yukon-Tanana composite and Dillinger terranes have been near the northwest part of the North American craton since the Precambrian. In the late Precambrian, the Wrangellia composite terrane began forming through volcanic activity far to the south in the proto-Pacific ocean. By the end of the Paleozoic the Wrangellia composite terrane was composed of two separate island arcs. Mesozoic By the end of the Triassic Period the Wrangellia composite terrane was composed of three separate arcs that had joined on the Farallon Plate. The Farallon Plate was moving towards the Mesozoic North America margin. During this time Mesozoic North America was within an equatorial region. In the Late Triassic, on the west side of the Wrangellia composite terrane, a subduction zone formed and is recorded in trench sediments that were compressed onto Wrangellia forming an accretionary prism. The subduction zone is recorded in the Border Ranges fault. Large amounts of sediment were being eroded from the landward side of the Wrangellia composite terrane. These sediments along with sediments from the continental margin of Mesozoic North America filled the basin(s) that formed between the two. Turbidities record deposition before North America and Wrangellia collided. The basins were faulted and folded towards the end of the Jurassic, when the Wrangellia composite terrane eventually collided with the Yukon-Tanana composite terrane (Nokleberg and others, 2000) By the Early Cretaceous the Wrangellia composite terrane had come into contact with North America, near eastern Oregon and northeast California. By the mid-Cretaceous, the Kula Plate had rifted apart from the Farallon Plate and began to move the Wrangellia composite terrane northward. The northward movement of the Wrangellia composite terrane is recorded in portions that were scrapped off along the margin of the North American Plate (Nokleberg and others, 2000). By the Late Cretaceous, the Wrangellia composite terrane was sutured, onto North America. Subduction and collision resulted in active volcanism and metamorphism along the plate boundary between the Yukon composite terrane and the Wrangellia composite terrane. The Denali fault marks this collision zone (Nokleberg and others, 2000). Cenozoic Collision between the Wrangellia composite terrane and North America resulted in thickening and heating of the crust. Evidence for these events is recorded in the numerous Cenozoic granitic plutons that dot the south-central Alaska. By the Eocene most of the terranes of south-central Alaska were at their present latitude. Approximately 43 million years ago the modern subduction zone, the Aleutian trench was well developed. The formation of this subduction zone coincided with a change in the direction, from the north to the northwest, of the Kula-Pacific plate relative motions. The terranes of south-central Alaska continue to be accreted, resulting in a region that is characterized by mountain ranges, volcanoes, and frequent earthquakes (Nokleberg and others, 2000). Previous studies have identified nine terranes (Jones and others, 1982) in the Alaska Range of south-central Alaska. Jones and others (1982) identified nine major terranes, in the Alaska Range of south-central Alaska. These nine terranes line from north to south, within the Alaska Range suture zone, as follows; Yukon-Tanana, Pingston, McKinley, Dillinger, Windy, Mystic, Chulitna, Westfork, and the Broad Pass. The Mystic, Chulitna, and Westfork appear to be structurally emplaced within Jurassic-Cenozoic sedimentary basins (Jones and others, 1982). The Yukon-Tanana terrane is characterized by regionally metamorphosed sedimentary, plutonic, and volcanic rocks (Jones and others, 1982). The majority of units that compose this terrane are Paleozoic in age but Precambrian- Mesozoic strata also exists (Jones and others, 1982). The dominant lithologies of the Yukon-Tanana include quartzite, schist, amphibolite, and granitic rocks (Jones and others, 1982). The Pingston terrane and McKinley terranes occur as disconnected slivers along the southern side of the Hines Creek fault (Jones and others, 1982). The Pingston terrane is composed of tan weathered phyllite, minor radiolarian chert, limestone, and intrusive masses of gabbro and diorite (Jones and others, 1982). Pingston terrane units range in age from Pennsylvanian-Triassic (Jones and others, 1982). Strata of the McKinley terrane ranges in age from Mississippian- Cretaceous (Jones and others, 1982). Large blocks (up to 1m) of Devonian limestone occur in the Triassic part of the McKinley terrane stratigraphy (Jones and others, 1982). McKinley terrane strata is characterized by gabbro, basalt, submarine fan deposits with chert and fossil detritus, conglomerate, and Devonian and older rocks (Jones and others, 1982). Dillinger terrane clastic turbidities and carbonate basin fill occurs on both sides of the Denali fault as folded and faulted thick assemblages (Jones and others, 1982). Dillinger turbidities, shale, and limestone are lower Paleozoic strata that are overlain by Jurassic sandstone and Triassic volcanic strata. Mystic strata structurally overlie Dillinger rocks south of the Denali fault. Mystic strata are dominantly Paleozoic sedimentary rocks with minor amounts of Permian turbidities containing Devonian radiolarian chert (Jones and others, 1982) The Windy terrane occurs along the Denali faults and Baines Creek fault and is composed of rocks found nowhere else in Alaska (Jones and others, 1982). Windy strata are unique because large blocks of Devonian limestone float in sandstone, conglomerate, and argillite (Jones and others, 1982). The Windy terrane units range in age from Silurian to Cretaceous (Jones and others, 1982). Chulitna terrane rocks were considered an ophiollite or red beds (Jones and others, 1982). The southwest trending Paleozoic-Mesozoic internally folded and faulted strata is sub-parallel to the Denali fault. The Chulitnia terrane is 10 stratigraphically distinct from turbidities that surround it based on upper Devonian ophiolite, upper Paleozoic chert, volcanic, carbonate, and siliciclastic units are in contact with Upper Triassic siliciclastic, carbonate, and volcanic strata (Jones and others, 1982). More recent work by Nockelberg and others (2007) divide the south-central Alaska terranes based on tectonic environments and overlap assemblages. The recent stratigraphic summary of Nokleberg and others (2007) provides a basis to compare the Triassic stratigraphy of the Chulitna terrane with that of the Wrangellia island arc, the Yukon-Tanana terrane, and surrounding terranes in the Alaska Range. Refer to Figure 1.2 for location of terranes in the Alaska Range Suture Zone. Yukon-Tanana composite terrane strata is composed of part of an continental margin arc consisting of ancient island arc assemblage with Mississippian to Upper Triassic structural mélange of pillow basalt, basalt, mafic tuff, chert, argillite, and limestone with minor gabbro and diabase (Nokleberg and others, 2007). The Wrangellia composite terrane is composed of are related rocks and the Upper Triassic to Lower Jurassic part of the stratigraphy consists of metabasalt, argillite, and limestone (Nokleberg and others, 2007). McKinley terrane submarine fan deposits, chert, and pillow basalts are associated with an Upper Triassic seamount (Nokleberg and others, 2007). Windy terrane marine sedimentary and volcanic rocks are associated with oceanic fragments offset by the Denali fault (Nokleberg and others, 2007). The Dillinger terrane and Mystic terrane are connected with the Paleozoic to lower Mesozoic continental margin of 11 Table 1.1. Triassic stratigraphy found within the Alaska Range suture zone as summarized by Nokleberg and Richter (2008). These stratigraphic descriptions serve as a basis to evaluate the stratigraphy measured in the Chulitna terrane in comparison with Wrangellia island arc, North American basin fill and exotic terranes. The locations of these terranes are referenced in Figure 2 of this paper with the exception of the Nixon Fork, Pingston, Seventymile, and the Stinkina. The Pingston, Nixon Fork, and Seventymile have been grouped into the Yukon-Tanana composite terrane for simplicity and the Stikinia terrane is approximately 150 miles northeast of the point where the Denali fault splays into the Hines Creek fault. 12 25:3 23»: 25:8 >25on 2.2.3 3.5? 026:0“ «5:26 :.E 330 :oonm 07.8.2. 66.. 2392. 062.2 2393. 25m _E :53 95 35292 29598 new 2630 gamma 26:3 new tune .9683 :3 octmEnsm Table 1.1. 223E: mzmmmn ucm mmficoEmo 830925 ucm .moficoemo .mo_umm_o_o___m .mozmm_o_cmo_o> 23E... Emu cmaamumso 13 the North American craton (Nokleberg and others, 2007). Table 1.1 summarizes the Triassic stratigraphy from terranes of the Alaska Range suture zone. THE CHULITNA TERRANE The Chulitna terrane is located in the central part of the Alaska Range suture zone directly south of the Denali fault. The terrane is bounded by younger Upper Jurassic-Cretaceous siliciclastic strata of the Kahiltna assemblage and is located between the Wrangellia composite terrane to the south and Paleozoic continental margin to the north. Recent potential gravity and magnetotelluric data collected across the Chulitna terrane indicates that the contacts on both sides with Jurassic-Cretaceous strata are defined by faults (Glen and others, 2007). The Chulitna terrane apparently rests on top of these Jurassic-Cretaceous strata. The Chulitna terrane is defined as a ~50-km-long and ~15-km-wide, southwest-northeast trending belt of structurally imbricated Devonian (?)—Early Jurassic sedimentary and volcanic strata (Figure 1.3.). All units older than Upper Jurassic, within this assemblage, have been subjected to low-grade regional metamorphism (Clautice and others, 2001). Only Triassic and older units have experienced greenschist-facies metamorphism (Clautice and others, 2001). The Devonian strata include banded cherts, siliceous mudstone and sandstones. Shallow marine to marginal marine strata include Permian limestone and Upper Triassic volcaniclastic, sandstone, conglomerate, mudstone, and siltstone units referred to as red beds as well as Upper Triassic limestone and sandy limestone. There is also interbedded Upper Triassic basalt and limestone as well as Jurassic 14 Figure 1.3. Geologic map of Devonian (?)—Jurassic units of the Chulitna terrane. Modified from Clautice and others (2001 ). The focus of this study is on the sedimentology and stratigraphy of the Upper Triassic volcaniclastic, siliciclastic, and carbonate strata of the Chulitna terrane. Normal faults trend northeast across the Chulitna terrane and are shown in red. The outlined region in the Little Shotgun creek region represents the location where a majority of the sedimentologic and stratigraphic data was collected for this study. 15 UCretaceeue-Paleogene - Igneous rocks (K-P) “Jurassic-Cretaceous - Submarine tan deposites oi the Kahiltna assemblage (Jr-K) KJ Jurassic-Cretaceous (Jr-K) - Sandstone, argillite, sandy carbonate, limestone, and volcanic rocks (KJ) E :Trlassic (Norien) - Limestone and basalt (Trib) T" Triassic (Norian) - Sedimentary and volcaniclastic strata (sandstone and conglomerate) (Trr) Triassic (Nodan-Rhaetlan) - Sedimentary strata (calcareous sandstone, argillite, sandy limestone) (Trs) L Trl Triassic (Norian) - Fosslliierous limestone ('l’rl) Upper Permian ~ Limestone (Pi) fl Permian - Mudstona and siltstone (Ps) D Paleozoic? - Devonian? - Sedimentary, volcanic. and voicanlclastlc units (P2?) J Paleozoic? - Devonian? - Ultramaflc slivers. dikes. and plugs Figure 1.3. 16 turbidities. The Upper Triassic strata of the Chulitna terrane are widely considered to be one of the more enigmatic stratigraphic successions in southern Alaska given the occurrence of laterally continuous siliciclastic and volcaniclastic units that are interbedded with basalt and limestone in the Triassic part of the section. This is the only occurrence of Upper Triassic siliciclastic red bed units in the Alaska Range suture zone and thus provides an excellent opportunity to begin to understand the Triassic tectonic history of the continental margin as preserved in Triassic strata in southern Alaska. Age control for Chulitna terrane stratigraphy has been obscured and reset due to the complex Mesozoic —Cenozoic plutonic emplacement in the region combined with the complex history of folding, faulting, and metamorphism. PREVIOUS WORK Over the past 30 years, a range of geologic and geophysical data has been collected and used to designate terranes in south-central Alaska (Hillhouse and Gromme, 1980; Stone and others, 2001; Csejtey and others, 1976; Jones, and others, 1972, 1980, 1982; Nokelberg and others, 2007; Clautice and others, 1999, 2001; Blodgett and others, 2000; Glen and others, 2007; Ridgway and others, 2002). No distinguishable paleo-latitude for the Chulitna terrane has been determined (Hillhouse and Gromme, 1980; Stone and others, 2001). Instead the Chulitna terrane was distinguished based on the regional extent of a Mesozoic stratigraphic sequence that is markedly different from adjacent 17 stratigraphy (Hawley and Clark, 1968; Jones and others, 1972, Jones and others, 1980; Jones and others, 1982; Clautice and others, 2001). Some of the first geologic studies of the Chulitna terrane began in the early part of the twentieth century and were focused primarily on documenting the economic potential for the region (T hrumond, 1918; Townsend, 1925; Clautice and others, 2001). These studies consisted of small-scale mapping of fault-controlled mineralization, general identification of sedimentary and volcanic rocks and coal deposits, and engineering geology projects that were associated with the construction of the Alaska railroad (Thrumond, 1918; Townsend, 1925; Leigler, 1940; Moxam and others, 1951). In the early 1900’s Chulitna’s first mining claims received considerable attention based on the potential for ore production (Thrumond, 1918; Townsend, 1925). Metal was economically important between 1941 and 1942, when the Golden Zone Mine produced Au, Ag, Cu, and Pb (Leigler, 1940; Moxam and others, 1951). More recent work in this area has consisted of large-scale, regional mapping projects and geochemical studies that have identified the general stratigraphic assemblages and the occurrence of large-scale faults, both of which have resulted in the classification of the Chulitna region as a fault-bounded tectonic terrane (Jones and others, 1980; Jones and others, 1982; Clautice and others, 2001). The Chulitna terrane consists of stratigraphically imbricated Mesozoic and Paleozoic strata (Jones and others, 1972, 1980, 1982; Wilson and others, 1998; Clautice and others, 2001). Clautice and others (2001) traced the dominate faults in the area on the ground and on aeromagnetic maps, which 18 show that these faults are liner zones of high conductivity. This evidence suggests that these faults are high angle faults (Clautice and others, 2001). High-angle faults trend mainly northeast, northwest, and north-northeast in the Chulitna terrane. Northeast trending are the most continuous and define the Broad Pass graben and mark the northwestern margin of the Chulitna terrane (Clautice and others, 2001). The next set of faults that appear to have a significant effect on offset in the Chulitna terrane trend northwest and are approximately 20 to 30 km apart (Clautice and others, 2001). These two sets of faults are up to 70 My based on cross cutting relationships (Clautice and others, 2001). The most commonly occurring faults in the Chulitna terrane are the north-northeast-trending set of strike slip faults. However, these faults are thought to have minor movements (Clautice and others, 2001). A young age is indicated by cross cutting relationships with veins, skarns, and mineralized Late Cretaceous plutons. Although several sets of structures may pre-date the previously discussed sets of faults these older structure have yet to be defined or traced. Table 1.2. summarizes age constraints for the Chulitna terrane stratigraphy provided by previous workers and outlined in the following discussion. Devonian Stratigraphy A narrow, strong magnetic signature trending northeast across the Chulitna terrane produces an aeromagnetic high which has been correlated to ultramafic strata in outcrop (Clautice and others, 2001), see Figure 1.3. Hawley 19 and Clark (1968) documented the occurrence of a massive chromite body within scattered blocks of gabbro, basalt, diabase, and abundant red radiolarian chert. Most serpentinite is clinochrysotile and lizardite indicating a lack of high- temperature metamorphism (Jones and others, 1982). Lenticular and podifonn blocks of sheared serpentinite are intercalated with the chert and pillow basalt (Wilson and others, 1998). The serpentinite has a variable composition ranging from serpentinized, chromite bearing dunite to gabbro and transitions to silica- carbonate rocks (Clautice and others, 2001). Major and trace element similarities relate the gabbro to basalt clasts of the Devonian volcaniclastic unit (Clautice and others, 2001). Trace element analysis reveals the serpentinite is similar to andesitic tuff and flows (Clautice and others, 2001). A Devonian age is suggested based on spatial relationship to Devonian chert unit, but age is not well constrained (Clautice and others, 2001). Chert lenses occur within pillow basalt and as isolated blocks within serpentinite. Pillow basalt contains red chert that is thought to contain Devonian age radiolarian, classified as Entactinospaera (Jones and others, 1982). Clautice and others (2001) mapped bedded, Early (?) to Middle Devonian limestone. Rugose and tabulate corals, and brachiopods indicate a Middle Devonian while conodonts indicate Late Silurian to Early Devonian age (Clautice and others, 2001). Jones and others (1982) report Polygnathus a conodont of Late Devonian (Famennian age from nine localities, in the northwest, and on Long Creek. 