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DATE DUE DATE DUE DATE DUE 5/08 K:lProj/Acc&PresIClRC/DateDue,indd SEDIMENTARY DYNAMIC__S AND STRATINOMY OF A MIDDLE CAMBRIAN ICHNOFOSSIL LAGERSTATTE, GROS VENTRE FORMATION, WYOMING, USA By Jayme Day Csonka A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Geological Sciences 2009 ABSTRACT SEDIMENTARY DYNAMICS AND STRATINOMY OF A MIDDLE CAMBRIAN ICHNOFOSSIL LAGERSTATTE, GROS VENTRE FORMATION, WYOMING, USA By. Jayme Day Csonka The ichnofossil Rusophycus is the product of a typical behavior of trilobites, but is only preserved under unique conditions. A unique confluence of conditions existed in the Middle Cambrian Gros Ventre environment and resulted in the preservation of a Rusophycus Iagerstatte. These conditions include the background deposition of firm, cohesive muds possibly bound by bacterial mats. The muds had a firmness that allowed the burrows to maintain their shape until they were filled in and cast during the next phase of sand deposition, likely from a storm. The deposition of sands following episodic high energy events provided an ideal casting medium for Rusophycus . The absence of spreite and presence of individual storm events in sectioned Rusophycus support this conclusion. These results contradict previous assertions that the Rusophycus ichnofossil was excavated by trilobites burrowing through sand to a muddy interface. The Gros Ventre Formation falls in the middle of the Cambrian Substrate Revolution. Bacterial mats were present before, during, and after the deposition of the Gros Ventre Formation. Bioturbation indices for the Gros Ventre Formation are comparable to those documented for the Cambrian by other workers and support the idea of a gradual increase of bioturbation intensity during the Middle Cambrian. This project is dedicated in its entirety to the memory of Myles Alexander Redder (1984-2008), whose advice, support, and physical labor were sorely missed for the duration of this project. iii ACKNOWLEDGMENTS My utmost gratitude is given to my advisor, Dr. Danita Brandt (Michigan State University), and additional committee members, Dr. Robert Anstey and Dr. Brian Hampton (Michigan State University). Additional thanks particularly to Dr. Carlton Brett (University of Cincinnati) for numerous useful discussions. Thanks to Aric Veibel for help conducting fieldwork. Thanks to my network of friends and family for their advice and support. This project could not have been completed with the donation of funds from the Aureal Cross Fund for Fieldwork in Paleontology and the PringIe-Drake Fellowship. Thanks to Dan Scaife, Will Young, and Cheri Jones of the Big Horn National Forest Service for allowing fieldwork to be conducted on National Forest land. Thanks to Dr. Warren Wood (Michigan State University) for the loan of equipment used in the field. Thanks to the users of Lansing Michigan Freecycle and Mid-Michigan ReUselt Networks for the donations of many useful supplies used in this project. iv TABLE OF CONTENTS LIST OF FIGURES ................................................................................ vii INTRODUCTION .................................................................................... 1 The Cambrian Substrate Revolution/Agronomic Revolution 2 Geologic Setting ............................................................................ 5 MATERIALS AND METHODS ................................................................. 13 Description of Ichnofossils Present .................................................. 14 Rusophycus ............................................................. 14 Teichichnus ............................................................. 22 Chondrites ................................................................ 25 Skoiithos .................................................................. 26 Pseudoichnofossils Present ........................................................... 27 Kinneyia ................................................................... 27 Bacterial Mat Structures .............................................. 28 RESULTS ............................................................................................ 3O Rusophycus Biostratinomy ............................................................. 30 Sedimentary Dynamics in the Gros Ventre Formation.......................... 3O lchnofabric of the Middle Cambrian Gros Ventre Formation................... 37 Petrographic Data ........................................................................ 38 DISCUSSION ...................................................................................... 39 Rusophycus Biostratinomy ............................................................ 39 Sedimentary Dynamics in the Gros Ventre Formation 42 lchnofabric of the Middle Cambrian Gros Ventre Formation and Its Place in the Cambrian Substrate Revolution 46 CONCLUSIONS ................................................................................... 49 APPENDICES 1. Bed-by-bed descriptions of measured sections .................................. 51 2. Description of Localities Burgess Junction #1 68 Passage Resort#1 69 Passage Resort #2 ..................................................... 72 Bear Lodge #2 ........................................................... 75 NorthTongue#1 76 ' North Tongue #2 79 North Tongue #3 82 Steamboat Point 83 3. Global Positioning System Data 85 REFERENCES .................................................................................... 86 vi 6a. 6b. 8a. 8b. 8c. 8e. LIST OF FIGURES Agronomic Revolution. Image Source: Seilacher, 2007. This figure also documents the transition of matgrounds to mudgrounds (mixgrounds) during the Cambrian Substrate Revolution. Note transition from surface to vertical burrowing ............................................................................... .3 Generalized stratigraphic column summarizing the Paleozoic stratigraphy of the Big Horn Mountains. From Koucky and Cygan (1963) .......................... 6 Map of Wyoming highlighting Burgess Junction. Image Source: Google Eanh ................................................................................................ 7 Satellite image map of the Burgess Junction area highlighting studied exposures. Field of view is approximately 1 mile. Image source: Google Eanh ........................................................................................................... 7 Geologic Map of the Burgess Junction area. Note, Gros Ventre Formation is grouped with the Flathead Sandstone on this map. image source: Big Horn National Forest Service ................................................... 8 Trilobite hash from top of Flathead Sandstone at NT1. Scale bar is 2 cm ......................................................................................................... 10 Trilobite hash from top of Flathead sandstone. Scale bar is 1 cm .............. 10 Sepkoski’s (1982) paleoenvironmental interpretation. Note, the Pilgrim is a lateral equivalent to the Flathead Sandstone and the Deadwood Formation is a lateral equivalent to the Gallatin Limestone ..................... 12 Rusophycus from Passage Resort 2 (Gros Ventre Formation). Scale bar is 2 cm .................................................................................................. 15 Rusophycus in the field (highlighted with red oval), collected at Burgess Junction ................................................................................................ 15 Slab with multiple Rusophycus and other traces. Scale is 2 cm ................. 16 Rusophycus with pebble of potassium feldspar, highlighted with red oval. Collected at Bear Lodge 2 (Gros Ventre Formation). Scale bar is 2cm ...................................................................................................... 17 Rusophycus with large pebble (3 cm) cross-cutting specimen. Pebble is absent, but mold of it remains. Scale bar is 2 cm ................................. 18 8f. Smallest size Rusophycus collected in Burgess Junction. Scale bar is 1 cm .......................................................................................................... 19 vii 89. Small sized Rusophycus collected in Burgess Junction. Scale bar is 1 cm ....................................................................................................... .20 8h. Medium sized Rusophycus collected at Burgess Junction. Scale bar is 1cm ....................................................................................................... .20 9. Cross-sectioned Rusophycus Scale bars are all 2 cm ............................. 21 10a. Teichichnus from PR2 (Gros Ventre Formation), scale bar is 2cm .......................................................................................................... 23 10b. Teichichnus from PR2 (Gros Ventre Formation), scale bar is 2cm ....................................................................................................... 23 11a. Cross-sectioned Teichichnus. Scale bar is 1 cm .................................... 24 11b. Cross-sectioned Teichichnus. Scale bar is 1 cm24 12. Chondrites from PR2 (Gros Ventre Formation). Scale bar is 5cm ................ 25 13. Skolithos Burrows from NT1 (Flathead Sandstone). Scale bar is 2 cm ........ 26 14a. Kinneyia preserved in sandstone from NT1 (Flathead Sandstone). Scale bar is 10cm ................................................................................................. 27 14b. Kinneyia preserved in carbonate from NT3 (Gallatin Formation). Scale bar is 2 cm .................................................................................................. 28 15a. Bacterial mat structures (?) from BJ1 (Gros Ventre Formation), scale bar is 2cm ..................................................................................................... 29 15b. Probable bacterial mat structure from Gallatin Limestone. Scale is 2 cm...29 16a. Scour marks on sole of bed from BJ1. Scale bar is 5 cm 31 16b. Cross-sectional view of bed from BJ1 featured in Figure 16a (above). Scale bar is 5 cm31 17. Sole surface of bed collected at PR2 with multiple Rusophycus (highlighted in red ovals) and rip-up clasts (shown with red arrows). Scale bar is 5 cm ............................................................................................. 32 18a. Bed PR2+810 in the field. Notebook (12 cm wide) for scale. This bed is contains heavily bioturbated muds with a bioturbation index of 4 ............ 32 18b. PR2+810 (Gros Ventre Formation), densely bioturbated layer with multiple Rusophycus. Scale Bar is 5 cm ......................................................... 33 viii 19. Mud/siltlsand beds topped with rippled sandstone at PR2 (Gros Ventre Formation). Hammer (22 inches) for scale ................................................ 34 20a. Composite stratigraphic column of the Burgess Junction area ................. 35 20b. Correlated stratigraphic section of exposures in the Burgess Junction area. Lines are drawn correlating the Chondrites marker beds ....................................................................................................... .36 21. Histogram combining both the Droser-Bottjer (1986) and Miller-Smail (1998)bioturbation indices ......................................................................... 37 22. Thin section photomicrographs at 10x power ......................................... 38 23a. Seilacher’s (2007) diagram of Rusophycus burrowing techniques. Note, no cross-sectional detail grven40 23b. Rusophycus excavation techniques proposed by Seilacher (2007). Note, excavation is inferred to be done subsurface at a sand-mud interface ................................................................................................... 4O 24. lchnofacies. The Flathead Sandstone comprises the sandy shoreface Skoiithos ichnofacies. The Gros Ventre Formation comprises the sublittoral zone Cruziana ichnofacies, which includes the Rusophycus ichnofossil .................................................................................................. 43 25a. Bioturbation figure modified from Droser et al., (1988) noting average ichnofabric index for Cambrian and Ordovician and the interval in which the Gros Ventre of Burgess Junction is positioned ................................................................................................ 47 25b. Comparison of 5 meters of measured Gros Ventre Formation (a predominantly siliciclastic unit) with 5 meters of an approximately coeval carbonate—dominated section published in Droser, et al., (1988) ...................................................................................................... 48 26. Burgess Junction #1 ..................................................................... 68 27. Massive Sandstone at PR1. ................................................................ 69 28. Passage Resort #1 (Gros Ventre Formation) ........................................ 7O 29. PR1, (Gros Ventre Formation) pink to green sandstones at base of section. Hammer (22 inches) for scale ................................................................... 71 30. Passage Resort #2 (PR2) ................................................................. 73 ix 31. PR2 (Gros Ventre Formation), Rippled Sandstone Bed. Hammer (22 inches) for scale ........................................................................................................ 