20 Massive to bedded radiolarian chert can be red, brown, green or black and is up to 0.5 percent MnO (Clautice and others, 2001). Chert is also found as large lenses and blocks in fault zones and as clasts in Middle to Lower Triassic conglomerate and is interlayer at the lower contact (Clautice and others, 2001). Analyses of immobile element compositions, including Ti-Zr-Y, indicate an intro- oceanic island arc (Clautice and others, 2001). Upper Devonian pyroxene, andesite tuff and flows with compositions ranging from island-arc tholeiitic basalt to dacitic tuff were mapped by Clautice and others (2001). Detritus derived from this sequence is found in the Upper Triassic strata (Jones and others, 1982). The ultramafic, radiolarian chert, pillow basalt, and argillite stratigraphic sequence is interpreted as an ophiolite by Jones and others (1982). They presumed an arc became the dominate source after this time based on the introduction of coarse conglomerate, tuff, andesite, and chert (Jones and others, 1980). Clautice and others (2001) noted that previously mapped Devonian volcanic rocks and tuffs were correlated to units in Broad Pass just south of the Chulitna terrane, see Figure 1.1. Geochemical data from these volcanic rocks indicate an arc-related igneous origin which Clautice and others (2001) interpreted as a link to the Wrangellia Island Arc. Mississippian-Permian Stratigraphy Upper Paleozoic(?) argillite and tuff is interbedded with siltstone and chert. Radiolarians in the chert indicate a Mississippian to Permian age (Jones and others, 1982). Sedimentary rocks dominate this sequence, which may correlate 21 to Upper Paleozoic tuff found in the West Fork terrane and the chert and argillite of the Broad Pass terrane (Clautice and others, 2001) see Figure 1.2. The chert and argillite are overlain by massive conglomerate containing cobble to boulder size chert (Jones and others, 1982). Conodonts from the chert clasts include Polygnathus cf. P. glaber, P. eff. P. webbi which indicate a Famennian age (Jones and others, 1982). The volcanic conglomerate grades into sandstone, siltstone, and chert (Jones and others, 1982). Chondrites and Scalarituba, brachiopods, bivalves, and bryozoans near the top of sequence were interpreted to indicate a Permian age (Blodgett and Clautice, 2000). Abundant helminthoid feeding tracks overlie the chert but contain no datable fossils according to Jones and other (1982). This Permian unit conformable and gradationally underlies Permian limestone. Massive crinoidal bryozoan limestone is intercalated with red or brown fossiliferous argillite (Jones and others, 1982). Brachiopods, bryozoans, and rare corals of Permian age are found in massive limestone (Jones and others, 1982). Brachiopods were identified as Fimbrinia so, which indicates an Early Permian age (Jones and others, 1982). Limestone blocks yield Neogondolella polygnathiformis conodonts of Karnian age (Jones and others, 1982). Fragments of brachiopods and bivalves from limestone blocks suggest a Late Triassic age (Jones and others, 1982). Horridonid brachiopods are common in Upper Permian limestone, which contains abundant megafauna typical of the Arctic during the Permian, including brachiopods, bryozoans, pelmatozoan debris, solitary rugose corals, ostracods, and trilobites (Clautice and others, 2001). 22 More recent petrographic studies of this limestone document hetrotrophic bryozoans and echinoderms as the dominate assemblage while phototropic biota contribute minor amounts (Montayne and others, 2003). The lack of nonskeletal grains, carbonate mud, and presence of mechanical compaction features combined with dominate heterotrophic biota was cited as evidence of cool-water deposition during the Permian (Montayne and others, 2003). Triassic-Jurassic Stratigraphy According to previous workers sandstone mudstone and limestone are uncomfortably overlain by volcaniclastic redbeds (Clautice and others, 2001; Wilson and others, 1998). Early Jurassic and Late Triassic red and brown sedimentary rocks including sandstone, siltstone, argillite, and conglomerate grade into fossiliferous brown sandstone and brownish-gray siltstone and basalt (Clautice and others, 2001). Middle(?) to Lower(?) Triassic tuff, andesite, basalt, greywacke, and conglomerate characterized by a predominantly volcanic and volcaniclastic composition was mapped by other field workers as part of the hemaitic sandstone, siltstone, and argillite (Jones and others, 1982). Early Jurassic and Late Triassic strata are more than 2,000 m thick (Wilson and others, 1998). The red sandstone sequence mapped by Jones and others (1982) contained thin interbeds of brown fossiliferous sandstone, limestone, and intercalated basalt flows. Clasts documented were dominantly basalt cobbles and pebbles derived from underlying basalt and flows within this sequence. In 23 lesser amounts quartzite pebbles, leuco-mica and red radiolarian chert clasts are present. However, there are thick lenses of up to 10 m or more of quartz pebble conglomerate. Jones and others (1982) cited this clast composition as evidence of nonmarine deposition in the Triassic when red beds appear. According to Jones (1982) this unit marks the destruction of a basin based on their interpretation of an ophiolite and the unconformity observed at the base of the red beds, cross-cutting units they had interpreted as older (Jones and others, 1982). Late Triassic units, of the Chulitna, show a southeast to northwest trend interpreted to represent a northeast trending paleo-shoreline (Jones and others, 1982). Jones and other (1982) described two coeval depositional settings; 1.)in the northwest: basalt, minor silicic volcaniclastics, and limestone and 2.) southeastern: nonmarine redbeds, minor basalt, and abundant silicic volcaniclastics. Clautice and others (2001) interpreted their compositional studies of the red beds as evidence that the “redbeds” of the Chulitna terrane are actually two distinct units. Clautice and others (2001) interpreted aeromagnetic data collected for the red beds as an indication that the diagenetic oxidation of primary Fe-Ti oxides resulted in a generally non-magnetic signature and red color (Clautice and others, 2001). When Clautice and others (2001) compared Ca/AI/Si standards for sandstones, Iimestones, shales, and mafic to intermediate igneous rocks the major element compositions of red clastic rocks of the Chulitna terrane they found that the red clastic rocks compositionally resemble mafic to intermediate igneous rocks and shales more so than sandstones. 24 Petrographic analysis of the framework grains shows that they are predominantly mafic volcanics, but also include quartz, felsic volcanics, plutonic and metamorphic rocks (Clautice and others, 2001). The conglomerate contains clasts of chert that contain upper Devonian radiolarian (Clautice and others, 2001). Point counts of these rocks yield values that range from high volcanic content and low in quartz to higher percentages of quartz and lower in volcanic content (Clautice and others, 2001). Clautice and others divided the red beds into Coarser volcaniclastic members that are often calcareous, with a predominantly medium to very coarse grained, poorly sorted matrix containing occasional feldspar, clinopyroxene, and hornblende crystals. Lithic clasts are subangular and consist of predominantly mafic volcanics frequently altered to chlorite, hematite, and calcite. Other lithic fragments include quartz, polycrystalline quartz, felsic volcanics, plutonic, and metamorphic rock. The compositions and lithologies of the lower portion indicate a volcanic arc affinity. The common red color of this unit is due to its andesitic-basaltic bulk composition and to syngenetic or diagenetic oxidation. According to Clautice and others (2001) they did not find evidence that this unit is hematitic sandstone or a "redbed" in the commonly accepted use of the term. The other unit was described by Clautice and others (2001) as red bed sandstone and conglomerate. Upper Triassic red bed sandstone and conglomerate described by Clautice and others (2001) includes siliciclastic rocks that are red-maroon, coarse-grained with local calcareous cement, and are matrix supported. Clast compositions recorded range from varying amounts of 25 quartz, basalt, and volcaniclastics. This was not placed within a stratigraphic context because no fossils were found and the contact with Upper Triassic limestone and basalt was uncertain (Clautice and others, 2001; Jones and others, 1982). A Late Triassic age was assigned to these rocks based on the lithologic similarity to Upper Triassic redbeds found in the central portion of the Chulitna (Jones and others, 1982). Along Long Creek the redbeds are overlain by thick silicic tuffs and breccias (Jones and others, 1982). Christy Creek samples contain juvavitid of latest Karnian to middle Norian age which were interpreted by Jones and others (1982) to represent contemporaneous deposition of these redbeds and the limestone and pillow basalt sequence. Red beds grade into brown fossiliferous sandstone and siltstone which contains marine mollusks and Heterastridium (Jones and others, 1982). Ammonites from fossiliferous locations, within this unit, indicate a Sinemurian and Early Jurassic age (Blodgett and Clautice, 2000). Colonial corals, snails, bivalves and Heterastridium in the lower part of the section indicate a Late Norian age (Jones and others, 1982, Blodgett and Clautice, 2000). Fossil evidence in the argillite includes limestone lenses containing ammonites and bivalves: Paracaloceras rursicostatum, Badouxia canadense, Weyla sp., Lima sp., and Eopecten sp. which suggest and early Sinemurian age (Blodgett and others, 2000). Lower Triassic limestone of Chulitna contains abundant megafossils (Nichols and Silberling, 1979). The most abundant of the 13 species found representing 13 genera include Meekoceras, Dieneroceras, Arctoceras, and Paranannites (Nichols and Silberling, 1979). All species found are indicative of 26 Early Smithian rocks associated with deposition along the North American craton at paleo-latitudes approximately 10° N (Nichols and Silberling, 1979). Petrographic analysis of intergranular acicular carbonate cement was also interpreted to indicate cementation during warm water, low latitude conditions (Nichols and Silberling, 1979). These ammonites were originally documented by Hawley and Clark (1968) as Euflemingites romunden' or Meekoceras gracilitatis which they interpreted to indicate deposition at northern and southern paleo- Iatitudes respectively. Geochemical analysis of Upper Triassic basalt indicates it is sub alkaline and tholeiitic by trace-element abundances typical of within plate settings (Clautice and others, 2001). Wilson and others (1998) assert that the chemistry of basalt is compositionally analogues with an ocean island shield volcano. Basaltic dikes and sills are indistinguishable compositionally from basalt flows of the same age (Wilson and others, 1998). However, the dikes and sills are at least 179 Ma based on Argon dating (Clautice and others, 2001). Limestone interbedded with the basalt is characterized by colonial scleractinian corals, megalodontid bivalves, and Spondylospira Iewesensi brachiopods, indicating Norian deposition. Upper Triassic limestone documented in the Long Creek area contains colonial scleractinian thicket reefs with well-preserved Norian age fauna including brachiopods, corals, bivalves, and gastropods (Blodgett and Clautice, 2000). Late Norian Monotis (Pacimonotis) subcircularis found at one location in the southwestern edge of the terrane (Siberling and others, 1997). This Triassic bivalve Monotis is the largest, most widespread Monotis in North America 27 TRIASSIC AGE CONSTRAINTS FOR THE CHULITNA TERRANE STRATA PROVIDED BY PREVIOUS WORKERS Sub-unit (according to Age control Clautice and others, 2001) Trr Upper contact with TRlb observed by this team Red bed sandstone and Lower contact with TRlb observed by conglomerate Clautice(2001) Lower gradational contact with unit TRs TRs Norian to Rhaetian faunal assemblages Brown sandstone and Heterastridum, snails, and bivalves argillite TRIb Limestones composed of colonial scleractinian Basalt, Basaltic tuff, and corals, megalodontid bivalves, and limestone Spondylospira lewesensis brachiopod all indicate that the age is Norian Basaltic Dikes and sills 40Ar/39Ar on dikes and sills indicates a minimum age of 179 Ma - Compositionally similar to TRlb basalt Trl Bindstone composed of colonial scleracinian Limestone thicket reefs as well as Norian age brachiopods, scleracinian corals, bivalves(Monotis subcircularis), and Estropods Trr Locally cut by TRb=younger than 179 Ma Red-colored tuff, andesite, Overlies limestone with fossils in the Penn- basalt, mudstone, and Permian range conglomerate Basal contact is with conglomerate containing Devonian age radiolarian chert Trl 13 ammonite fossils of Olemekiam age Table 1.2. Summary of the age constraints on the Triassic stratigraphy of the Chulitna terrane provided by Blodgett and Clautice, (2000). 28 (Siberling and others, 1997). It is found adjacent to the Chulitna terrane in the Susitna, Nenena, and Wrangellia terranes (Siberling and others, 1997) see Figure 1.2. PURPOSE OF STUDY There are numerous occurrences of Triassic strata throughout the Alaska Range Suture Zone. Previous studies have focused on the accretionary history of the continental margin since Jurassic time, however, very little is known about the Triassic history of the continental margin and island arc prior to accretion. Numerous Triassic and older, fault-bounded terranes occur north of the Wrangellia composite terrane and south of the Mesozoic continental margin. Each of these terranes has been studied within the general context of regional mapping projects and, although there are some stratigraphic similarities shared between terranes, there is little consensus on the tectonic history of the North American Cordillera, as preserved in Triassic stratigraphy, due primarily to a lack of detailed study. A second factor that has limited our understanding of the Triassic tectonic evolution of the North American Cordillera is the lack of exposed Triassic age siliciclastic strata in southern Alaska. Siliciclastic strata preserved in mountain belts are often the only remaining record of exhumation associated with tectonic activity along active margins. Many of the terranes in this region share a common stratigraphic link that includes interbedded basalt and limestone units, however, the occurrence of Triassic siliciclastic strata is rare if not absent. One 29 exception is the Chulitna terrane in south-central Alaska. Based on findings from previous studies, three hypotheses have been proposed to explain the stratigraphic history, tectonic evolution, and present day occurrence of the Chulitna terrane in the Alaska Range suture zone. The hypotheses include: 1.) The Chulitna terrane represents an “exotic” tectonic terrane that has no prior association or affiliation with the North American margin or the Wrangellia Composite Terrane. In this model, the Chulitna terrane is considered truly “exotic” or “suspect” within the context of the Alaska Range suture zone and the North American Cordillera. Jones et al. (1982) have proposed that much of the older parts of the Chulitna terrane may represent part of an ophiolite sliver that was obducted to the margin during accretion of the terrane. The enigmatic occurrence of red bed units together with isolated biostratigraphic data have been used as the main evidence for an exotic origin for the Chulitna terrane. 2.) The Chulitna terrane represents a sliver of stratigraphy that was inboard of the Wrangellia composite terrane and possibly accreted to the island arc prior to its eventual accretion to the North American margin (Csejtey and others, 1992). In this model, the Chulitna stratigraphy was thrust northward to its present position and represents a faulted-bounded klippe within the Alaska Range suture zone. Clautice et al. (2001) present a similar model by which the Chulitna stratigraphy correlates with strata of the Wrangellia composite terrane and represents a fault-bounded portion of Wrangellia that has undergone northward structural emplacement. Geochemical studies of parts of the Chulitna stratigraphy indicate a large volcanic component which resembles calc-alkaline 30 arc rocks associated with Wrangellia (Clautice and others, 2001). Geochemical data combined with biostratigraphic similarities between the Chulitna terrane and Wrangellia are cited as evidence that links the two terranes. 