74 32. Bear Lodge #2 (Gros Ventre Formation) .......................................................... 75 33. North Tongue #1 (Flathead Formation) ................................................... 77 34. North Tongue #1(Flathead Formation) ............................................................. 78 35a. Flat pebble conglomerate from NT2 (Gallatin Limestone). Side and top view. Top of scale bar is 4 cm 79 35b. Flat pebble conglomerate from NT2 (Gallatin Limestone) Top view. Scale baris 10cm... ... .. . ... .... ... .........80 36. Slab of flat pebble conglomerate collected in float at NT2. Hammer (22 inches) for scale ..................................................................................... 80 37. Arrow pointing to exposure at North Tongue #2 Locality (NT2). Gravel road to right of arrow for scale .............................................................................. 81 38. Flat pebble conglomerate from NT3 (Gallatin Formation). Scale bar is 2 cm .............................................................................................................. 82 39. Steamboat Point (Middle Cambrian Flathead Formation through Ordovician Bighorn Formation) ....................................................................................... 84 INTRODUCTION This project describes animal-sediment relationships, biostratinomy, and sedimentary dynamics in the Middle Cambrian Gros Ventre Formation of north- central Wyoming. The Middle Cambrian is an interval of interest because it spans the transition from Late Neoproterozoic matground substrates to Late Cambrian mudground substrates in what has been called the Cambrian Substrate Revolution (Bottjer, et al., 2000) and Agronomic Revolution (Seilacher & Pfluger, 1994), events that may be partially documented in exposed sections of the Gros Ventre Formation in the Big Horn Mountains of Wyoming. The Gros Ventre Formation is also interesting because it contains an unusual abundance of the trilobite trace fossil, Rusophycus. This abundance of trace fossils from the Gros Ventre Formation offers an exceptional window into early Paleozoic sedimentary dynamics and ecological change following the Cambrian Explosion (Runnegar, 1982) and subsequent Agronomic Revolution (Seilacher and Pfluger, 1994) (Figure 1). Substantial work has been done on the association of ichnotaxa and substrates of the Proterozoic/Paleozoic boundary (Bottjer, et al., 2000) and the Cambrian/Ordovician boundary (Droser and Bottjer, 1989). Less work has been done on the significance of Middle Cambrian ichnotaxa and the substrate relationships of their trace-makers. In this project the Gros Ventre Formation was examined at three scales, which include from smallest to largest: (1) description of the dynamics of Rusophycus excavation and preservation, (2) description and interpretation of the sedimentary conditions that resulted in the exceptional preservation of the ichnofossils, and (3) evaluation of how these ichnofossils contribute to our understanding of the transition of marine substrates from undisturbed bacterial matgrounds at the Proterozoic/Paleozoic boundary to completely bioturbated mudgrounds at the base of the Ordovician System. Three hypotheses, one corresponding to each scale of investigation, were tested: (1) Rusophycus formation is the result of active backfilling by trilobite trace-makers (Seilacher, 2007); (2) ichnofossil preservation in the Gros Ventre is the result of episodic storm deposits and tidal deposition; (3) at the transition from Late Neoproterozoic matgrounds to Late Cambrian mudground, Middle Cambrian strata will show a mix of bacterial matgrounds and bioturbation, and that depth and intensity of bioturbation will be greater in the Middle Cambrian than described for the Proterozoic/Paleozoic transition, but less than that of the Cambrian/Ordovician transition. The Cambrian Substrate Revolution/Agronomic Revolution Marine substrates of the Late Neoproterozoic are typically characterized by abundant bacterial mats (Bottjer, et al., 2000). These bacterial matgrounds formed a sharp sediment-water interface and thrived in the absence of vertical bioturbation. The Cambrian Explosion, of the early Cambrian (Series Two, Stage Three—informally called the Atdabanian) (lntemational Commission on Stratigraphy, 2008), in which there was a presumptively rapid diversification of phyla (Runnegar, 1982), resulted in fundamental change in the trophic structure of seafloor faunal communities. Bacterial mats were an abundant, but diminishing biotope that also hosted predators, which could feed on the organisms that fed on the bacterial mats. By the end of the Cambrian, an increase in predation and relocation of microbes from bacterial mats to the coatings of clastic grains due to vertical bioturbation led to a decline in abundance of bacterial matgrounds, and a subsequent increase in water content within the substrate, causing a less distinct sediment-water interface and less cohesive, muddier substrates (Droser et al, 1999; Bottjer, et al., 2000, Seilacher, 2007). By the earliest Ordovician, the majority of bacterial matgrounds had been converted to mudgrounds (Droser and Bottjer, 1989). This change in substrates is referred to as the Cambrian Substrate Revolution (Bottjer, et al, 2000)(Figure 1), which can be considered a sedimentary effect of the Agronomic Revolution (Figure 1), the process by which the substrate-linked feeding patterns of organisms were permanently altered due to changes in substrate consistency, location of food sources, and safety from predation (Seilacher, 2007). Agronomic Revolution Garden of Edlacara '. , mo 'scglatlbn documents the transition of matgrounds to mudgrounds (mixgrounds) during the Cambrian Substrate Revolution. Note transition from surface to vertical burrowing. Ichnofossils (e.g., tracks, trails, or burrows) are a key tool for understanding the Cambrian Substrate Revolution. They provide evidence for the behavior of organisms, and are extremely valuable for studying deposits void of body fossils, but rich in ichnofossils, a typical paradoxical characteristic of many ichnofossil deposits in the sedimentary record. This study examines a critical window of the Middle Cambrian, which is less well-documented than the earliest stages of the Cambrian and Ordovician Periods. The sediment-animal dynamics of the Middle Cambrian is of particular importance for understanding the chronological development of the Cambrian Substrate Revolution. The results of this study will help paleontologists and stratigraphers understand whether the transformation from Proterozoic sedimentary fabric to Ordovician fabric was transitional or whether the middle Cambrian may have marked an abrupt “tipping point" (sensu Gladwell, 2002) in the evolution of the sedimentary fabric of the marine seafloor. Geologic Setting The seven localities studied in the area around Burgess Junction in the Big Horn Mountains of Wyoming (USA) were all Cambrian exposures (Figures 2, 3, 4, and 5). The primary stratigraphic interval of interest was the Middle Cambrian Gros Ventre Formation, but the underlying Flathead (sandstone) and overlying Gallatin (limestone) Formations were also examined in the area near Burgess Junction for contextual data. The Gros Ventre Formation was first formally described by Blackwelder (1918) as having: Greenish and gray calcareous shales, with gray, striped conglomeratic and oolitic limestones, separating overlying Gallatin Limestone from underlying Flathead quartzite. Contains Middle Cambrian fossils. The type section of the Gros Ventre is exposed on the western slope of Doubletop Peak in the Gros Ventre Range of western Wyoming (Blackwelder, 1918). The Gros Ventre of the Big Horn Mountains is primarily composed of mudstones, silts, and sands that were deposited during the Sauk Sea transgression. GENERALIZED PALEOZOIC SECTION along US-I4. East flank of BIG HORN MOUNTAINS. WYOMING Bighorn dolomite — 368’ ‘ _ Ch‘ugwater Fm Lander ss — 10’ Permian unit — 56’ Harding ss — 33’ Tensleep ss - 108' Gallatin limestone - 168’ Amsden Fm - 247’ Darwin 33 - 78’ Gros Ventre shale - 615' Madison Fm -— 870' fiChert in limestone ' CI] Sandstone Wes in dolorhiTe SE! Siltstone 350mm Conglomerate breccia -Shale IEGypsum & anhydrite Figure 2. Generalized stratigraphic column summarizing the Paleozoic stratigraphy of the Big Horn Mountains. From Koucky and Cygan (1963). ' ’ m ~29 ' mid r, ’1': ‘ IT. : f 7 , -. - .Rlv :ll/ Iwmne C Figure 3. Map of Wyoming highlighting Burgess Junction. Arrow Points to Burgess Junction. Image source: Googl eEa Passage Resort #2 ('0‘ {Id—I Figure 4. Satellite' Image map of the Burgess Junction area highlighting studied exposures. Field of view is approximately 1 mile. Image source: Google Earth Bear Lodge Formation Amsden Formation ., , , Bighorn Dolomite - Flathead Sandstone U Gallatin Limestone Glacial Deposits ‘ _: Landslide Deposits r l . 7 Madison Limestone (v_» Precambrian Granite i -, Terrace Gravel i“, ._—.—— Figure 5. Geologic Map of the Burgess Junction area. Note, Gros Ventre Formation is grouped with the Flathead Sandstone on this map. Image source: Big Horn National Forest Service. The Gros Ventre Formation of the Burgess Junction area of the Big Horn Mountains is not particularly well-exposed. The Burgess Junction exposures are of approximately nine meters of exposed section, which is a small window of the lower portion of the approximately 200 meters of the Gros Ventre Formation mapped in the Big Horn Mountains (Koucky and Cygan, 1963). The predominantly muddy to silty formation is exposed almost exclusively in drainages and forms low rolling topography in contrast to the more resistant overlying carbonate and underlying sandstone units. The ichnofossils are preserved in sandstone and weather out in abundance on the surface. The Flathead Sandstone was first described by AC Peale (1893) as having: Remarkably persistent quartzite or sandstone, which has long been recognized In [the] Rocky Mountain region as lying in most cases at [the] base of Paleozoic section...ln places rests on Belt series and in places on Archean schists or gneisses. Koucky and Cygan (1963) describe the contact at the top of the Flathead with the base of the Gros Ventre formation in the Big Horn Mountains as a series of brown limestone trilobite hash beds (Figure 6a and 6b). See Appendix 2 for field photos of the Flathead Sandstone. The Gallatin Limestone was first described by AC Peale (1893) as mainly calcite...[and] conformably overlain by Jefferson Limestone and rests on Flathead Shales...Named for typical occurrence in Gallatin Range (the southern extension of which is in [the] Northwest corner of Yellowstone Park...lt is essentially a series of limestones, more massively bedded than those of underlying Flathead Formation, and forms first prominent limestone bluff that rises above the Archean areas. Koucky and Cygan (1963) described the contact at the top of the Gros Ventre Formation with the base of the Gallatin Limestone as the first prominent limestone ledge creating a break in the slope. The Gallatin is described having more massive limestone beds and fewer mudstone beds, in contrast to the muddy underlying Gros Ventre Formation. See Appendix 2 for field photos of the Gallatin Limestone. Figure 6a. Trilobite hash from top of Flathead Sandstone at NT1. Scale bar is 2 cm. Figure 6b. Trilobite hash from top of Flathead Sandstone. Scale bar is 1cm The Flathead, Gros Ventre, and Gallatin Formations are all part of a facies transition from a sandy shoreface during the time of Flathead deposition to an open marine carbonate shelf during the deposition of the Gallatin Formation. Morgan (1998) described the Flathead Formation and Wolsey Formation (a lateral equivalent of the Gros Ventre Formation in the nearby Clarks Fork region of Wyoming), as a classic transgressive succession for the Middle Cambrian, which occurred from west to east with a shoreline trending North-South. The thickness of the Flathead Formation is variable depending on the underlying topography created by the 2.7 Ga Precambrian granite complex (Middleton, 1980; Sepkoski, 1982). Morgan (1998) described the Wolsey as a transition from predominant deposition of sands to predominant offshore muds at fair-weather wave base with sand layers incorporated into the Wolsey during periods of storm activity causing increased wave energy. Sepkoski (1982) wrote about the Sauk Sea transgression, focusing on the flat-pebble conglomerates present in the Gallatin Formation, but described an overall depositional system for the Cambrian in this region as a storm dominated shallow-water depositional environment and described the paleoenvironment of the Gros Ventre Formation as a subtidal lagoon (Figure 7). 11 A. Stratigraphic cross-section. upper PILGRIH FORMATION middle lot-u I-IASMARK 'Gnos VENTRE' DEADWOOD romn‘nou FORMATION FORMATION (“999! 9°88. . K233; x2 1‘. - . . . "—~ 0774‘“- 5— ww _“ mSholy Focies: —~- if“ N“ 7‘.“ a} ’ ‘E-L— W ...—‘- ~O- I ' _ Jjfl’aflfi” / ,/ ' 25m [00 krll B. Depositional environments. PERITIOAL BANK INTERTIDAL SUBTIDAL LAGOON 'COASTAL COMPLEX” SWN' Iatar- Infra- WME Sher-aloe. Sanction, boys, etc. W“— .F" ‘Q‘SV' ‘—'j"'_'.n,- Figure 7. Sepkoski’s (1982) paleoenvironmental interpretation. Note, the Pilgn'm is a lateral equivalent to the Flathead Sandstone and the Deadwood Formation is a lateral equivalent to the Gallatin Limestone. 