3.) The Chulitna terrane may represent a displaced sliver of continental crust associated with the western margin of North America. This model implies that the Chulitna stratigraphy may share some affinity with the North American margin. It is important to note that this is an emerging model based on preliminary U-Pb detrital zircon data from the Chulitna terrane that potentially suggests a stratigraphic link to the North American Cordillera by the end of the Triassic (Hampton and others, 2007). LITTLE SHOTGUN CREEK STUDY AREA The Chulitna terrane is oblique to the Denali fault, running NE to SW. At the southwestern corner of the terrane there are several ridges and valleys that provide excellent exposures of the Chulitna terrane stratigraphy. Little Shotgun creek (Figure 1.4.) is the ideal study location to address the hypotheses of this project due to the excellent exposure of Triassic stratigraphy that is part of the Chulitna terrane. Figure 1.4 shows the Paleozoic-Mesozoic stratigraphy that is present along the ridges of the Little Shotgun Creek catchment. It is important to note that within the 10 km2 study area, located in the western region of the Chulitna terrane, red beds and metavolcaniclastic units described by previous workers are exposed. Siliciclastic and carbonate units make up the bulk of the Little Shotgun creek measured sections shown in the Little Shotgun sections. Figure 1.4 shows the location of measured sections for this study and the general 31 geology of the Little Shotgun creek study area. Jurassic, Permian, and Devonian strata are exposed at the bottom of section LS-062907 for 132.5 meters. The basal Triassic is a conglomerate with Devonian boulders of chert. Serpentinite is highly weathered in this rubble-crop exposure and it appears to be gradational with the Triassic conglomerate. The contacts between Triassic units also appeared gradational in many places. Few erosional contacts were noted, although they could not be traced due to rubble-crop exposure. Overall measured sections show 650 meters of coarsening upward sequences. 550 meters of the total has been connected with the Triassic units. Triassic basalt and tuff units measured approximately 25 meters total in the area. Little Shotgun creek Triassic stratigraphy can be divided into three major units: siliciclastics, carbonates, and interbedded basalt and limestone. These units have been further subdivided by previous workers based on varying interpretations (Jones and others, 1982; Clautice and others, 2001; Blodgett and Clautice, 2000). METHODS AND APPROACH In order to determine the significance of the Chulitna terrane in terms of its relationship to Wrangellia, North America, or other terranes within the suture zone a detailed study of the Triassic stratigraphy of the Chulitna terrane was the primary goal of this study. The purpose of this project is to conduct a geologic field and laboratory study to document the sedimentologic and stratigraphic history and determine the general provenance of Triassic strata from the Chulitna terrane in the Alaska Range suture zone. Measured stratigraphic sections provide the data 32 Figure 1.4. Geologic map of Devonian (?) - Jurassic units in the Little Shotgun creek study area in the Chulitna terrane. Modified from Clautice and others (2001). Note the location of three measured sections in the Little Shogun creek study area. 33 MAP LEGEND 3” Faults .Ouaternary - Alluvium (Gal) “Cretaceous-Paleogene - Igneous rocks (K-P) “Jurassic-Cretaceous - Submarine lan deposites of the Kahiltna assemblage (Jr-K) KJ Jurassic-Cretaceous (Jr-K) - Sandstone, argillite, sandy carbonate, limestone, and volcanic rocks (KJ) “Triassic (Norian) - Limestone and basalt (Trlb) T" Triassic (Norlan) - Sedimentary and volcaniclastic strata (sandstone and conglomerate) (Trr) ‘i'rs Triassic (Norian-Rhaetlan) - Sedimentary strata (calcareous sandstone, argillite, sandy limestone) (Trs) Trl Triassic (Norian) - Fossllllerous limestone (Trl) E Upper Permian - Limestone (PI) fl Permian - Mudstone and siltstone (Ps) Paleozoic? - Devonian? - Sedimentary, volcanic, and volcaniclastic units (Pz?) _- Paleozoic? - Devonian? - Ultramaflc slivers, dikes, and plugs Figure 1.4. 34 needed to determine facies associations and develop a depositional model for the Chulitna terrane. Descriptive petrography along with geochemical studies provided by previous workers will be utilized to suggest provenance within a stratigraphic context. The field portion of this study included 30 days in the Chulitna terrane and consisted of geologic mapping, measuring stratigraphic sections, and sample collection from Triassic stratigraphy. Geologic mapping is done in the field area to compare with previous geologic maps and make corrections based on field observations. Measured stratigraphic sections aided in providing the first detailed stratigraphic picture of the Chulitna terrane. In general, working knowledge of stratigraphy allows for upsection (temporal) and along-strike (spatial) comparison of sedimentary depositional environments. As a whole, mapping and measured sections provide the basis for comparing the Chulitna stratigraphy with the Triassic units of the Wrangellia composite terrane, North American continental margin, and other tectonic terranes in the Alaska Range suture zone. A total of 74 sandstone and limestone samples were collected within the context of measured sections. These samples were prepared for thin sections and descriptive petrography data was collected. Outcrop, hand sample, and petrographic descriptions provide the basis to classify facies. Describing facies within a stratigraphic context results in facies associations. A depositional model for the facies encountered within the measured sections of Little shotgun creek will be developed. Once a depositional model for the Chulitna terrane has been established the environment implicated can be compared with the existing tectonic 35 models. The results of this study will provide a stratigraphic context to compare and contrast the western region of the Chulitna terrane with future stratigraphic measurements of the central and eastern portion of the Chulitna terrane. These results can also be applied to existing geochemical and paleontological studies to add a stratigraphic constraint. Measured Chulitna terrane stratigraphy can be compared and contrasted with the Triassic strata of other terranes within the Alaska Range suture zone measured by previous workers. 36 CHAPTER II: SEDIMENTOLOG Y, STRA TIGRAPH Y, AND DEPOSITIONAL SYSTEMS INTRODUCTION Chulitna terrane stratigraphy is characterized and defined primarily by Paleozoic to Mesozoic volcanic rocks and carbonate and siliciclastic sedimentary strata. Paleozoic units include bedded chert, siliceous mudstone, fine-grained sandstone, limestone, and basalt. Mesozoic (Upper Triassic) units make up some of the more enigmatic strata in the Chulitna terrane and are reported to consist of continental red beds and volcaniclastic strata (Clautice and others, 2001; Jones and others, 1982). Triassic strata of the Chulitna terrane also include interbedded limestone, basalt, and tuff. Although previous workers have provided general descriptions of Upper Triassic strata exposed throughout the Chulitna terrane these units have never been measured or described within a sedimentologic and stratigraphic context. Moreover, the nature of the contacts between these units as well as a general depositional environment has not been constrained for these strata. The purpose of this study is to measure, document, and classify the Upper Triassic siliciclastic and carbonate sedimentary strata encountered in the Little Shotgun Creek study area and Chulitna terrane for the purpose of better understanding the stratigraphic and tectonic implications of these strata in the context of the Alaska Range suture zone. Consensus regarding a depositional model for these units has eluded previous workers but for the first time this study anchors the depositional model on measured stratigraphy. Any tectonic 37 implications suggested by the resultant depositional model can be reviewed to determine general relationships of the new data to test existing tectonic models for the Chulitna terrane. GENERAL FACIES METHODOLOGY The feedback between sedimentary environments and sedimentary facies relies on physical properties. Physical properties as outlined in this study include documentation of sedimentary structures and textures, geometry, thickness, and stratigraphic and structural relationships between individual units. Measurement and description of these properties have be used in this study to constrain facies associations and ultimately to determined depositional environments that were active during deposition of Upper Triassic strata of the Chulitna terrane. Sedimentary facies are classified based on geometry of the deposit, primary sediment properties, and derived sediment properties. In order to work backward from observed facies to specific processes it is necessary to group facies according to environmental or genetic similarities into facies associations. In order to determine the depositional environment for facies encountered in the Chulitna terrane this study focuses on grain size, sorting, shape, the presence of matrix, general grain composition, fossil content, thickness and lateral extent, geometry of the beds, and sedimentary structures. Documentation of each of these characteristics in the context of measured stratigraphic sections can be used to infer a temporal change in depositional environment for each Upper Triassic siliciclastic unit from the Chulitna terrane. 38 STRATIGRAPHIC OVERVIEW Sedimentologic and stratigraphic data were collected from three measured stratigraphic sections in Upper Triassic stratigraphy from the Little Shotgun Creek study area of the Chulitna terrane (Figure 2.1). The Little Shotgun Creek study area is located in the southwestern extent of the Chulitna terrane (Healy A-6 1:36.360 quadrangle) and represent some of the best exposed Upper Triassic strata in the Alaska Range (Figure 2.3, 2.4, 2.5). Figure 2.2 represents a summary of three new measured stratigraphic sections from Upper Triassic strata of the Chulitna terrane. Refer to Appendix A of this document for a detailed, meter-by-meter summary of sedimentologic and stratigraphic data from measured sections. Overall, Upper Triassic strata measured from the field area consist primarily of two distinct successions (Figure 2.3, 2.4, 2.5). The lowermost succession is characterized by interbedded conglomerate, sandstone, siltstone, and fossiliferous and sandy limestone (Figure 2.4). The upper succession consists primarily of interbedded basalt and limestone (Figure 2.3, 2.5). Previous studies have identified the general lithology of each of these successions; however, this is the first study to present Upper Triassic strata in the context of an upper and lower succession. This study is focused largely on documenting the sedimentologic and stratigraphic trends of interbedded siliciclastic and 39 Figure 2.1. Generalized geologic map of the Little Shotgun Creek study area in the southwest part of the Chulitna terrane. The focus of this study is on Upper Triassic (Norian—Rhaetian) siliciclastic and carbonate strata (red and orange map units, respectively). The location of stratigraphic sections measured as a part of this study is denoted by numbered circles. Geologic map is modified from Clautice et al., 2001. 40 Ye -Setion 1: 1 " k . . fiL-JS’ . réfitmwww MAP LEGEND 3". Faults by Quaternary - Alluvlum (Gal) ”Cretaceous-Paleogene - igneous rocks (it-P) “Jurassic-Cretaceous - Submarine tan deposites of the Kahiltna assemblage (Jr-K) KJ Jurassic-Cretaceous (Jr-K) - Sandstone, argillite, sandy carbonate, limestone, and volcanic rocks (KJ) “Triassic (Norian) - Limestone and basalt (Trlb) T" Triassic (Norian) - Sedimentary and volcaniclastic strata (sandstone and conglomerate) (Trr) Triassic (Noflan-Rbaetlan) - Sedimentary strata (calcareous sandstone, argillite, sandy limestone) (Trs) Trl Triassic (Norian) - Fossliltereus limestone (Trl) “Upper Permian - Limestone (Pl) fl Permian - Mudstone and siltstone (Ps) mPaleozoic? - Devonian? - Sedimentary, volcanic, and volcaniclastic units (Pa?) _. Paleozoic? - Devonian? - Ultramaflc silvers, dikes, and plugs Figure 2.1 41 Figure 2.2 Measured stratigraphic sections from Upper Triassic strata in the Little Shotgun Creek study area, Refer to figure 2.1 for a location of measured sections in the Little Shotgun Creek study area and the Chulitna terrane. Note that the majority of measured sections document Upper Triassic siliciclastic and carbonate units, however stratigraphic relationships have been documented with overlying, younger basal and limestone units. A structural relationship has been observed between Upper Triassic siliciclastic and carbonate units and adjacent Paleozoic (?)—Jurassic (?) siliceous mudstone, pillow basalt, and serpentinite units. 42 cflfl :33 9532.. .m wcoumoE: mu. 298an.80 E macawucmw E Begafl ocosmuzz - >8. .3sz :28me $2550 znmom . Figure 2.2. 43 Figure 2.3. Exposure of upper and lower successions of Upper Triassic stratigraphy in the Little Shotgun Creek study area and surrounding regions. (A) View to the north of interbedded siliciclastic and carbonate strata with Devonian (??) basalt and serpentinite (SP/BS) (view to the north). Red arrows denote location of Upper Triassic strata in photo B. (B) View to the east of upright, dipping beds off siliciclastic (Trr) and carbonate strata (T rs) and overlying basalt and limestone (Trlb)). Thick red arrow denotes a thrust fault in the Trr and Trs units. 44 79% . - 'r‘ ""1 I, 1‘, (If? ' 90:“ rr":"”"' "rift/[l ‘- ‘. \. 1 // I . 1",” s‘ (r, // '4. I .. , ' - f r 1' A / ~\\ “ RIASSIC SECTIO .t\ Figure 2.3 45 Figure 2.4 Upright, dipping strata comprised of interbedded siliciclastic redbed units (Trr) and limestone (Trl) that make up measured section 2 (refer to figure 2.2. for location of measured section). Measured stratigraphic sections reveal the occurrences of stratigraphic contacts between siliciclastic and carbonate strata and are reported here to represent a continuous stratigraphic succession. View to the north and strata are dipping into the photo. The Trl unit in this figure is ~60 mm thick for scale. Refer to measured section 2 and Appendix for detailed sedimentologic and stratigraphic descriptions from these strata. 46 Figure 2.5. Upright, dipping beds of Upper Triassic strata consisting of a lower succession of interbedded redbed (Trr) and limestone (Ts) units and overlying succession of interbedded basalt and limestone (Trlb). This photo was taken in the central to northeastern parts of the Chulitna terrane. Note the similarity in stratigraphic relationships between Upper Triassic strata in this location as compared with documented measured strata from the Little Shotgun Creek field area of this study. 