12 MATERIALS AND METHODS Seven exposures (Figure 4, Appendix 2) were studied in the area around Burgess Junction, Wyoming. Four of these exposures were of the Gros Ventre Formation (Burgess Junction 1, Bear Lodge 2, Passage Resort 1 and Passage Resort 2). All of the Gros Ventre exposures were measured. Three sections were sampled at a 5-10 cm interval: Burgess Junction 1, Passage Resort 1, and Passage Resort 2. Ichnofossils found in situ were also collected and recorded in the stratigraphic chart (see Appendix 1 and Figures 203 and 20b). An abundance of ichnofossils found in float were collected, though a substantial number were left at the exposure because of the overwhelming number of ichnofossils in float. The samples were unpacked and stored in the Paleontology Laboratory at Michigan State University’s Department of Geological Sciences. Twenty specimens of Rusophycus and ten specimens of Teichichnus ichnofossils were cut in cross section in multiple orientations with a diamond-blade rock saw and then polished to examine internal structure. Twenty-two samples were prepared and shipped to Vancouver GeoTech Labs to be prepared as thin sections. A thin section sample was selected for each of the major sedimentary packages and marker beds represented in the exposed sections. The majority of the thin section samples were selected from beds that could have a thin section made of both the bedding and an in situ ichnofossil (the use of in situ implies that the sample was collected and documented while measuring the exposed section). Bioturbation intensity was determined using the index of Droser and Bottjer l3 (1986) for within-bed bioturbation and that of Miller and Smail (1997) for bioturbation on bedding surfaces. Images in this thesis are presented in color. DESCRIPTION OF ICHNOFOSSILS PRESENT Rusophycus The ichnofossil Rusophycus (Figures 8a-h) is a bilobed, hypichnial ichnofossil found as casts on the sole surface of sedimentary beds, and was originally described in 1852 by James Hall as being a “fucoid” with a presumed algal origin (Hall, 1852) followed by a later interpretation as an ichnofossil produced by an arthropod trace-maker (Dawson, 1864; Nathorst, 1883, 1886). Subsequent ethological interpretations of Ru30phycus as an arthropod ichnofossil include its possible origin as a nesting structure (Fenton and Fenton, 1937), a burrow for hiding from predators or hunting for prey (Seilacher, 1953, 1955, 1959; Osgood, 1970; Bergstrom, 1973; Osgood and Brennan, 1975; Brandt, et al., 1995) or a structure excavated for feeding (Glaessner, 1957; Whittington, 1980; Seilacher, 1985). For a more detailed description of Rusophycus ethology, see Brandt’s (2008) description of multiple Rusophycus assemblages. It is not the purpose of this thesis to review the taxonomy of Rusophycus (e.g., placement of Rusophycus in synonomy with Cruziana or the number of ichnospecies present). For more detail on Rusophycus taxonomy, see Osgood (1970), Birkenmajer and Bruton (1971), Crimes (1975), Bergstrom (1976), Bromley and Asgaard (1979), Pollard (1985), Buatois and Mangano (1993), Keighley and Pickerill (1996), Bromley (1996), and Seilacher (2007). The 14 definition of Rusophycus used in this thesis follows that described by Hanzschel (1975) as short, bilobate traces, with or without transverse striae or wrinkles. The probable Rusophycus-makers in the Gros Ventre Formation were Elvinia, Irvinge/Ia, Parabolinoides, Ehmania, Arapaho/a, Cedaria, and Tricrepicephalus. Figure 8a. Rusophycus from Passage Resort 2 (Gros Ventre Formation). Scale bar is 2 cm. 15 ., . , l,’ , Figure 8b. Rusophycus in the field (highlighted with red oval). collected at Burgess Junction 1 (Gros Ventre Formation). Hammer (12 inches exposed) for scale. Figure 8c. Slab with multiple Rusophycus and other traces. Scale is 2 cm Figure 8d. Rusophycus with pebble of potassium feldspar, highlighted with red oval. Collected at Bear Lodge 2 (Gros Ventre Formation). Scale bar is 2cm. Figure 8e. Rusophycus with large pebble (3 cm) cross-cutting specimen. Pebble is absent. but mold of it remains. Scale bar is 2 cm “<( N .- - t” Figure 8f. Smallest size R usophycus collected in Burgess Junction. Scale bar is 1 cm Figure 89. Small sized Rusophycus collected in Burgess Junction. Scale bar is 1 cm Figure 8h. Medium sized Rusophycus collected at Burgess Junction. Scale bar is 1cm. 20 Figure 9. Cross-sectioned specimens of Rusophycus. Scale bars are all 2 cm. 21 In the Gros Ventre Formation at Burgess Junction, the Rusophycus occur as isolated sand lenses in muds, or on the sole surfaces of sandstone beds. The Rusophycus tend to be more concentrated in horizons that are stratigraphically above sandstone beds (see ichnofossil occurrences noted in stratigraphic columns, Figures 20a and 20b). In cross-section (Figure 9), the specimens show fine sandy to silty laminae with mud drapes; some specimens yield cross- bedding. Teichichnus The ichnofossil Teichichnus (Figures 10a and 10b) is also found in abundance at the Burgess Junction Gros Ventre exposures. Teichichnus is commonly attributed to the work of worms, but sometimes interpreted as the work of arthropods simply because of the breadth of the structure, which has been considered too wide for the body of an annelid or any other worm-like organisms (Seilacher, 2007). Teichichnus bears a morphologic similarity to the ichnofossil Diplocraterfon with its serial repetition of spreiten, but the tubes of the Teichichnus ichnofossil have a J-shape, rather than the U-shape seen in Diplocraterion. Seilacher (2007) described Teichichnus as: characterized by transversal backfill structures that are not evenly draped between the two shafts of a U-tube. Rather, they were generated by J-tubes, very shallow U-tubes, or only in a limited stretch of an undefined tunnel system. Most of them have a retrusive backfill system. Teichichnus is one of the most abundant ichnofossils found at the Burgess Junction exposures. They tend to occur in the thinner sandy lenses in the silty to 22 muddy beds (see ichnofossil occurrences noted in stratigraphic columns in Figures 20a and 20b). Teichichnus is usually not found in conjunction with Rusophycus at the studied exposures. In cross-section (Figures 11a and 11b), the specimens show concave up spreiten. Figure 10a. Teichichnus from PR2 (Gros Ventre Formation), scale bar is 2cm. Figure 10b. Teichichnus from PR2 (Gros Ventre Formation), scale bar is 2cm. 23 Figure 11a. Crosst—secioned Teichichnus. Scale bar is 1 cm. Figure 11b. Cross-sectioned Teichichnus. Scale bar is 1 cm. 24 Chondrites Seilacher (2007) described Chondrites (Figure 12) with a stratigraphic range of Ordovician to Paleogene in shelf facies, and in Recent deep sea muds, with newer discoveries of Chondrites in modern deep sea muds. Hantzschel (1975) listed a lower Cambrian occurrence of Chondrites. These simple, branching burrows are often considered an indicator of low-oxygen conditions and typically occur along bedding planes in turbidite series, shallow marine shales, and storm sands (Seilacher, 2007). A Chondrites-bearing bed is a distinct stratigraphic unit within the Gros Ventre Formation at the Burgess Junction localities that serves as a marker bed for correlation between exposures with its green siltstones, mudstones, and abundant Chondrites ichnofossils (Figures 20a and 20b). Figure 12 Chondrites from PR2 (Gros Ventre Formation). Scale bar is 5cm. 25 Skolithos Simple cylindrical, vertical burrows found extending to both the top and sole surfaces of sandstone beds in the Flathead Sandstone (Figure 13). These types of burrows are typical of the Skolithos ichnofacies (figure 24), which characterizes the Flathead sandstones as originating from a sublittoral, high energy, sandy environment. Skolithos burrows are also present in the Gros Ventre, but are not as abundant. Figure 13. Skolithos Burrows from NT1 (Flathead Sandstone). Scale bar is 2 cm. 26 PSEUDOICHNOFOSSILS PRESENT This section is designated for sedimentary structures associated with bacterial mats. Kinneyia The wrinkle structure, Kinneyia (Figures 14a and 14b), is not an ichnofossil, but rather a sedimentary structure often interpreted as a pseudofossil and formerly treated as problematica (Hantzchel, 1975). Kinneyia is presumed to form in the earlier stages of bacterial mat development, and not formed by sediment loading or burial processes on the bacterial mat, but rather, by oscillations of water flowing beneath a partially liquefied area at the interface between the bacterial mat and the underlying sediment (Porada, et al, 2008). 27 Figure 14b. Kinneyia preserved in carbonate from NT3 (Gallatin Formation). Scale bar is 2 cm. Bacterial Mat Structures A number of specimens were collected that were interpreted by this author as bacterial mat structures (Figures 15a and 15b), but are different from Kinneyia, as these structures are likely either fossilized bacterial mats or casts of bacterial mats. They differ from Kinneyia because they are not interpreted as the results of fluid dynamics (Porada, et al,. 2008). 28 Figure 15a. Bacterial mat structures (?) from BJ1 (Gros Ventre Formation), scale bar is 2cm Figure 15b. Probable bacterial mat structure from Gallatin Limestone. Scale is 2 cm. 29 RESULTS Rusophycus Biostratinomy Rusophycus occur as discrete sand lenses within beds of micaceous silty mudstone (shale classifications are based upon the descriptions of Potter, Maynard, and Pryor, 1980), as well as casts on the sole of sandstone beds. Preservation of the Rusophycus ichnofossils ranges from indistinct bilobed structures to specimens showing impressions of the ventral anatomy, outline of the cephalon, and detailed scratch marks. Several Rusophycus have large angular rocks fragments of arkosic (primarily potassium feldspar and quartz) incorporated into the trace (Figures 80 and 8E). The sectioned and polished specimens (Figure 9) show an internal structure of fine laminae of sand and silt with mud drapes. Sedimentary Dynamics in the Gros Ventre The base of the exposed sections of Gros Ventre have well-sorted sandstones that grade into green silty mudstones with abundant Chondrites. The Chondrites beds served as a stratigraphic marker for correlating the four exposures of Gros Ventre studied in the Burgess Junction area. Some very poorly preserved Rusophycus were found in these lower intervals, though they become abundant and better preserved in the upper portions of the stratigraphic sections. The beds have an overall fining upwards trend (Figures 203 and 20b) with more abundant silt beds draped in mud with regularly spaced sandy lenses 30 towards the top of the exposed sections. The thicker sand beds have graded bedding with sole marks (Figure 16a) and mud lenses and rip-up clasts (Figure 17) within the bedding as well as mud drapes around the beds. Additionally, the thicker sand beds are almost always topped with a densely bioturbated layer (Figures 18a and 18b) of silt draped in mud. This very rhythmic pattern of sandy, silty, and muddy layers (Figure 19) repeats itself all the way to the top of the section. T BJltlzolt‘m , C M. Figure 16b. Cross-sectional view of bed from BJ1 featured in Figure 16a (above). Scale bar is 5 cm 31 K“ V Figure 17. Sale surface of bed collected at PR2 with multiple Rusophycus (highlighted in red ovals) and rip-up clasts (shown with red arrows). Scale bar is 5 cm. ' V» 2' *1.» +2. I ~. Figure 183. Bed PR2+81O in the field. Notebook (12 cm wide) for scale. This bed is contain heavily bioturbated muds with a bioturbation index of 4. S 32 Figure 18b. PR2+810 (Gros Ventre Formation), densely bioturbated layer with multiple Rusophycus. Index of Bioturbation for this bed is 4. Scale Bar is 5cm. 33 . .. ' '.; .mia‘ in; ' . Figure 19. Mud/silt/sand beds topped with rippled sandst Hammer (22 inches) for scale. one at PR2 (Gros Ventre Formation). 34 , .4 - ———-‘—‘._____~___,-_J - " m “ r" I ‘q‘:" ”7/ I ”My ‘\.I-,IJ 1' ‘5 P H“-‘ .J ‘ W... —. (g ! 1 i . - _ (f . ‘. ‘J" i . ) \~__, .. P‘w'v; ’* "N -"f i . ~ ' I * ' / 2L... ' I ,~__'_,.. . -a. --'"'.' I' - 2%... ~- ~ ‘ j I ‘ 1 .. . ' ‘ ‘ ‘ . . 3 ' i t ... .. Figure 20. Composite stratigraphic column (hand drawn) of the Burgess Junction area. 35 .5 a? w m M , . -. :0 " lat / '1' [15,12 :2“ :r i J44 “ $1.“; . 1) ) .4." - V ." - ' ' L-~..- Lv‘ f1 Y xv . i- w . ' « I, ,v i: V, , IV c2“ ‘7’ l: ”LE- , “l ‘ I, 49 [+2 I 1v / , , ,L. ‘."'_1\’,\ I = 20c 7 Bear Lodge #2 (3L2) Thickness: 430 cm Passage Resort #1 (PR1 Thickness: 1250 cm ' =20cm = 200m Passage Resort #2 (PR2) Thickness: 900 cm = 20cm ,. Burgess Junction #1 “’ (BJ1) (5, Thickness: 380 cm Key Sandstone Silstone Mudstone Covered Ripple Surface Mud Lenses Rusophycus Teichichnus Skolithos Chondrites Figure 20b. Correlated stratigraphic section (hand drawn) of exposures in the Burgess Junction area. Lines are drawn correlating the Chondrites marker beds. No body fossils were found in the measured sections of the Gros Ventre Formation, though fragments of trilobites were collected from the Flathead Sandstone, and body fossils are present in the overlying Gallatin Limestone. The lowermost exposed portions of the section contained well-sorted sandstones that grade into muddier green shale/silt with abundant Chondrites (Figure 20a and 20b). 