47 carbonate strata that are preserved in the lowermost succession of the Upper Triassic strata of the Chulitna terrane. While the study summarizes trends from the southwestern area of the Chulitna terrane, similar stratigraphic relationships have been observed in the northeastern region of the Chulitna terrane (Figure 2.5). The following text provides a generally summary of stratigraphic trends from base-to-top of Upper Triassic siliciclastic and carbonate strata of the upper succession in the Little Shotgun Creek study area. Table 6 is a summary of sedimentary facies classifications that have been documented from the Chulitna terrane in this study. Upsection Stratigraphic Trends The lowermost portion of Upper Triassic strata consists primarily of interbedded red and green conglomerate, sandstone, and mudstone. These strata are documented in the upper 75 m of measured section 3 (Figure 2.2) and are some of the most coarse-grained strata reported from the Chulitna terrane consisting primarily of interbedded conglomerate and coarse-grained sandstone. Overall, conglomeratic units have tabular geometries and are characterized largely by matrix- and Clast-supported gravel deposits that contain outsized clasts of red chert. Outsized chert boulders when measured along their long axis, range in size from 0.15—1.0 m in length and are on average ~0.25 m long. The occurrence of matrix— and Clast-supported conglomerate is common throughout the lower succession of Upper Triassic strata in the study area. However, this is 48 the only location in measured stratigraphic sections where outsized chert clasts have been observed and documented in this study. The majority of measured sections from the Little Shotgun Creek study area reveal stratigraphic relationships between interbedded Upper Triassic siliciclastic conglomerate and sandstone units and carbonate strata. Siliciclastic units range in thickness between 1-3 m thick while carbonate strata is typically <1 m thick. These strata have been documented in measured section 1 and 2 (refer to Figure 2.1. and 2.2. for location and general upsection trends of measured sections). Section 3 documents a stratigraphic relationship between older Upper Triassic siliciclastic and carbonate strata and younger uppermost Triassic basalt and limestone (Figure 2.2.). Prior to this study, much of these siliciclastic and carbonate strata where divided into 3-4 separate units that were thought to represent several very different depositional environments (Clautice et al, 2001). In addition, given that much of the stratigraphy of the Chulitna terrane has been structurally deformed, many of the contacts between Upper Triassic siliciclastic and carbonate strata were thought to possibly represent structural contacts rather than stratigraphic contacts. Previous studies have also suggested that Upper Triassic strata from the Little Shotgun Creek region are overturned with the oldest beds being interbedded basalt and limestone and younger units being siliciclastic redbed units and carbonate strata. Measured sections and stratigraphic relationships documented hear reveal that (1) Upper Triassic redbed and carbonate strata are interbedded and potentially reflect a depositional system that includes a strong 49 siliciclastic and carbonate component and (2) that interbedded basalt and carbonate strata that were originally proposed to be older than redbed and carbonate units are actually younger and stratigraphically overlie older siliciclastic and carbonate strata. SEDIMENTARY FACIES Sedimentologic and stratigraphic descriptions from measured sections reveal a total of 21 individual lithofacies with 14 representing siliciclastic transport and deposition and 7 representing some combination of carbonate transport, deposition, and in situ precipitation. Table 2.1 and 2.2 present a summary of each facies documented from Upper Triassic siliciclastic strata and carbonate units from the Little Shotgun Creek study area and the corresponding facies codes for each lithofacies. Siliciclastic facies are made up primarily of gravel, sand, silt, and mud facies. Carbonate lithofacies consist largely of muddy limestone, sandy limestone, and fossiliferous limestone. Both siliciclastic and carbonate, Upper Triassic, facies can be found interbedded from base-to-top in the study area. The coarsest-grained siliciclastic lithofacies consist of massive, matrix- supported pebble and cobble conglomerate (Gmm), matrix-supported conglomerate with crude normal grading (Gmg), and massive, Clast-supported conglomerate (Gcm) (Figure 2.6 A, B, C, D). Clast-supported, bedded gravel (Gh), as well as planar (Gp) and trough (Gt) cross-stratified gravel (Figure 2.6. C, 50 SILICICLAS TIC LITHOFACIES Facies codes Sedimentary structures and facies Interpretation 6 mm Massive-structurless; matrixnsupported. Pebble Sediment gravity flow (plastic); high strength, and cobble conglomerate viscous Gm Matrix-supported. Pebble and cobble conglomerate; Sediment gravity flow (pseudoplatic); low 9 crude normal grading strength, viscous G Massive-structureless; Clast-supported. Pebble Sediment gravity flow (pseudoplastic); inertial cm and cobble conglomerate bedload, turbulent flow) Gh Clast-supported, bedded gravel. Pebble and Longitudinal bedform migration; lag deposits; cobble conglomerate; horizontal bedding; imbrication bedload, turbulet flow Gp Planar cross-stratification. Pebble and cobble Longitudinal to transverse bedform migration conglomerate Trough cross-stratification. Pebble and cobble . . . Gt conglomerate; no grading; individual beds are typically Deposrtronof gravel under upper flow regime . condrstron rn scours and small channels <05 m thick Sm Massive-structuriess. Fine- to medium-grained Sediment gravity flow; near-surface turbidity sandstone; no grading or stratification currents in fluid flow Sh Horizontal stratification. F ine- to medium-grained Deposition under upper and/or lower plane- sandstone; no grading; flat parallel lamination bed conditions (critical to sub-critical flow) Ripple cross-stratification. Fine- to medium- Deposrtrondurrng “9’3"“ Of WW“ Sr . j . . . (asymmetric) ripples. lower flow regime grained sandstone, no grading, asymmetnc geometry . . conditions Low-angle cross-stratification. Fine- to medium- Migration of small-scale 20 dunes and/or SI grained sandstone; no grading; cross-stratification plane-bed macroforms in shallow watter; dips <10° with respect to bedding deposition on a sloped surface planar c‘ ass-stratification. Frne- to medrum- Migration of small-scale 2D dunes in shallow Sp grained sandstone, no grading, occur in beds that are . . . . . water, transverse and lmguord bedforms typically <0.25 m thick Trough cross-stratification. F ine- to medium- Migration of 3D dunes in shallow water; St grained sandstone; no grading; beds contain trough traction processes during lower to upper flow sets that are <05 m thick regmine conditions; linguoid bedforms Pedogenic carbonate. Mudstone and claystone; Soil development of mud deposits resulting P massive aggregated texture; sphereoidal (5-7 cm die.) from infiltration, leaching, and carbonate and elongate (10-25 cm long) calcareous nodules precipitation during subaerial exposure . . , Suspension fallout; deposition during lower Fsm Massive-structureless. Srltstone and mudstone, no flow regime conditions; waning flow and grading or stratification . . fallout In standing water Table 2.1 Summary of siliciclastic sedimentary facies classifications and interpretations from Upper Triassic strata of the Chulitna terrane, utilizing and adapting facies codes from Miall (1977). 51 CARBONA TE LITHOFACIES [Facies codes Sedimentary structures and facies Interpretation Mudstone (massive). Muddy limestone; Suspension fallout or possible sediment gravity "3 isolated fine-grained (<0.03 mm) biogenic and elastic flow (plastic); components not organically components “a POLEEEUEDQEGEOSWO" Wackestone (massive to laminated). Matrix- [Sediment gravity flow (plastic) or deposition by WS supported muddy limestone with fine-grained suspension fallout; components not organically (<0.03 mm) biogenic and elastic components bound during deposition Packstone (massive). Grainsupported limestone; Sediment gravity flow “(psudneoplasticl or minor PS lime mud matrix with fine- to course-grained sediment .reworkrng/wrnnowrng; components __ ____uz,-L0-O3_:_2. mmtclestigano t109erigcompent-rots... __, Mm: °§3E°E£Y b09051 “1’"‘19 “£99999; Sandy limestone (massive to stratified). Grain- Sediment reworking/winnowing or sediment SIS supported limestone; fine- to course—grained gravity flow (pseudOplastiC); components (0.03-2 mm) Clastic an_d biogg'iic components _ not organically bound during deposition Floatstone (massive). Matrix supported muddy . Sediment gravity flow (plastic); components FS limestone with coarse-grained (>2 mm) biogenic were not bound organically during deposition 3099929933- RS Rudstone (massive). Grain-supported limestone; Sediment gravity flow (pseudoplastic); , coarse-grained (>2 mm) biogenic components components were not bound organically during deposrtron BS Boundstone (blndstone). biogenic component bound r Q6905i,ti°" by binding and encrusting Of and encrusted togther by lime mud biogenic organism; components were bound organically during deposrtion Table 2.2 Summary of carbonate sedimentary facies classifications and interpretations from Upper Triassic strata of the Chulitna terrane, utilizing and adapting carbonate classification from Dunham (1962). 52 Figure 2.6. Summary of all gravel facies documented from Upper Triassic siliciclastic strata of the Chulitna terrane. Conglomerate and sandstone units are interbedded throughout measured sections and nearly all siliciclastic and carbonate facies have been documented from base-to-top of measured sections. (A) Massive, matrix-supported pebble conglomerate (Gmm) overlying a massive sandstone bed (hand lens for scale). (B) Interbedded matrix-supported conglomerate (Gmg), and massive (Sm) and horizontally stratified (Sh) medium-grained sandstone (coin for scale). (C) Interbedded massive, Clast-supported pebble to cobble conglomerate (Gcm) and bedded pebble conglomerate (Gh) (rock hammer for scale). (D) Lenticular bed of matrix-supported pebble conglomerate (Gmg) and horizontally-stratified (Sh) coarse-grained sandstone bound by massive, Clast-supported pebble conglomerate (Gcm) (rock hammer for scale). (E) Trough cross-stratified pebble conglomerate (Gt) bound by massive (Sm) and horizontally-stratified (Sh) medium-grained sandstone (rock hammer for scale). (F) Interbedded planar cross-stratified pebble and cobble conglomerate (Gp) and horizontally-stratified (Sh) medium-grained sandstone (coin near center of photo for scale). 53 GRA VEL FACIES Figure 2.6. 54 E, F). Bedded gravel is characterized by horizontal bedding and isolated occurrences of crude pebble imbrication. Sandstone facies that occur in Upper Triassic strata consist primarily of trough cross-stratification (St), horizontal stratification (Sh), planar cross-stratification (Sp), as well as low-angle cross- stratification (SI) (Figure 2.7. A, B, C, F). The most commonly occurring facies is massive, fine- to medium-grained sandstone (Sm) (Figure 2.7. B, F), Ripple cross-stratification (Sr) is rare and occurs sporadically throughout these strata (Figure 2.7 F). The majority of conglomerate and sandstone deposits are deep red in color; however there are repeated occurrences of green to gray conglomerate deposits throughout Upper Triassic stratigraphy. F ine-grained siltstone and mudstone occurs throughout measured sections and is most commonly massive to slightly laminated (Fsm) and often exhibits an aggregated texture (P) (Figure 2.8. A, B, C, D). Spheroidal (3-8 cm dia.) to elongate (6-20 cm long) carbonate nodules are common in aggregated mudstone deposits (Figure 2.8. E, F). Siltstone and mudstone beds are typically <3 m thick, however deposits can be up to 15 m thick. Fine-grained strata throughout the Upper Triassic succession of redbeds is characterized by dark red colors as well as isolated occurrences of light green aggregated deposits with carbonate nodules (Figure 2.8. C, D, E, F). Siltstone and mudstone are interbedded with conglomerate and sandstone facies from base to top of measured sections in both siliciclastic and carbonate strata. Carbonate lithofacies observed from Upper Triassic strata in the study area consist largely of muddy limestone (MS), sandy limestone (Sls), massive to 55 Figure 2.7. Summary of all sandstone facies documented from Upper Triassic siliciclastic strata of the Chulitna terrane. Sandstone and conglomerate units are interbedded throughout measured sections and nearly all sandstone facies have been documented from base-to-ntop of measured sections. (A) Trough cross-stratified, medium- to coarse-grained sandstone (coin for scale). (B) Interbedded horizontally-stratified (Sh) and planar cross-stratified (Sp) fine-grained sandstone (hammer for scale). (C) Interbedded low-angle, cross-stratification (SI) and horizontal stratification (Sh) (coin for scale). (D) Interbedded horizontal stratification (Sh), matrix-supported pebble conglomerate (Gmg), and massive sandstone (Sm) (rock hammer for scale). (E) Massive sandstone (Sm) beds ranging from 0.25-1 m thick (rock hammer for scale). (F) Individual beds of horizontally-stratified (Sh), ripple cross-stratified (Sr), and planar cross-stratified, medium- to coarse-grained sandstone interbedded with planar cross-stratified conglomerate (Gp), and massive, matrix-supported conglomerate (Gmm) (rock hammer for scale). 56 SANDS TONE FA CIES Figure 2.7. 57 Figure 2.8. Summary of all mudstone and siltstone facies documented from Upper Triassic siliciclastic strata of the Chulitna terrane. Mudstone and siltstone is interbedded with sandstone and conglomerate units throughout measured sections. (A) Interbedded massive sandstone (Sm) and massive mudstone (Fsm); individual bed is~0.75 m thick (hammer for scale). (B) A thick (~15 m) succession of red to yellow aggregated mudstone (P) with carbonate nodules (red and yellow oxidized region represents ~15 m of aggregated mudstone for scale). (C) Interbedded red (oxidized) and light green (slightly reduced) aggregated mudstone (P) (hammer for scale). (D) Interbedded sandy limestone (Sls) and light green (reduced) aggregated mudstone with gravel stringers (rock hammer for scale). (E) Massive mudstone and siltstone (Fsm) overlain by aggregated mudstone with yellow spheroidal (3-8 cm dia.) carbonate nodules (P) (rock hammer for scale). (F) Dark red (oxidized) aggregated mudstone with yellow elongate (6-15 cm long) carbonate nodules (hammer for scale). 58 MUDS TONE/S IL TS TONE FA C IE 8 P oxldf ed '\4 . Figure 2.8. 59 Figure 2.9. Summary of all carbonate facies documented from Upper Triassic siliciclastic strata of the Chulitna terrane. Carbonate strata are sporadically interbedded with siliciclastic redbeds units from the middle to upper parts of measured sections. Carbonate facies are primarily indicative of sedimentation by transport of eroded carbonate material. There are repeated occurrences of in situ carbonate precipitation as well. (A) Tabular beds of muddy limestone (MS); beds range are on average ~0.10-0.25 m thick (hammer for scale). (B) Tilted interbedded succession of muddy limestone (MS) and massive wackestone (VVS) (hammer for scale). (C) Interbedded sandy limestone (Sls), packstone (PS), and fossiliferous floatstone (FS) (hammer for scale). (D) Thin (0.10-0.25 m), tabular beds of sandy limestone (Sls) (rock hammer for scale). (E) Interbedded packstone (PS) and fossiliferous floatstone (FS); fossil hash in fioatstone is typically coarse-grained ( >2mm) (coin for scale). (F) Rudstone (RS) consisting largely of fossil hash with lesser amounts of lime mud matrix (coin for scale). (G) Interbedded, tabular beds of sandy limestone (Sls) and fioatstone (F S); beds are typically <0.25 m thick (rock hammer for scale). (H) Tabular to irregular beds of Boundstone (bindstone) (BS); primary fossil occurrences include hydrozoan Heterastridium (round gray limey nodules in bed just above rock hammer). 60 CARBONA TE FACIES Figure 2.9. 61 laminated, matrix-supported wackestone (VVS), and matrix-supported, massive floatstone (FS), as well as massive, grain-supported packstone (PS) and rudstone (RS) (Figure 2.9. A, B, C, D, E, F, G). With the exception of muddy limestone, the majority of carbonate lithofacies contain some component of fossil hash. Fossiliferous boundstone (BS) is commonly found interbedded with the carbonate facies listed above and contains abundant occurrences of the hydrozoan Heterastridium (Figure 2.9 H) FACIES ASSOCIATIONS Documentation of measured stratigraphic sections, including bed thickness and geometry, and sedimentary facies identification have resulted in classification and interpretation of six distinct facies associations that occur throughout Upper Triassic strata in the Little Shotgun Creek study area and the greater Chulitna terrane. Facies associations were determined by grouping sedimentary facies (Table 2.1. and 2.2.) that reflect similar depositional patterns and environments. It is important to note that any one individual facies may occur in more than one facies association. Table 2.3. provides a summary of facies associations identified from siliciclastic and carbonate strata of the Chulitna terrane and includes descriptions on bed thickness, lateral extent, geometry (tabular vs. lenticular), and facies distribution as well as general descriptions on lithology and color. The following text accompanies Table 2.3. and provides a summary for each facies association. 62 Facies Association 1 (FA 1) Description. Individual beds in Facies Association 1 (FA 1) are tabular and lenticular and consist largely of interbedded fine- to coarse-grained sandstone and conglomerate bodies. Beds are laterally extensive for 75-150 m and are typically 2-3 m thick. Lenticular beds exhibit with-to-thickness ratios of >25:1. Overall, sandstone and conglomerate deposits of FA 1 range from light to dark red to green in color. Conglomerate units consist primarily of pebble and cobble conglomerate bodies that are made up primarily of matrix-supported, massive gravel deposits (Gmm) and massive gravel with isolated yet regular occurrences of crude normal grading (Gmg). Clast-supported massive gravel deposits (Gcm) occur sporadically throughout these deposits and are interbedded with massive gravel deposits and sandstone. Sandstone bodies consist primarily of fine— to medium-grained massive sandstone (Sm) and are interbedded with conglomerate throughout measured sections. Thin deposits (<1 m thick) of massive and structureless mudstone (Fsm) are sporadically interbedded with sandstone and conglomerate units from base to top of measured sections. Interpretation - Debris flow (clastic). Facies Association 1 is interpreted here to represented deposition by punctuated sediment gravity flows characterized by plastic/pseudoplastic deposition. Conglomerate, sandstone, and mudstone facies distribution is likely due to (1) initial high-energy debris flow events followed by (2) subsequent waning flow and nondeposition (Collinson, 63 1996). The red color of the majority of these strata are interpreted to reflect oxidation in a nonmarine setting. The green color of strata are thought to reflect sedimentation in slightly-reduced shallow marginal marine conditions (Collinson, 1996; Bridge and Demicco, 2008). Overall sediment gravity flows are interpreted to have occurred under plastic to pseudoplastic conditions based on the distribution of facies reflecting massive to crudely graded bedding. Individual events likely varied from low to high strength debris flow events. Individual flow events were commonly viscous with isolated occurrences of near-surface turbidity as evidence by isolated occurrence of clast supported conglomerate. Debris flow events were characterized by gravel, sand, silt, and mud size fraction. Deposition of mudstone likely occurred during the waning flow stages of sediment gravity flows during low flow regime conditions and/or suspension fallout (Collinson, 1996; Boggs, 2001) . Facies Association 2 (FA 2) Description. Facies Association 2 (FA 2) consist entirely of sandstone and conglomerate bodies that exhibit lenticular geometries. Individual units are laterally extensive for 50-75 m and beds are typically 1-3 m thick exhibiting with- to-thickness ratios of >15:1. Deposits range from dark red to light green in color. The coarsest grained lithology in FA 2 includes pebble and cobble conglomerate bodies that consist of Clast-supported, bedded gravel (Gh) and planar cross- stratified (Gp), and trough-cross stratified (St) gravel. Gravel stringers are common and crude pebble imbrication can be observed in isolated instances. 