36 lchnofabric of the Middle Cambrian Gros Ventre Formation 121 samples of beds from the Gros Ventre Formation were collected from the Burgess Junction area. Each sample was described in detail for composition, grain size, sorting, rounding of grains, and bioturbation index of both the bedding plane and cross-section of the sample (detailed descriptions given in Appendix 1). The average bioturbation index for the bedding planes is 2.332 (on a scale of 0 through 5), using the Miller and Small (1998) index of bioturbation. The average bioturbation index for the cross section of samples is 2.105 (on a scale of 0 through 5), using the index of bioturbation developed by Droser and Bottjer (1986). Bioturbation was most well-developed in the beds with abundant Chondrites (Figure 20a and 20b), which have an average cross-sectional and bedding plane index of 3.5. These results are shown in the histograms below. Index of Bioturbation so — —- n =121 Miller-Smail average = 2.332 .5 O Droser-Bottjer average 1 05 tamer—Ema’rtégai’ ; L- Dffier'fljmfi (1,985) i Number of Occurrences N bu o o ... O I Bioturbation Index—Droser-Bothier, 1986 and Miller-Small (1998) Figure 21. Histogram combining both the Droser-Bottjer (1986) and Miller-Smail (1998) bioturbation indices.‘ 37 Petrographic Data The ichnofossil and associated bedding thin sections were composed of sandstones that were dominantly monocrystalline quartz and potassium feldspar with some polycrystalline quartz and zircon in a matrix of fine-grained pyrite and clay present as black stringers and as brown coatings around the quartz and feldspar grains. Comparisons were made between ichnofossils samples and associated bedding from the same horizon, specifically focusing on Rusophycus and Teichichnus.(Figure 22). The Rusophycus thin sections show a similar grain size as well as a comparable amount of brown coating on the clastic grains than the associated bedding from the same horizon. The Teichichnus thin sections have a smaller grain size and less of the brown coating on the elastic grains than the associated bedding from the same horizon. Rusophycus in situ Associated bedding . I'd Teichichnus in situ Associated Bedding Figure 22. Thin section photomicrographs at 10x power. 38 DISCUSSION Rusophycus Biostratinomy The method of excavation by the Rusophycus trace-maker has been a subject of debate in the ichnofossil community. Some ichnologists have inferred that Rusophycus was excavated on the surface of muddy substrates and subsequently infilled by sedimentation (Osgood, 1970; Crimes, 1975; Baldwin, 1977). Other ichnologists have proposed an intrastratal origin for Rusophycus, claiming that the organism burrowed in sand to a muddy interface and actively backfilled the trace as it was being excavated (Seilacher, 1955, 1970, 1983, 1985; Birkenmajer and Bruton, 1971). The abundance of Rusophycus in the Gros Ventre Formation made it possible to section and polish 20 specimens (Figure 9) and test the interstratal hypothesis by looking for the spreiten (internal structures) predicted by Seilacher’s scenario. The sectioned and polished specimens showed fining upward laminae of sand and silt with mud drapes, indicative of episodic deposition and sedimentary processes. Spreiten would look like concentric U-shaped laminae created by the trilobite trace-maker. The Rusophycus appear to be filled by multiple sedimentary events (three or more pulses of sedimentation) following the excavation of a burrow in exposed muds. 39 WT; W 1rw I'I‘V‘W‘t" w W «ll _ M Alkali-4M1 KT... W VMF'I""' . “‘3‘“ . . I 54—4..“ but llthth Frgure 23b. Rusophycus excavation techniques proposed by Seilacher (2007). Note, exeevation is inferred to be done subsurface at a sand-mud interface. Additional support for the scenario that the Rusophycus were passively filled comes from the grain size of some of the sand beds in which the Rusophycus are preserved (Figures 8d and 8e). These Rusophycus have large angular grains cross-cutting the specimen and were clearty deposited after the trace was excavated. Evaluation of the photomicrographs (Figure 22) taken from thin sections of ichnofossils and their associated bedding from the same horizon show sandstones with clastic grains of quartz and potassium feldspar, matching the composition to the underlying and nonconforming Precambrian granites. The brown coatings on the grains are organic matter derived from the abundant bacterial mats present in the Sauk Sea. The brown coatings were evaluated by comparison to photomicrographs of similar sandstone facies (Scholle, 1979) and photomicrographs of Kinneyia (Porada, et al., 2008). Other supporting evidence for the presence of organic matter (bacterial mat-derived) coating the clastic grains include the presence of bacterial mat structures in the Gros Ventre Formation and overlying units and the abundance of Rusophycus ichnofossil, which suggests an abundant food source (bacterial mats) in the Sauk Sea. In thin section, the Rusophycus ichnofossils show a similar grain size to associated bedding of the same horizon. This observation suggests that the Rusophycus contain a casting medium that is the result of sedimentary deposition, rather than biologic reworking of the sediment by the Rusophycus trilobite trace-maker. In contrast, Teichichnus ichnofossils in thin—section show less organic material and smaller grains than the associated bedding of the same horizon. The Teichichnus ichnofossils show substantially less organic matter in the matrix or on the coatings of clastic grains. This suggests that the Teichichnus ichnofossil is the product of active work of the Teichichnus trace-maker burrowing down through the sediment, producing spreite and ingesting organic matter (Figure 22). 41 These results are congruent with other results from this project derived from evaluation of hand samples, cross-sectioned hand samples, and the sedimentary dynamics of the Sauk Sea transgression. The Rusophycus ichnofossils were excavated in open muds and subsequently filled by sedimentation. Sedimentary dynamics in the Gros Ventre Formation The fining upward rhythmic pattern of the sandy, silty, and muddy Gros Ventre Formation is consistent with previous descriptions (Sepkoski ,1982; Morgan, 1998; Middleton, 1980) of this sequence as a fining upward transgressive system. The underlying Flathead Formation is primarily characterized by shoreface sands and ichnofossils typical of the Skolithos ichnofacies (Figure 24). The depositional environment transitions to a sublittoral zone during the deposition of the Gros Ventre Formation. The Gros Ventre is characterized by mud-drapes and flaser-bedded sands, silts, and muds and contains ichnofossils typical of the Cruziana ichnofacies (Figure 24) with particularly abundant and excellently preserved Rusophycus, Teichichnus, Skolithos, and Chondrites ichnofossils. During the deposition of the stratigraphically overlying Gallatin Formation, the transition to an open marine carbonate zone is completed. There is an overall transition from near shore sands of the