64 Sandstone units of FA 2 are made-up of fine- to coarse-grained sandstone characterized by horizontal (Sh), ripple (Sr), low-angle (sl), planar (Sp), and trough (St) cross-stratification. Thin deposits (<0.10 m) of massive and structureless mudstone (Fsm) are sporadically interbedded with sandstone and conglomerate throughout measured sections Interpretation - Channels and bars (fluvial/deltaicltidal). Facies Association 2 (FA 2) is interpreted to represent deposition by (1) ephemeral flow and bar migration in poorly-confined, fluvial channels (nonmarine), (2) small- scale delta mouth bar migration (marginal marine) of lateral and transverse bars, and/or (3) lateral and transverse bar migration in tidal channels (marginal marine) (Collinson, 1996; Reading and Collinson, 1996). Nonmarine fluvial deposits likely owe their red color to oxidization while deltaic and tidal deposits are typically green in color, which is attributed to sedimentation in a slightly less- oxidizing shallow, marginal-marine environment. Stratified conglomerate and sandstone is interpreted to reflect migration in channels of gravel and sand bars (Reading and Collinson, 1996; Boggs, 2001) . Bar migration reflects downstream and lateral migration of longitudinal to transverse bars during upper flow regime conditions. Mudstone deposits are observed in fluvial channels and likely represent sedimentation during waning channel flow and/or suspension fallout in standing water that remains in ephemeral channels after flow events. Much of the stratified sandstone and conglomerate deposits may be the result of fluvial reworking of the top of debris flow deposits (FA 1). 65 Facies Association 3 (FA 3) Description. Deposits of Facies Association 3 (FA 3) consist entirely of massive- to slightly laminated mudstone (Fsm) and massive, aggregated mudstone that occurs in conjunction with extensive carbonate nodule horizons (P). Overall these deposits are tabular and individual units are laterally extensive for >150 m. Beds are typically 2-3 m thick, however, interbedded fine-grained deposits can range in thickness from 5-15 m. All mudstone units have nonerosive contacts at base of beds and deposits are dominantly dark red with isolated occurrences of light green aggregated mudstone. Carbonate nodules are pervasive throughout aggregated fine-grained deposits and are typically yellow in color with individual nodules are spheroidal (3-8 cm dia.) to elongate (6-20 cm long) Interpretation - Floodplain/tidal flat. Facies Association 3 (FA 3) is interpreted to represent deposition of silt and mud during extensive, unconfined sheet flooding in nonmarine floodplain (red) and marginal-marine tidal flat (red to green) regions. Red coloration is attributed to extended periods of nondeposition, subaerial exposure, oxidation, and pedogenic soil formation (Collinson, 1996; Reading and Collinson, 1996) . Light green aggregated mudstone deposits are interpreted to reflect soil development in tidal regions where deposits are initially subaerialy exposed during low tide condition and subsequently submerged during high tide and storm surge conditions. Overall deposits of FA 3 are interpreted to reflect sedimentation in overbank floodplain and tidal flat regions 66 that are adjacent to fluvial or tidal channel and/or distal, waning debris flow (Collinson, 1996; Reading and Collinson, 1996; Johnson and Baldwin, 1996) . In either case prolonged subaerial exposure and soil formation occurred after deposition. Facies Association 4 (FA 4) Description. Facies Association 4 (FA 4) consist largely of tabular, very well-sorted bodies of fine-grained sandy limestone (Sls). Individual units are laterally extensive for 75-150 m and consist largely of low-angle cross- stratification. Beds are 1-3 m thick and composed entirely of fine- to medium- grained sandy limestone (Sls) that is bound by calcite cement. Individual units are stratified and very well sorted and sand grains are rounded to subrounded. These strata are typically gray to yellow in color and occur sporadically in conjunction with other carbonate facies throughout measured sections. Interpretation — Beach/barrier island. Facies Association 4 (FA 4) is interpreted to represent deposition in the presence of high- to medium energy wave action along a beach/barrier island setting. Facies distribution reflects a beach front environment where sedimentation and reworking of sand occurred during high-energy wave action in foreshore beach and upper shoreface environments (Reading and Collinson, 1996; Johnson and Baldwin, 1996; Pettijohn et al., 1987). Facies are consistent with wave action above mean fairweather wave base. Stratified beach deposits may represent reworked debris 67 flow deposits and/or reworked fluvial, deltaic, and tidal deposits of FA 1 and FA 2, respectively (Reading and Collinson, 1996; Johnson and Baldwin, 1996) . Facies Association 5 (FA 5) Description. Facies Association 5 (FA 5) is characterized by tabular beds of interbedded fossiliferous boundstone (BS) and muddy limestone (MS). Individual units are laterally extensive for >150 m and beds are typically 0.25-1 m thick. Deposits are gray to yellow in color. Boundstone (bindstone) units are bound and encrusted by lime mud and contain fossiliferous horizons that consist primarily of the hydrozoan Heterastridium. Muddy limestone deposits are typically <1 m thick and have very rare occurrence of primary fossils or fossil hash. Interpretation - Carbonate reef/bank - Lagoon. Facies Association 5 (FA 5) is interpreted to reflect in situ precipitation and deposition by binding and encrusting of biogenic organisms in a low energy, shallow marine carbonate reef/bank to Iagoonal environment. All biogenic components were originally bound during deposition and are interpreted to reflect development of isolated, small-scale, reefs and banks (Wright and Burchette, 1996; Boggs, S., 2001). Reef/bank development was likely characterized by in situ precipitation and deposition in low energy shelf-edge reef and/or Iagoonal carbonate bank environment that may be part of a rimmed carbonate platform to ramp depositional system (Wright and Burchette, 1996). Muddy limestone was likely deposited during suspension settling and or the waning stages of submarine 68 sediment gravity flow (Reading and Collinson, 1996; Wright and Burchette, 1996). Very little to no siliciclastic strata is found interbedded with FA 5. Facies Association 6 (FA 6) Description. Facies Association 6 (FA 6) consists primarily of tabular units of interbedded muddy limestone (MS) and massive to laminated wackestone (VVS) as well as massive floatstone (FS), and rudstone (RS). Individual beds are laterally extensive >75 m and are typically 0.1-0.5 m thick; gray to green—yellow in color. Fossil hash is common throughout these deposits and consist primarily of diarticulated bivalve shells. F ine-grained units of FA 6 consist of massive- to laminated, matrix-supported, muddy limestone with fine- grained (0.03-2 mm) to coarse-grained (>2 mm) fossil hash. Packstone, and rudstone units consist of massive, grain-supported limestone with fine—grained (0.03-2 mm) to coarse-grained (>2 mm) fossil hash. Muddy matrix is present in packstone deposits. Floatstone units are massive, matrix-supported with >2mm components. Deposits of lime mud (MS and WS) also occur throughout this facies association and are typically <1 m thick. Primary fossils and fossil hash is rare in muddy limestone deposits. Interpretation - Debris flow (carbonate). Facies Association 6 (FA 6) is interpreted to represent, punctuated submarine sediment gravity flows in a mid to outer ramp region of a rimmed carbonate platform/ramp shallow marine environment. Fossil debris (hash) is likely derived from mid- to inner ramp reefs/banks that have been eroded by wave energy. Debris flows originated on 69 unstable slopes in mid-ramp regions and may have undergone further gravity flow to deeper outer ramp regions (Wright and Burchette, 1996; Boggs, S., 2001) . Due to the amount of mud preserved in these deposits, debris flows likely deposited sediment below storm wave base and have not been reworked by wave action. Wackestone and floatstone are interpreted to reflect sediment gravity flow under plastic to pseudoplastic conditions. Flows were likely high strength and viscous due to high amount of mud throughout these deposits. Packstone and rudstone are interpreted to represent pseudoplastic, turbulent sediment gravity flows; dominated by coarse-grained fossil hash and lesser amounts of mudstone. In this case turbulence during transport would winnow mud resulting in preservation of primary fossil hash. Muddy limestone units are interpreted to be the result of suspension settling after debris flows or the waning stages of sediment gravity flow. Overall these deposits are interpreted to have accumulated below storm wave base (Wright and Burchette, 1996). DEPOSITIONAL SYSTEM Upper Triassic strata from the Little Shotgun Creek study area and greater Chulitna terrane clearly reflect a depositional system that is dominated by mixed siliciclastic and carbonate environments. Energy during deposition ranged from very high in the case of sediment gravity flows and channels to very low in the case of carbonate bank and reef environments. Previous studies have provided 70 Table 2.3 Summary of descriptions and interpretations of siliciclastic and carbonate sedimentary facies and facies associations for Upper Triassic strata of the Chulitna terrane. 71 TABLE 2.3. DESCRIPTION INTERPRETATION FACIES ASSOCIATION 1 (FA 1) Tabular and lenticular sandstone and conglomerate bodies; individual units are laterally extensive for 75-150 m and beds are typically 2-3 m thick and exhibit with-to-thickness ratios of >25:1; overall. deposits range from light to dark red to green in color (A) Conglomerate. Pebble and cobble conglomerate bodies are made up of primarily of matrix-supported. massive gravel deposits (Gmm); crude, normal grading (Gmg) is common; clast- supported massive gravel deposits (Gcm) occur sporadically throughout these deposits (B) Sandstone. F ine- to medium-grained sandstone bodies are interbedded with conglomerate and primarily massive and structureless (Sm) (C) Mudstone. Thin deposits (<1 m thick) of massive and structurelss (Fsm) mudstone are sporadically interbedded with sandstone and conglomerate DEBRIS FLOW (SUBAERIAUSUBA QUEOUS) Punctuated sediment gravity flows characterized by plastic/pseudoplastic deposition; facies distribution is likely due to (1) initial high-energy debris flow events event and (2) subsequent waning flow and nondeposition; red deposits were oxidized (nonmarine); green strata were slightly reduced (marine) (A) Sediment gravity flow. Low to high strength debris flow events (plastic to pseudoplastic); individual flow events were commonly viscous with isolated occurrences of near-surface turbidity currents; gravel-size fraction (B) Sediment gravity flow. Debris flow events characterized by near surface turbidity currents in fluid flow; sand-size fraction (C) Sediment gravity flow (waning stages). Deposition during the waning flow stages of sediment gravity flows during low flow regime conditions and/or suspension fallout FACIES ASSOCIATION 2 (FA 2) Lenticular sandstone and conglomerate bodies; individual units are laterally extensive for 50-75 m and beds are typically 1-3 m thick and exhibit with-to-thickness ratios of >15:1; overall, deposits range from dark red to light green in color (A) Conglomerate. Pebble and cobble conglomerate bodies are made up Clast-supported, bedded gravel (Gh) and planar cross-stratified (Gp), and trough-cross stratified (St) gravel (B) Sandstone. F ine- to medium-grained sandstone bodies consist largely of horizontal (Sh), ripple (Sr), low-angle (sl), planar (Sp), and trough (St) cross-stratification (C) Mudstone. Thin deposits (150 m and beds are typically 2-3 m thick; fine—grained deposits are 5-15 m thick; nonerosive contacts at base of beds deposits range from dark red to light green in color (A) Mudstone. Fine-grained deposits (1-3 m thick) of massive and structurelss mudstone (Fsm) (B) Aggregated mudrock. Extensive tine—grained deposits consisting of massive mudstone and spheroidal (3-8 cm dia.) to elongate (6-20 cm long) yellow nodules FL OODPLAIN - TIDAL FLAT Deposition of silt and mud during extensive, unconfined sheet flooding in nonmarine floodplain (red) and marginal-marine tidal flat (red to green) regions; overbank channel and/or distal, waning debris flow; prolonged subaerial exposure and soil formation after deposition (A) Suspension fallout (floodplain). Unconflned flow and suspension settling (B) Pedogenic alteration. Soil formation in floodplain and tidal flat regions resulting from subaerial exposure possibly during arid conditions (carbonate nodules) FA CIES ASSOCIATION 4 (FA 4) Tabular. very well-sorted bodies of fine-grained sandy limestone; individual units are laterally extensive for 75-150 m and consist largely of low-angle cross-stratification; beds are 1-3 m thick (A) Sandstone. F inecgrained sandy limestone (Sls) with calcite cement; individual units are stratified and very well sorted; sand grains are rounded to subrounded; gray to yellow in color BEA CH - BARRIER ISLAND High- to medium energy wave action along a beach/barrier island; Beach strata may represent reworked debris flow deposits and/or reworked fluvial, deltaic, and tidal deposits (A) Beach front. Deposition and reworking during high-energy wave action foreshore beach and upper shoreface regions; wave action above mean fairweather wave base FACIES ASSOCIATION 5 (FA 5) Tabular beds of interbedded fossiliferous boundstone (BS) and muddy limestone (MS); individual units are laterally extensive for >150 m and beds are typically 0.25-1 m thick; gray to yellow in color (A) Boundstone (bindstone). Biogenic components are bound and encrusted by lime mud; fossiliferous horizons consist primarily of the hydrozoan Heterastridium (B) Muddy limestone. Deposits of lime mud that are typically <1 m thick; fossils occurrence is rare CARBONATE REEF - BANK - LAGOON In situ deposition by binding and encrusting of biogenic organims in a low energy, shallow marine carbonate reef/bank to Iagoonal environment; all components were originally bound during deposition (A) Reef/bank framework. In situ precipitation and deposition in low energy shelf-edge reef and/or Iagoonal carbonate bank environment; rimmed carbonate platform to ramp environment (B) Suspension settling/sed. gravity flow. Deposition of lime mud in reef/bank/lagoon setting 73 TABLE 2.3. continued DESCRIPTION INTERPRETATION FACIES ASSOCIATION 6 (FA 6) Dominantly tabular units of interbedded muddy limestone (MS) and massive to laminated wackestone (WS) as well as massive floststone (FS). and rudstone (RS); individual beds are laterally extensive >75 m and typically 0.1-0.5 m thick; gray to green-yellow in color; fossil has is common (A) Wackestone and floatstone. Massive to laminated, matrix-supported, muddy limestone with tine-grained (0.03-2 mm) to coarse-grained (>2 mm) fossil hash (B) Packstone and Rudstone. Massive, grain-supported limestone with fine—grained (0.03-2 mm) to coarse-grained (>2 mm) fossil hash; muddy matrix is common in packstone deposits (C) Muddy limestone. Deposits of lime mud that are typically <1 m thick; fossils occurrence is rare DEBRIS FLOW (SUBAQUEOUS) Punctuated submarine sediment gravity flows in a mid to outer ramp region of a rimmed carbonate platform/ramp environment; fossil debris is likely derived from mid- to inner ramp reefs/banks; the majority of debris flow deposits accumulated below storm wave base and have not been reworked (A) Wackestone and floststone. PlasticIPseudoplastic sediment gravity flows; high strength; viscous (B) Packstone and Rudstone. Pseudoplastic, turbulent sediment gravity flows; dominated by coarse-grained fossil hash (C) Suspension settling/sediment gravity flow. Deposition below storm wave base (no reworking) Table 2.3. 74 general stratigraphic descriptions for redbed and carbonate units, however the relationship and interplay between siliciclastic and carbonate strata has gone undocumented until this study. Based on the 21 total sedimentary facies, bedding thickness, geometry, and lateral extent, as well as the 6 facies association identified here, Upper Triassic redbed and carbonate units of the Chulitna terrane are interpreted to reflect (1) nonmarine siliciclastic sedimentation as a result of subaerial debris flows, fluvial channels, and overbank floodplain regions, (2) marginal marine sedimentation in tidal channels, deltaic front bar migration, tidal flat, and beach/barrier island regions, and (3) marine sedimentation by carbonate reef and bank development and submarine slumps and debris flows of fossil hash and fine-grained muddy limestone. Figure 2.10. provides a conceptual diagram and summary on how all six facies associations are related and interact in a mixed siliciclastic-carbonate fan-delta depositional system. PROVENANCE OVERVIEW In addition to sedimentary facies provenance trends were documented from conglomerate and sandstone units from base to top of measured sections. The summary presented here represents a preliminary overview of mineralogy observed from the base to top of each measured section. This report is not intended to be a comprehensive petrographic and provenance analysis for Upper Triassic strata, but rather a general description and documentation of compositional trends from these strata. Table 2.4., 2.5., and 2.6. present an 75 Fluvial channels and bars Fluvial/tidal channels and bars Fluvial channels deltaic mouth bars Debris flow (subaqueous) Carbonate reef/bank FA1 - Debris flow (subaerial) W FA 2 - Channels and bars “A (fluvial - tidal) FA 3 - Floodplain - tidal flat El IKE FA 4 - Beach - barrier island FA 5 -Carbonate reef/bank - lagoon FA 6 -Debris flow (subaqueous) Figure 2.10. Conceptual model for a mixed siliciclastic-carbonate fan-delta depositional system that includes sedimentation in the following environments: (1) subaerial debris flows (FA 1), (2) fluvial/deltaic/tidal channels (FA 2), (3) overbank floodplain and tidal flat (FA 3), (4) beach/barrier island. (5) carbonate reef/bank - lagoon (FA 5), and (6) submarine debris flows (FA 6). Modified from Nemec and Steel (1988). 76 overview of sandstone composition as observed from sandstone samples collected from throughout measured sections. The following text provides an upsection summary of composition from Upper Triassic siliciclastic and sandy carbonate strata from the Little Shotgun Creek study area. From base-to-top of measured Section 1 (Table 2.4.) sandstone composition is characterized by occurrence of chert grains (C) and monocrystalline quartz (Qm) with isolated intervals that contain lithic volcanic grains (Lv) and plagioclase grains (P). A majority of sandstone in the lower part of Section 1 have a mud/clay matrix and calcite cement is common throughout these strata. The upper portion of measured Section 1 contains similar occurrence of chert (C), monocrystalline quartz (Qm), lithic volcanic grains (Lv) and also exhibits isolated occurrence of polycrystalline quartz (Qp) and rare, isolated lithic metamorphic grains (Lm). Grains of serpentinite (8) also occur sporadically throughout measured Section 1. Measured Section 2 represents the thickest documented stratigraphic interval from This study and contains occurrences of monocrystalline (Qm) and polycrystalline (Qp) quartz, red and green chert (C), lithic volcanic (Lv) and lithic metamorphic (Lm) grains, as well as isolated chlorite grains (Table 2.5). Serpentinite occurs sporadically but throughout measured Section 2. A majority of these samples are characterized by calcite cement. Isolated occurrences of silica cement do occur sporadically in several samples where calcite cement is not observed. The middle portion of measured Section 2 contains primarily chert 77 Table 2.4. Description of samples from Section 3. Appendix A shows the exact location where each sample was collected. 78 Little Shotgun Creek, Healy A-6, south-central Alaska Collected by Jennifer DeLoge, Brian Hampton Collected June 2007 Section 3 LS-062907 Sample ID Hand sample description Petrographic description Plagioclase and Lv are dominate with minor amount LS-062907-01 Greenish gray matrix with vf-c of Qm, Do, and chert. 100% clasts of black and green chert and calcite cement and 75% quartz. Slight reaction with HCL. calcite grain coating. Chert and Qp dominate with Greenish gray matrix with vf-c lesser amounts of LS.O62907-02 clasts of black and green chert and serpentinite and Lv. 100% quartz. No reaction with HCL. calcite cement. Greenish gray matrix with vf-g LS-062907-04 clasts of quartz and plagioclase laths. No reaction with HCL. Volcaniclastic Greenish gray matrix with vf-c clasts of black and glassy red chert. LS-062907'06 Pyrite flakes in matrix. No reaction with HCL. Volcaniclastic Greenish gray matrix with vf-vc LS-062907-07 clasts of black, brown, and red Serpentinite dominate with chert. Highly reactive with HCL. plagioclase, Qm, and chert. Red matrix with vf-g size clasts of Cherts dominate with LS—062907-09 quartz, black and red chert, and plagioclase and Qm. Clay plagioclase. matrix. Black and tan banded chert. Tan is LS-062907'11 chert and black is siliceous mud. Banded chert Red matrix with vf-g size clasts of LS-062907-10 quartz, black and red chert, and plagioclase. HCL reacts with grain edges only. Cherts dominate with minor amount of Qm. Clay matrix. Table 2.4. 79 Table 2.5. Description of samples from Section 2. Appendix A shows the exact location where each sample was collected. 80 Little Shotgun Creek, Healy A—6, south-central Alaska Collected by: Jennifer DeLoge, Brian Hampton Collected: June 2007 Section 2 LS-063007 LS-062607 LS-061 907 Cherts dominate with Lv and LS-061907-03 Gray, sandy matrix with f-c size clasts plagioclase laths. 95% calcite of green and red chert cement. Gray, sandy matrix with f-c size clasts Cherts dominate with Qp, Qm, LS-061907-04 of green and black chert. Red chert Lm, and serpentinite. 75% only makes up 1%. calcite cement. Dark gray matrix with vf black chert. LS-061907-05 Highly reactive with HCL. Possible Chert framework grains with trace fossils. 95% calcite cement Dark gray mudstone. Highly reactive LS’OG‘IQW-OS with HCL. Individual grains to small to see. Red matrix with f—g size clasts of sub LS-061907-07 angular red and green chert. Moderate Cherts dominate with Lm and reaction with HCL. Op. 85% calcite cement. Red matrix with vf-vc size clasts of sub- LS-061907-08 angular red and green chert. Slight Cherts dominate with Op. 75% reaction with HCL. calcite cement. Red matrix with f-vc size clasts of sub- LS-062607-01 rounded red and green chert. Slight Cherts dominate with Qm. reaction with HCL. 50/50 calcite and clay cement. Red matrix with vf-c size clasts of sub- Chert, Lv, and plagioclase LS-062607—02 angular-sub-rounded red and green dominate with minor amounts of chert. Slight reaction with HCL. Qp and Qm. 5% calcite cement 5 D5 Red matrix with vf-f size clasts of green Cherts dominate with Qp. Qm, L 2607.03 chert. Slight reaction with HCL. and Lv. No calcite cement. Green matrix with cs-g size clasts of LS-062607-04 angular-sub-rounded quartz and red, Cs- G size Chert, Qp, and Qm. brown, and green chert No calcite cement. 81 Table 2.5. continued LS-062607-05D Green - gray matrix with vf black chert clasts and f-m red chert and m-vc quartz. Slight reaction with HCL. Tuff Red matrix with vf-m sized quartz and Cherts dominate with Qp, Qm, LS-062607-06 red and green chert clasts. Slight and plagioclase. No calcite reaction with HCL cement. Dark gray matrix with vf clasts of black 7 and gray chert and quartz. Microfolds $062607 07 in secondary calcite fracture fill. Highly Cherts dominate with Qm. 75% reactive with HCL. calcite cement. Dark gray matrix with vf clasts of black Chert and Qm dominate with LS—062607-09 and gray chert and quartz. Highly minor amount of Op. 75% reactive with HCL. calcite cement. Dark gray matrix with vf clasts of black LS-063007-01 and gray chert and quartz. Highly Cherts dominate with Qm. 75% reactive with HCL. calcite cement. Dark gray matrix with vf clasts of black Cherts dominate with Qm with LS-063007-02 and gray chert and quartz. No reaction minor amount of chlorite. No with HCL. calcite cement. Chert and Qm dominate with Dark gray matrix with vf clasts of black minor amounts of Qp and LS-063007-03 and gray chert and quartz. Slight plagioclase. 50% calcite reaction with HCL. cement. Chert and Qm dominate with Dark gray matrix with vf clasts of black minor amounts of Qp and 6063007 04 and gray chert and quartz. Slight plagioclase. 50% calcite reaction with HCL. cement. Dark gray matrix with vf clasts of black, Cherts dominate with Qm with LS—063007-05 green, and brown chert and quartz. No minor amount of chlorite. No reaction with HCL. calcite cement. Dark gray matrix with vf clasts of black Cherts dominate with Qm with LS-063007-06 and green chert and quartz. Slight minor amount of chlorite. Minor reaction with HCL. calcite cement. 82 Table 2.5. continued Cherts dominate with Qm with Dark gray matrix with vf clasts of black minor amount of chlorite, Qp 606300707 and green chert and quartz. Slight and plagioclase. Minor calcite reaction with HCL. cement. Dark gray matrix with vf clasts of black Cherts dominate with Qm with LS-063007-08 and green chert and quartz. Slight minor amount of chlorite. Minor reaction with HCL. calcite cement. Cherts dominate with Qm with LS-063007-09 Dark gray matrix with vf clasts of black minor amount of plagioclase, and green chert and quartz. Slight Qp, Lm, and Lv. Minor calcite reaction with HCL. cement. Cherts dominate with Qm with LS -063007- 1 0 Dark gray matrix wrth vf clasts of black minor amount of plagioclase, and green chert and quartz. Slight reaction with HCL. Qp, Lm, and Lv. Minor calcite cement. Table 2.5. 83 Table 2.6. Description of samples from Section 1. Appendix A shows the exact location where each sample was collected. 84 Little Shotgun Creek, Healy A-6, south-central Alaska Collected by: Jennifer DeLoge, Brian Hampton Michigan State University chert. Pyrite flakes in matrix. Collected: June 2007 Section 1 LS-062707 Qp and Qm dominate with LS-062707- Dark greenish gray matrix with vf-m size minor amount of chert. 04 quartz. Plagioclase laths. Slight reaction 50/50 clay and calcite with HCL. cement Qm and chert dominate LS-062707- Dark greenish gray matrix with fgreen with minor amount Op. 05 chert, quartz, and feldspar. No reaction with 50/50 clay and calcite HCL. cement Plagioclase, Qm, Lv, and LS-062707- Dark greenish gray matrix with vf-f tan and chert in approximately 06 black chert and quartz. Pyrite flakes equal amounts. Clay matrix present in matrix. No reaction with HCL. and grain coating. LS-062707- . . . Cherts dominate wrth minor 07 Dark greenish gray matnx wrth vf-vc chert, amount of Qm and Qp. feldspar, and quartz. No reaction with HCL. 80/20 clay/calcite cement. Cherts dominate with minor LS-0(6)§707- Red matrix with vf chert, quartz and amount of Qp, Qm, and feldspar. Only fractures react with HCL. plagioclase. Clay matrix. Cherts dominate with minor LS-062707- Green matrix supported conglomerate. amount of Qp and Lv. Clay 09 Clasts of quartz and unidentified black laths. matrix with (40%) calcite Some grains react slightly with HCL. replacement. LS -062707- Gray matrix with vf-f aqua green, black, and Cherts dominate with Qm, 10 red chert. Pyrite flakes present in matrix. Qp, Plagioclase, and Lm. No reaction with HCL. 50% calcite cement. Chert dominate with minor LS-O$12707' Dark gray matrix with vf clasts of red, green, amount of Qm and (Zip. and black chert. No reaction with HCL. 50% calcite cement. Cherts dominate with Qm LS-0$§707- Red matrix with vf-f clasts of green and red and plagioclase. 50I50 calcite and clay matrix. 85 Table 2.6. continued LS-062707- 1 3A Red matrix with vf-f clasts of green, black, and red chert. Pyrite flakes in matrix. No reaction with HCL. Cherts dominate with Qm, Qp, and plagioclase. Clay matrix. LS-062707- 1 38 Red matrix with vf-f clasts of green, black, and red chert. Pyrite flakes in matrix. No reaction with HCL. Cherts dominate with Qm, Qp, and plagioclase. Clay matrix. LS-O62707- 14 Light greenish gray matrix with vf-g size clasts of green, red, and tan chert. No reaction with HCL Chert and Qm dominate with minor amounts of plagioclase and Qp. Clay matrix. LS-062707- 1 5 Greenish gray matrix with vf—vc size clasts of chert and quartz. Cherts dominate with Qm, Qp, Lm, and oversized serpentinite. 50/50 calcite and clay matrix. Table 2.6. 86 (C) with minor components of monocrystalline quartz (Qm), and lithic volcanic grains (Lv). Polycrystalline quartz (Qp) and lithic metamorphic grains (Lm) are observed near the top of Section 2. From base-to-top of measured Section 3, grains consist primarily of monocrystalline (Qm) and polycrystalline quartz (Qp), chert (C), plagioclase (P), and lithic metamorphic (Lm) and lithic volcanic (Lv) components (Table 2.6). Chert and plagioclase are most common throughout the middle and upper portions of measured Section 3. Overall sandstone are characterized by mud/clay matrix with calcite being the dominant cement. Isolated occurrence so sandstone with silica cement do occur throughout measured Section 3 but are isolated. Preliminary provenance summary Upper Triassic siliciclastic strata from the Little Shotgun Creek study area potentially reveal a mixed provenance history that include derivation of volcanic detritus from a possible arc source area and metamorphic detritus that may have been derived from recycled orogen-continental block regions. Further study will be needed to to provide a robust provenance analysis of Upper Triassic strata in the Chulitna region. However, preliminary results presented here are the first approach at documenting upsection trends in the modal composition of siliciclastic strata from this region of the Alaska Range suture zone. While descriptions presented here are only a first-order look at compositional trends, it is clear that these strata contain detritus that was derived from both magmatic 87 source regions, as evidenced by lithic volcanic and plagioclase grains, as well as metamorphic source areas, evidenced by lithic metamorphic and polycrystalline quartz grains. Based on this preliminary study, at least two source areas may have contributed material during Upper Triassic time which could likely have been magmatic arc regions and metamorphosed continental margin-recycled orogen regions. DISCUSSION Upper Triassic siliciclastic and carbonate strata in the Chulitna terrane are interpreted here to represent nonmarine, marginal marine and marine sedimentation in a fan-delta style depositional system (Figure 2.10.). Deposition of siliciclastic and carbonate material appears to have been coeval and linked in a mixed system characterized by high energy erosive subaerial debris flow events and fluvial/deltaic/tidal transport and adjacent low-energy floodplain and tidal flat sedimentation. At the same time this depositional system allowed for isolated carbonate reef and bank development, likely in distal low-energy environments. The occurrence of all six facies associations throughout the Little Shotgun Creek measured sections suggest that high and low energy conditions persisted throughout deposition of these strata. This may be a reflection of continually changing locus of nonmarine sedimentation and may also reflect periods of rising or falling sea level. Another likely scenario for the ubiquitous distribution of these facies throughout measured sections is tidal influenced in marginal marine settings that would drive regular temporary changes in sea level 88 along the margin of nonmarine and marginal marine components of this depositional system. Preliminary provenance analysis potentially reveals possible magmatic and metamorphic source areas for Upper Triassic siliciclastic strata in the Chulitna terrane. These strata have been described by previous workers to consist of dominantly volcaniclastic deposits that were derived from magmatic sources (Clautice, et al., 2001). Isolated siliciclastic strata have been reported from this region but are not considered to be a major component to this stratigraphy. Preliminary findings from this study support a largely volcaniclastic stratigraphic framework, however the repeated occurrence of metamorphic grains and polycrystalline quartz throughout measured sections suggest that a second metamorphic source area was likely contributing detritus to this region during Late Triassic time. While past studies have provided general descriptions for volcaniclastic and siliciclastic units, these have previously been mapped separate from each other and independent from Upper Triassic carbonate strata. Based on finding from this study, it is proposed that Upper Triassic siliciclastic and carbonate strata of the Chulitna terrane consists of interbedded volcaniclastic, siliciclastic, and carbonate units and should be treated as one depositional unit that makes up a mixed fan-delta depositional system. Preliminary basin model and tectonic framework Clearly much more work is needed in the Chulitna terrane to determine a robust basin model for Upper Triassic strata exposed in the Alaska Range suture 89 zone. Based on the data presented here, one possible model is proposed to account for the sedimentology and stratigraphy documented herein, and regional geology and stratigraphy that is exposed above and below the stratigraphy of interest in this project. Figure 2.11. outlines a backarc tectonic setting that respects the finding from this study as well as the regional Paleozoic-Jurassic stratigraphy of the Chulitna terrane. As a review, the Chulitna terrane is considered one of the more enigmatic terranes in the North American Cordillera due largely to the unique occurrence of Upper Triassic siliciclastic, volcaniclastic, and carbonate stratigraphy. Directly above the stratigraphy of interest of this project lies uppermost Triassic basalt and limestone (Figure 2.1.). Stratigraphically older stratigraphic units in the Chulitna terrane consist of sheared serpentinite and pillow basalt (Figure 2.1.). Isolated occurrences of andesite have also been reported from Chulitna terrane. In order to account for bimodal volcanism as well as pillow basalt and serpentinite occurrences in the Chulitna terrane, a backarc basin model is proposed hear as one possible explanation to account for the diverse lithologies observed from the Chulitna terrane. It is important to note that while much of the Late Paleozoic and Mid Mesozoic tectonic history of the North American Cordillera is widely thought to be characterized by compressional tectonics, Late Triassic (Norian) basalt has been reported from many terranes in the Alaska Range suture zone throughout southern Alaska as well as locations further south in Yukon, and western British Columbia. Back arc basins are typically dominated 90 a sin (extension) Figure 2.11. Conceptual model for basin development during the Triassic development of the Chulitna terrane. In this scenario, mixed nonmarine, marginal marine, marine fan delta development would take place along the margin of a continental block. This model, while non-unique, accounts for the depositional facies observed in Upper Triassic stratigraphy from the Chulitna terrane and allows for magmatic source areas (blue and green) as well as metamorphosed continental source areas (orange). Yellow deposits depict expected depocenters in this tectonic configuration. Backarck extension allows for the occurrence of bimodal volcanism (basalt and andesite) observed in the Chulitna terrane. This tectonic setting also allows for a mechanism (rifting) to explain older occurrences of serpentinite and pillow basalt that are reported form the Chulitna terrane. Backarc extension allows for one possible explanation for the pervasive occurrence of Paleozoic and Triassic basalt that is observed throughout the Chulitna terrane and nearly all other terranes in the Alaska Range suture zone. 91 by sediments derived from the arc and the deposits are commonly associated with immature clastic deposits in ocean-continent settings (Allen, Allen, 2005). As arcs converge on continental margins the following may occur; 1. volcanism generally becomes enriched in silica, 2. volcaniclastic sandstones, ash falls and flows, and reworked volcaniclastic sediments are deposited (Pettijohn, et al., 1987; Einsele, 2000; Bridge and Deminnico, 2008). Although sedimentary rocks directly related to Wrangellia Island Arc should be compositionally different, based on silica content, from those related to the continental margin reworking, mixing and diagenesis can make it difficult to distingue in ancient deposits (Pettijohn et al., 1987; Fisher, 1994). Future studies should clearly be aimed at determining the continental block source along the inboard margin of this system as well as the arc source along the outboard margin of the basin. 92 CHAPTER III: CONCLUSIONS SUMMARY OF STUDY The unique occurrence of Upper Triassic siliciclastic, volcaniclastic, and carbonate stratigraphy, of the Chulitna terrane, distinguishes this terrane within the collage of terranes that compose the North American Cordillera. Stratigraphically older stratigraphic units in the Chulitna terrane consist of serpentinite, volcanics, sedimentary, and carbonate strata. New geologic mapping and measured stratigraphic sections of enigmatic Upper Triassic strata from the Chulitna terrane reveal a continuous record of competing volcaniclastic, siliciclastic, and carbonate sedimentation. Sedimentation is interpreted to represent a combination of nonmarine siliciclastic sedimentation, marginal marine siliciclastic-carbonate sedimentation, and marine carbonate sedimentation. The purpose of this study is to measure, document, and classify the Upper Triassic siliciclastic and carbonate strata encountered in the Little Shotgun Creek study area and Chulitna terrane for the purpose of better understanding the stratigraphic and tectonic implications of these strata in the context of the Alaska Range suture zone. Consensus regarding a depositional model for these units has eluded previous workers but for the first time this study anchors the depositional model on measured stratigraphy. Prior to this study, much of these siliciclastic and carbonate strata where divided into 3-4 separate units that were thought to represent several very 93 different depositional environments (Clautice et al, 2001 ). However, measured sections reveal that carbonate and siliciclastic successions are interbedded throughout. In addition, given that much of the stratigraphy of the Chulitna terrane has been structurally deformed, many of the contacts between Upper Triassic siliciclastic and carbonate strata were thought to possibly represent structural contacts rather than stratigraphic contacts. Previous studies have also suggested that Upper Triassic strata from the Little Shotgun Creek region are overturned with the oldest beds being interbedded basalt and limestone and younger units being siliciclastic redbed units and carbonate strata. Measured sections and stratigraphic relationships documented hear reveal that (1) Upper Triassic redbed and carbonate strata are interbedded and potentially reflect a depositional system that includes a strong siliciclastic and carbonate component and (2) that interbedded basalt and carbonate strata that were originally proposed to be older than redbed and carbonate units are actually younger and stratigraphically overlie older siliciclastic and carbonate strata. Sedimentologic and stratigraphic descriptions from measured sections reveal a total of 21 individual lithofacies with 14 representing siliciclastic transport and deposition and 7 representing some combination of carbonate transport, deposition, and in situ precipitation. Based on the 21 total sedimentary facies, bedding thickness, geometry, and lateral extent, as well as the 6 facies association are identified and interpreted. Upper Triassic redbed and carbonate units of the Chulitna terrane are interpreted to reflect (1) nonmarine siliciclastic sedimentation as a result of subaerial debris flows, fluvial channels, and 94 overbank floodplain regions, (2) marginal marine sedimentation in tidal channels, deltaic front bar migration, tidal flat, and beach/barrier island regions, and (3) marine sedimentation by carbonate reef and bank development and submarine slumps and debris flows of fossil hash and fine-grained muddy limestone. Upper Triassic siliciclastic strata from the Little Shotgun Creek study area potentially reveal a mixed provenance history that include derivation of volcanic detritus from a possible arc source area and metamorphic detritus that may have been derived from recycled orogen-continental block regions. While descriptions presented here are only a first-order look at compositional trends, it is clear that these strata contain detritus that was derived from both magmatic source regions, as evidenced by lithic volcanic and plagioclase grains, as well as metamorphic source areas, evidenced by lithic metamorphic and polycrystalline quartz grains. While past studies have provided general descriptions for volcaniclastic and siliciclastic units, these have previously been mapped separate from each other and independent from Upper Triassic carbonate strata. Based on finding from this study, it is proposed that Upper Triassic siliciclastic and carbonate strata of the Chulitna terrane consists of interbedded volcaniclastic, siliciclastic, and carbonate units and should be treated as one depositional unit that makes up a mixed fan-delta depositional system. Based on the data presented here, one possible model is proposed to account for the sedimentology and stratigraphy documented herein, and regional geology and stratigraphy that is exposed above and below the stratigraphy of interest in this project. A back arc 95 tectonic setting respects the findings from this study as well as the regional Paleozoic—Jurassic stratigraphy of the Chulitna terrane. The general back are basin model would account for the diverse lithologies observed from the Chulitna terrane. Much of the Late Paleozoic and Mid Mesozoic tectonic history of the North American Cordillera is widely thought to be characterized by compressional tectonics, however Late Triassic (Norian) basalt has been reported from many terranes in the Alaska Range suture zone throughout southern Alaska as well as locations further south in Yukon, and western British Columbia. CONCLUSIONS The Chulitna terrane of the southern Alaska Range is defined in part by the occurrence of Upper Triassic red bed and carbonate units that occur in the Alaska Range suture zone. Measured sections and stratigraphic relationships documented in this study reveal that (1) Upper Triassic redbed and carbonate strata are interbedded and potentially reflect a depositional system that includes a strong siliciclastic and carbonate component and (2) that interbedded basalt and carbonate strata that were originally proposed to be older than redbed and carbonate units are actually younger and stratigraphically overlie older siliciclastic and carbonate strata. 21 facies identified in the Little Shotgun Creek study area are grouped into 6 facies associations. Facies association one includes subareial to subaqueous debris flows that are interpreted to represent a delta-fan system. Mudstones, 96 sandstones, and matrix supported, poorly sorted conglomerates are typical of facies association one strata. Facies association 2 includes mudstones, sandstones and conglomerates that are interpreted as fluvial and tidal channels and bars. Facies association 3 includes mudstones, siltstones, and paleosols which are interpreted to represent overbank deposition and subareial exposure on a floodplain and tidal flat. Facies association 4 includes fine grained, well sorted sandy limestones interpreted to represent deposition within a beach and barrier Island. Facies association 5 includes bindstones and limey mudstones which are interpreted to represent deposition on a carbonate reef/bank and within a lagoon. Facies association 6 includes rudstones, floatstones, wackestones, and mudstones which are interpreted to represent debris flows at the back reef and fore reef. Based on this preliminary study of provenance, at least two source areas that may have contributed material during Upper Triassic time could likely have been magmatic arc regions and metamorphosed continental margin-recycled orogen regions. Future study is needed in order to constrain the source of these sediments. Quantitative data is needed in order to compare with findings from similar studies of terranes within the Alaska Range suture zone in order to compare results. Based on finding from this study, it is proposed that Upper Triassic siliciclastic and carbonate strata of the Chulitna terrane is one interbedded unit that makes up a mixed fan-delta depositional system. Previous tectonic models for the Chulitna terrane were tested minding the constraints provided by this 97 study and the known plate configurations during the collision of the Wrangellia Composite terrane and the Yukon-Tanana Composite terrane. Although a back are basin fits the tectonic and stratigraphic constraints it is only one possible explanation and requires further investigation. 98 APPENDIX A LITTLE SHOTGUN CREEK DETAILED MEASURED SECTIONS 99 24 20 16 12 08 Little Shotgun Creek Chulitna terrane measured section #062907 page 1/9 0-24/195 meters Siliceous mudstone/chert and siltstone Sample LS—062907-O1 Siliceous mudstone and banded chert Bedded siltstone with faint cross—bedding '_|_I_l'_] M SVFFSCVCG 100 48 4O 36 32 28 24 Little Shotgun Creek Chulitna terrane measured section #062907 page 2/9 24-48/195 meters LS-062907-02 Siliceous mudstone, bedded siltstone and very fine to coarse sandstone (all sandstone is massive) M SVFFSCVCG 101 Little Shotgun Creek Chulitna terrane measured section #062907 page 3/9 48-72/ 195 meters 72 Siliceous mudstone, bedded siltstone and minor volcaniclastic unit 5"- LS-062907-03 56 52 48 M SVFFSCVCG 102 Little Shotgun Creek Chulitna terrane measured section #062907 page 4/9 72-96/ 195 meters 92 88 Fine grained siliceous mudstone and volcaniclastic 80 76 LS-062907-04 72 M SVFFSCVCG 103 Little Shotgun Creek Chulitna terrane measured section #062907 page 5/9 96-120/195 meters 120 _ Gr en tuff LS-062907-06 1 16 _, 1 12 _. _, Sheared Basalt LS-062907-05 108 __1 104 fi 100 Fine grained black siliceous mudstone and siltstone with chert-like apperance 96 M SVFFSCVCG 104 Little Shotgun Creek Chulitna terrane measured section #062907 page 6/9 120—144/195 meters 144 LS-062907-07 140 Siliceous mudstone and siltstone Random oversized grades into green mudstone at —- red chert clasts top of sandstone 136 132 _—F " 128 __ Sheared Basalt 124 _ 120 IlllIl] M SVFFSCVCG 105' 168 164 160 156 152 148 144 Little Shotgun Creek Chulitna terrane measured section #062907 page 7/9 144-168/195 meters Green silioeous mudstone, siltstone, and very fine to ooarse sandstone withred chert clasts Tuff LS-062907-08 Siliceous mudstone, siltstone and very fine to coarse sandstone random oversized red chert clasts M SVFFSCVCG 106 Little Shotgun Creek Chulitna terrane measured section #062907 page 8/9 168-192/195 meters 192 LS-062907-1 0 188 184 180 176 7-09 172 168 M SVFFSCVCG 107 Red siliceous mudstone, siltstone, very fine to coarse sandstone and conglomerate with green and red chert clasts Green and red fine-grained matrix with oversized (1/2 m diamter) chert clasts Little Shotgun Creek Chulitna terrane measured section #062907 page 9/9 192-195/195 meters 216 212 208 204 200— END OF SECTION Red siltstone, very fine to coarse sandstone [“17 'M SVFFSCVCG green and red chert clasts | | I 108 24 20 16 12 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 1/17 0-24/ 394 meters Cross stratification Chert 80% Quartz 20% _ _ _ ' Matrix/Clast supported _ <3 cm diameter, qzt. __ dominant with minor red chen #- LS-061 907-02 Quartz and red/green chert grains approximatly 15cm of fossil hash Matrix supported, 80%Quartz ' with 20% red chert clasts mudstone with calcretions and very-fine sandstone I I j 3: Quartz dominant with minor ' ‘ red & green chert grains Interbedded mudstone and very fine _ grained sandstone LS-061 907-01 I I 'n r' D.- — I I II III M SVFFSCVCG sandstone 109 Massive mudstone - fine sandstone with gravel stringers, approximatly 10cm - thin, in very-fine Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 2/17 2448394 meters Tan to red sandstone containes quartz, red and green chert grains LS-061 907-03 Interbedded fine-coarse grained sandstone and pebble conglomerate (<0.5 cm diameter). Quartz and chert present throughout. 32 Mudstone and very-fine grained sandstone (0.5-20 cm thick beds). Medium to fine 28 gravel size chert and quartz 24 MSVF SCVCG 110 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 3/17 48-72/394 meters 72 68 Green interbedded very fine grained sandstone, siltstone, and mudstone Interbedded very fine grained sandstone, siltstone, and Green interbedded medium-coarse sandstone and mudstone with limey cement 52 LS-061907-04 48 111 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 4/17 72-96/394 meters 96 :9— T £1771, Limestone _T 1 it: 1} j 92 _E-gTL Tan limestone LS-061907-05 a??? fine grained containing possible blastoid 88 84 Bivalves Monotis subcircularis Tan weathering rind 80 Massive fossilferous siltstone and very fine grained sandstone 76 Transitional stratigraphic contact with TRvs 72 M SVFFSCVCG 112 116 _»H p 104 100 96 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 5/17 96—120/394 meters Gray massive limestone beds 0.10-0.50 m on average Interbedded gray micrite - and tan limestone 113 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 6/17 120-144/394 meters 144_ like: 140 _ £112? '13- E Gray limestone -ilff. 1 36— E if? ” Ti 1.: 132 7 1F _ s: ‘1 E5 :[ :‘ 128_ 5&1 X F vi Gray massive limestone — J41; - beds 0.10-0.50 m on average 2%; LS-061 907-06 124 _ :L :w n Tl' TEL 120 l . I I l I I I I M SVFFSCVCG 114 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 7/17 144—168/394 meters 168 164 Matrix supported conglomerate angular-subrounded 1-3 cm red/green chert and quartz(<10%) clasts 160 156 152 148 LS-061907-07 Gray limestone MSVF SCVC 115 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 8/17 168-192/394 meters 192 Fine grained matrix supported sandstone with very coarse—granual size red/green/black chert clasts 180 Red mudstone with calcareous nodules increasing in size upsection 1-6 cm diameter 176 172 168 116 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 9/17 192-216/394 meters 216 212 Waypoint N 63 03.498' W 149 53.717' 4590' 208 204 Fine grain sandstone with matrix supported coarse-granual size (1-6 cm diameter) red/green/black chert clasts 200 196 192 MSVF SCVC 117 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 10/17 216-240/394 meters 240 LS-062607—06 . ‘ Tuff with red rare sparry concretions 236 J j I . - 1 Green LS-062607-04 LS—062607-05 _ LS-062607-03 232 All fine grained mudstone/siltstone Concretrons has limestone nodules 2-6 cm diameter 228 Very coarse sandstone to mudstone and poorly sorted, matrix supported chert and quartz conglomerate 224 220 LS-062607-01 LS-061907-08 II'III.IIIII M SVFFSCVCG 118 264 260 256 252 248 244 240 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 11/17 240—264/394 meters TRs Fine grained limestone and micrite LS-062607-07 Fossil hash CONTACT WITH TRs Conglomerate with boulder to granual size clasts M SVFFSCVCG 119 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 12/17 264-288/394 meters 288 . Waypornt 160 N 63 03.560' W 149 53.851' 4561' 284 I Carbonates LS-062607-09 280 276 TRs Fine grained limestone and micrite 272 268 264 I III M SVFFSCVCG 120 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 13/17 282-306/394 meters 306 II.— m m .m- LS-063007—02 302 -_ m Mudstone and very—fine grained “ sandstone m 298 n— “— —_— III-— 294 _ _ _ _ In.“ In.“ 290 “- LS-063007-O1 II...— Il—n- TRs _;i; i; :er — Fine grained sandstone, mudstone 2 m and minor limestone 86 Ian—— Ian—- m— 282 II III M SVFFSCVCG 121 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 14/17 306-330/394 meters 330 326 III-In- m— nul— 322 II—l- III—- III—— III—— 318 Mudstone and very-fine grained sandstone Ill—— m— III-— III-— 314 m- unlu- Ian-— m III-Il— 310 Ill-I— Inns-I— unn- m- 306 ir'iir M SVFFSCVCG 122 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 15/17 330—354/394 meters % f l a 7:3 354 I L. 3g. -- . , '. 1' qulose-up of limestone with fossil hash 1 _Heterastridium(TRs) and limestone nodules 350 m 3'6 cm diameter 1.8-063007-04 III—— II"— Interbedded m sandStone and mudstone 346 m— with minor amount of III—— limestone containing LS‘063007'03 fossi' hash 342 338 H.— Mudstone 334 330 FIIIIII MSVFFSCVCG 123 Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 16/17 354-378/394 meters 378 LS-063007-07 374 370 LS-063007-06 366 362 LS—063007-05 358 354 MSVF SCVC 124 Sandstone, mudstone and limestone with fossil hash Close-up of limestone with fossil hash Little Shotgun Creek Chulitna terrane measured section #061907, 062607, 063007 page 17/17 378-394/394 meters 402 398 __ END OF SECTION 394 386 382 _ 378 LS-063007-10 Sandstone. mudstone and limestone with fossil hash LS-063007-09 LS-063007-08 w Close—up of limestone $53: With fOSSII hash II CVCG Little Shotgun Creek Chulitna terrane measured section #062707 page 1/ 14 0-24/325 meters Green fine-very coarse sandstone — and conglomerate Red coarse sands‘one LS-O72707-02 Red mudstone with calcretions Limestone - gray micrite Red mudstone with calcretions 08 Green mudstone and siltstone m LS-062707-01 Mudstone with Calcareous (lime) nodules M SVFFSCVCG 126 Little Shotgun Creek Chulitna terrane measured section #062707 page 2/ 14 24-48/325 meters 48 36 32 28 24 Red mudstone with calcretions Green mudstone siltstone and M SVFFSCVCG 127 _ very fine . grained sandstone _- I-é , - J ' ' ' ° . ' s I .' ".j V j 1 Green sandstone and ‘ ' - . y ' . fl LS-062707-04 conglomerate I j ' ' ' _ conglomerate A 1 . .. ' . ] contains quartz .' , '. *1 'J and chert : . . I ' ‘e T L ' e 1 '. .j . " . I - l Fossrl hash ' g ' 1 ° pectin, bivalves, pelcopods - . ' . 1 - .7 '- 1 .I l I .. 1 I . . l . . _ Green fine-very coarse sandstone and conglomerate Little Shotgun Creek Chulitna terrane measured section #062707 page 3/14 48-72/325 meters Micrite and fine grain limestone and — green mudstone, siltstone and very fine grained sandstone Red massive siltstone with with calcareous nodules M SVFFSCVCG 128 Little Shotgun Creek Chulitna terrane measured section #062707 page 4/ 14 72-96/325 meters Green-yellow siltstone, sandstone and LS'062707'07 minor conglomerate LS-062 707-06 Red mudstone, siltstone, and very fine sandstone with calcretions Limestone interbedded with mudstone 80 76 72 _r—I M SVFFSCVCG 129 Little Shotgun Creek Chulitna terrane measured section #062707 page 5/14 96—120/325 meters 120 Conglomerate 116 Calcretions 112 LS-062707-08 1 08 100 Red siltstone and sandstone with calcretions in top 5 meters of section M SVFFSCVCG Green-yellow siltstone sandstone and limestone 130 Little Shotgun Creek Chulitna terrane measured section #062707 page 6/ 14 120-144/325 meters Green-yellow siltstone sandstone and conglomerate LS-062707-10 LS-062707-09 120 IIIIII M SVFFSCVCG 131 Little Shotgun Creek Chulitna terrane measured section #062707 page 7/ 14 144468/325 meters 168 LS-062707-12 Red siltstone and sandstone with calcareous nodules 164 160 156 Green-yellow siltstone sandstone and conglomerate 152 148 Calcareous green sandstone LS-062707-1 1 144 M SVFFSCVCG 132 Little Shotgun Creek Chulitna terrane measured section #062707 page 8/14 168-192/325 meters Red conglomerate lenticular, pinchout unit Red siltstone and sandstone — with calcareous nodules >3cm diameter Red siltstone sandstone and conglomerate M SVFFSCVCG 133 Little Shotgun Creek Chulitna terrane measured section #062707 page 9/14 192—216/325 meters LS-062707—13a LS-062707-13b Red mudstone, siltstone, sandstone with calcareous nodules and massive conglomerate Red conglomerate lenticular, pinchout unit IIIIII M SVFFSCVCG 240 236 232 228 224 220 216 Little Shotgun Creek Chulitna terrane measured section #062707 page 10/ 14 216-240/325 meters Green massive conglomerate and green siltstone Red siltstone and sandstone Green siltstone and sandstone Red siltstone Green sandstone LS-062707-14 Red siltstone, sandstone with calcareous nodules and green conglomerate M SVFFSCVCG 135 Little Shotgun Creek Chulitna terrane measured section #062707 page 11/14 240-264/325 meters 264 260 256 252 248 244 240 _I_ Red siltstone and sandstone T Massive conglomerate with rounded to subrounded red/green chert and angular quartz clasts _ Red siltstone, sandstone and minor white limestone M SVFFSCVCG 136 Little Shotgun Creek Chulitna terrane measured section #062707 page 12/14 264-288/325 meters 288 284 280 Limestone Gray micrite and Biomicrite 276 272 Green siltstone 268 Red siltstone 264 M SVFFSCVCG 137 Little Shotgun Creek Chulitna terrane measured section #062707 page 13/14 288-312/325 meters 312 308 _ LS-O62707-150 CONTACT 304 _J. " 300 _— 296 _f r, , “1'1“ . ILI‘ bs Ll J1 292 _ L n 288 l Green basalt fl LS-062707-15a LS-062707-15b if m " A” L15 . I4 I r LT iii ~ l :L r1 1 if 4i— F I_ I J - H I ._( . ”'1 H Limestone Gray micrite and Biomicrite H ll I“. U . l l a L, Li} _J _L L Li . IlIITl M SVFFSCVCG 138 Little Shotgun Creek Chulitna terrane measured section #062707 page 14/ 14 312—325/325 meters 336 _ 332 .— 328 _ 312 ? Basalt continues upsection-not measured END OF SECTION \ Basalt Limestone M SVFFSCVCG 139 REFRENCES 140 REFERENCES Berg, H.C., Jones, D.L., and Coney, P.J., 1978, Map showing pre-Cenozoic tectonostratigraphic terranes of southeastern Alaska and adjacent areas: US. Geological Survey Open-File Report 78-1085, 2 sheets, scale 1:1,000,000 Blodgett, RB, and Clautice, K.H., 2000, Fossil locality map of the Healy A-6 Quadrangle, south-central Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2000-5, 42 p, 1 sheet, scale 1263,360 Blodgett, R.B., in press, Pre-, syn-, and post-collisional stratigraphic framework and provenance of Upper Triassic—Upper Cretaceous strata in the northwestern Talkeetna Mountains, Alaska, In Ridgway, K.D., Trop, J.M., Glen, J., and O'Neill, J.M., eds., Tectonic growth of a collisional continental margin: Crustal evolution of southern Alaska: Geological Society of America Special Paper Blodgett, R.B., Fryda J, Chulitnacula, a new paleobiogeographical distinictive gastropod genus from upper Triassic strata in accreted terranes of southern Alaska: Journal of the Czech Geological Society, v. 46, no. 3-4, p 299-306 Boggs, S., 2001, Principles of Sedimentology and Stratigraphy, 3rd ed., Prentice- Hall Bridge, J., Demicco, R., 2008, Earth surface processes, landforms and sediment deposits: London, Cambridge University Press, 830 p Csejtey, B., Cox, D.P., Evarts, R.C., Stricker, G.D., Foster, H.L., 1982, The Cenozoic Denali Fault System and the Cretaceous Accretionary Development of Southern Alaska, Journal of Geophysical Reseach, v. 87, n. B5, p 3741-3754 Clark, A.L., Clark, S.H.B., and Hawley, CC, 1972, Significance of upper Paleozoic oceanic crust in the upper Chulitna District, west-central Alaska Range, in US. Geological Survey Staff, Geological survey research; Chapter C: US. Geological Survey Professional Paper 800-C, p C95-C101 Clautice, K.H., Newberry, R.J., Blodgett, R.B., Bundtzen, T.K., Gage, B.G., Harris, E.E., Liss, S.A., Miller, M.L., Reifenstuhl, R.R., Clough, J.G., and Pinney, 0.8., 2001, Bedrock geologic map of the Chulitna region, southcentral Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2001-1A, 31 p, 1 sheet, scale 163,360 141 Clautice, K.H., Newberry, R.J., Pinney, D.S., Blodgett, R.B., Bundtzen, T.K., Gage, B.G., Harris, E.E., Liss, S.A., Miller, M.L., Reifenstuhl, RR, and Clough, J.G., 2001, Geologic map of the Chulitna region, southcentral Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2001-18, 32 p, 1 sheet, scale 1:63,360 Collinson, J.D., Alluvial sediments: Sedimentary Enviroments: Processes, Facies and Stratigraphy, ed., Reading, H.G., 1996, p 37—82 Coney, P.J., Jones, D.L., Monger, J., 1980, Cordilleran suspect terranes: Nature, v. 288, p 329-333 Dickinson, W.R., 2004, Evolution of the North American Cordillera: Annual Review of Earth and Planetary Sciences, v. 32, p13-45 Eastham K.R., Ridgway K.D., 2002, Stratigraphic and provenance data from the Upper Jurassic to Upper Cretaceous Kahiltna assemblage of south-central Alaska: United States Geological Survey Professional Paper 1662, p 45-53 Einsele, G., 2000, Sedimentary basins: Evolution, facies, and sediment budget: Springer, 628 p Fisher, R.V., 1961, Proposed classification of volcaniclastic sediments and. rocks: Geological Society of America Bulletin, v. 72, p 1409—1414 Gage, B.G., and Newberry, R.J., 2003, Ore mineralogy and mineral compositions from Golden Zone Mine, southcentral Alaska, in Clautice, K.H., and Davis, P.K., eds., Short Notes on Alaska Geology 2003: Alaska Division of Geological & Geophysical Surveys Professional Report 1208, p 21-33 Gehrels, GE, and Berg, HQ, 1988, A review of the regional geology and tectonics of southeastern Alaska: US. Geological Survey Open-File Report 88- 659,40 p Glen, J.M.G., Schmidt, J., Pellerin, L., McPhee, D.K., O’Neil, J.M., 2007, Crustal structure of Wrangellia and adjacent terranes inferred from geophysical studies along a transect through the northern Talkeetna Mountains: Tectonic growth of a collisional continental margin: Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, 21 p Haeussler P.J., Bradley D.C., Wells RE, and others, 2003, Life and death of the resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time: Bulletin of the Geological Society of America, v. 115, no.7, p 867-880 142 Hawley, CC, and Clark, A.L., 1968, Occurrences of gold and other metals in the upper Chulitna District, Alaska: US. Geological Survey Circular 564, 21 p Hillhouse, J.W., and Gromme, S., 1980, Paleomagnetism of Triassic red beds and basalt in the Chulitna terrane, South—central Alaska: US. Geological Survey Open- File Report 80-368, 12 p Johnson, H.D., Baldwin, C.T., Shallow clastic seas: Sedimentary Enviroments: Processes, Facies and Stratigraphy, ed., Reading, H.G., 1996, p 232-280 Johnston ST, 2001, The Great Alaskan Terrane Wreck: Reconciliation of paleomagnetic and geological data in the Northern Cordillera: Earth and Planetary Science Letters, v. 193, no.3-4, p 259-272 Jones, D.L., lrvvin, W.P., Overshine, AT, 1972, Southeastern Alasks; a displaced continental fragment?: US. Geological Survey Professional Paper 800-8, p 3211- 8217 Jones, D.L., Siberling, N.J., Csejtey, B., Jr., Nelson, W.H., Blome, CD, 1980, Age and structural significance of ophiolite and adjoining rocks in the upper Chulitna district, south-central Alaska: US. Geological Survey Professional Paper 1 121-A, p A1—A21 Jones, D., Siberling, N.J., Gilbert, W.G., Coney, P.J., 1982, Character, distribution, and tectonic significance of accretionary terranes in the central Alaska Range, Journal of Geophysical Research, v. 87, p 3709—3717 Leigler, G.E., August 10, 1940, Internal Communication: Teritory of Alaska, Department of Mines, lR-194-02, 2 p Monger, J.W.H., and Berg, HQ, 1987, Lithotectonic terrane map of Western Canada and southeastern Alaska: US. Geological Survey Miscellaneous Field Studies 1874-B, 12 p, 1 sheet, scale 12,500,000 Montayne, Simone, and Whalen, MT, 2003, A Permian cool-water limestone from the Chulitna terrane, southcentral Alaska, in Clautice, K.H., and Davis, P.K., eds., Short Notes on Alaska Geology 2003: Alaska Division of Geological & Geophysical Surveys Professional Report 120D, p 53-62 Moxham, R.W., Eckhart, R.A.., Cobb, E.H.., West, W.S., Nelson, AB, 1951, Geology and raw materials of the Windy Creek area, south-central Alaska: United States Department of the Interior, Geological Survey, 58 p 143 Nichols, KM, and Silberling, N.J., 1979, Early Triassic (Smithian) ammonites of paleoequatorial affinity from the Chulitna terrane, South-central Alaska: US. Geological Survey Professional Paper 1121—B, p B1-B5 Nokleberg, W.J., Richter, DH, 2007, Origin of narrow terranes and adjacent major terranes occurring along the Denali fault in the eastern and central Alaska Range, Alaska: Tectonic growth of a collisional continental margin: Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p 129 O'Neill J.M., Ridgway K.D., Eastham KR, 2003, Mesozoic sedimentation and deformation along the Talkeetna thrust fault, south-central Alaska - New insights and their regional tectonic significance: United States Geological Survey Professional Paper 1678, p 83-92 Pettijohn, F.J., Potter, P.E.P., Siever, R., 1987, Sand and sandstone: Springer, 553 p Pinney, 0.8., 2001, Engineering—geology map of the Chulitna region, southcentral Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2001-1 D, 5 p, 1 sheet, scale 1:63,360. Plaflrer, G., Berg, H. C., 1994, Overview of the geology and tectonic evolution of Alaska, in Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Boulder, CL, Geological Society of America, Geology of North America, v. G-1, p 989—1 021. Reading, H.G., Levell, B.K., Controls on sedimentary rock record: Sedimentary Enviroments: Processes, Facies and Stratigraphy, ed., Reading, H.G., 1996, p 5- 36 Reading, H.G., Collinson, J.D., Clastic coasts: Sedimentary Enviroments: Processes, Facies and Stratigraphy, ed., Reading, HQ, 1996, p 154-231 Ridgway K.D., Trop J.M., Sweet AR, 1997, Thrust-top basin formation along a suture zone, Cantwell Basin, Alaska Range: implications for development of the Denali fault system, Geological Society of America Bulletin 109, no.5, p 505-523 Ridgway, K.D., Trop, J.M., Nokleberg, W.J., Davidson, CM, and Eastham, KR, 2002, Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range: Progressive basin development and deformation in a suture zone: Geological Society of America Bulletin, v. 114, p 1480-1504 Stanley, G.D., Jr., and Yarnell, J.M., 2003, New paleontological investigations of the Triassic carbonate rocks in the Upper Chulitna district (Chulitna terrane), southcentral Alaska, in Clautice, K.H., and Davis, P.K., eds., Short Notes on 144 Alaska Geology 2003: Alaska Division of Geological & Geophysical Surveys Professional Report 120K, p 109-116 Townsend, H., 1925, Confidential communication, Geological Survey: Department of the Interior, lR-195-48 Thurrnond, FL, 1918, Report on Ready Cash Group, Ohio River, Broad Pass Region, Alaska: Alaska Territorial Department of Mines Miscellaneous Report 67- 1, 8 p Trop J.M., Ridgway K.D., Manuszak JD, and others, 2002, Mesozoic sedimentary-basin development on the allochthonous Wrangellia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction: Bulletin of the Geological Society of America, v. 114, no.6, p 693-717 Veever, J.J., Powell, C.McA., and others, 1994, Permian-Triassic Pangean basins and foldbelts along the Panthalassan margin of Gondwanaland: Synthesis: Geological Society of America Memoir 184, p 331 White, J.D.L., and Houghton, B.F., 2006, Primary volcanistic rocks: Geology, v. 34, p 677—680 Wilson, F.H., Dover, J.H., Bradley, D.C., Weber, F.R., Bundtzen, T.K., and Haeussler, P.J., 1998, Geologic map of Central (Interior) Alaska: US. Geological Survey Open-File Report 98-133, 63 p Wright, V.P., Burchette, T.P., Shallow-water carbonate enviroments: Sedimentary Enviroments: Processes, Facies and Stratigraphy, ed., Reading, H.G., 1996, p 325-394 145