Flathead Sandstone grading to sublittoral lagoonal muds, silts, and sands of the Gros Ventre deposition, and ultimately to open marine carbonates 42 during the deposition of the Gallatin Limestone at the end of the Middle Cambrian. Semi- Sandy Shore consolidated Substrate Bathynl Zone “try... - Abyssal SerI'iI'f-tw" Iéfimldfed SandyShore SubllttoralZone ‘3: 2°“ Trypanites (Glossilungr‘tesI Skolithos Cruziana Zoophycos Nereites Figure 24. lchnofacies. The Flathead Sandstone comprises the sandy shoreface Skolithos ichnofacies. The Gros Ventre Formation comprises the sublittoral zone Cruziana ichnofacies, which includes the Rusophycus ichnofossil. Gros Ventre desposition compares closely to the model proposed for the coeval Wolsey Formation (Middleton, 1980; Morgan, 1998). The Wolsey comprises predominantly fine-grained muds and silts interspersed with coarse- grained arkosic sands, which are interpreted as derived from the nearby granitic shorelines. The large, subangular grains seen in many of the sandstone beds of the Gros Ventre Formation suggest derivation from a nearby sediment source. Geologic maps (Figure 5) show the Flathead Sandstone and Gros Ventre 43 Formation in contact with Precambrian granites, implicating the granites as the shorelines in the Sauk Sea. Middleton (1980) described the Precambrian granites as “crystalline highs [that] were islands in an epeiric sea.” Sepkoski’s (1982) description of the Gros Ventre Formation as deposited in a lagoonal setting fits well with the observations made of the Gros Ventre at Burgess Junction. The predominant rock type is mudstone to siltstone. The large, subangular to subrounded arkosic sand grains found in the sandstones and even in the Rusophycus ichnofossils imply a nearby sediment source. The lagoon within the Sauk Sea would have allow for sequestering of the coarser-grained sediments during episodic storms in what is othenrvise a lower energy environment in which muds and silts were deposited during periods of relative quiescence. The isolated Rusophycus in mudstone beds provide additional evidence of episodic storms that swept arkosic sands from nearby exposed Precambrian granitic shorelines into this shallow littoral environment, possibly being transported in tidal channels as a storm surge ebb deposit. The fine detail in many of the Rusophycus implies a cohesive substrate and suggests the presence of bacterial mats in the Wyoming Sauk Sea during the Cambrian, which is supported by the presence of Kinneyia and bacterial mat structures in stratigraphically adjacent strata. The bacterial mats likely contributed to firm, cohesive muds, which allowed excavated structures (e.g. Rusophycus) to survive long enough to be filled and cast by sands sweeping in during episodic storm deposition. The bacterial mats may have also been the source of food mined by the trace-makers. It is unlikely that trilobites would excavate burrows in such coarse material. One specimen (Figure 8e) has an impression from a pebble (3 cm) in the middle of one of the Rusophycus lobes, cross-cutting the Rusophycus scratch marks. The resolution of time between the sedimentary events that filled and cast the Rusophycus was probably narrow. Episodic storms initially deposited sand, silts and muds were deposited as energy waned. Deposition of the mud resulted in (1) better preservation of the ichnofossils and (2) opportunistic exploitation of the substrate by trilobites and other mud-dwelling/feeding organisms (eg. the Teichichnus trace-maker). Brett and Seilacher (1991) described lagerstatten developing as a result of event sedimentation. The exposures at Burgess Junction have numerous event deposits that preserve an anomalously large number of ichnofossils, thus warranting the description of the Burgess Junction Gros Ventre Formation as an ichnofossil lagerstatte. The presence of a meters-thick Chondrites unit provides evidence of the oxygenation conditions during Gros Ventre deposition. Seilacher (2007) interpreted Chondrites as indicative of low levels of oxygenation. The transition from beds with abundant Chondrites to beds with abundant Rusophycus may therefore indicate an increase in oxygen levels. The return of burrowing trilobites above the Chondrites beds likely reflect this increase in oxygenation. No trilobite body fossils were found in the exposures of the Gros Ventre Formation, suggestion that trilobites either ( 1) 45 escaped the environment after excavating the Rusophycus burrows, but before storms swept through and cast their burrows with sands and lags of silt and mud, or (2) were killed during the storms that deposited the Rusophycus-casting sandstones, but did not fossilize because of taphonomic biases due to the poor chance of preservation in sandstones. By the time of Gallatin deposition, lithologies were carbonate-dominated, indicating less siliciclastic input and a more open marine environment. lchnofabric of the Middle Cambrian Gros Ventreiand its place in the Cambrian Substrate Revolution The average bioturbation index for the bedding planes in the Gros Ventre Formation is 2.332, (on a 0-5 scale) using the Miller and Small (1998) index of bioturbation. The average bioturbation index for the cross section of beds is 2.105 (on a 0 through 5 scale), using the index of bioturbation developed by Droser and Bottjer (1986). Bioturbation was most abundant in the Chondrites beds (Fig 20. Composite strat column), which have an average cross-sectional and bedding plane index of 3.5 (on a 0 through 5 scale). Droser, et al., (1988) recorded an average lchnofabric index of 1.02 in the Early Cambrian, and increase to 3.1 in later Early Cambrian strata and a maximum of approximately 3.5 in Late Cambrian strata of the inner and middle shelf carbonate facies of the Great Basin of California, Nevada, and Utah. The average index of 2.11 observed in the Gros Ventre Formation of Burgess Junction falls between the Early Cambrian and Late Cambrian lchnofabric values 46 of Droser and Bottjer (1988) and support the idea that the Cambrian Substrate Revolution was a gradual transition, rather than an abrupt “tipping point” (sensu Gladwell, 2002). The slightly higher average index of 2.33 derived using the Miller and Smail (1998) index for bedding planes is consistent with the hypothesis that during this interval, most biotic activity took place at or near the substrate, and therefore is found on bedding planes, and not within the bedding. N. —\Gros Ventre ”ET 1 1 fi 7 *1 Cambrian Ordovician Average lchnofabric index 3 Figure 25a. Bioturbation figure modified from Droser et al., (1988) noting average ichnofabric index for Cambrian and Ordovician and the interval in which the Gros Ventre of Burgess Junction is positioned. 47' Droser—Bottjer MIIIrr-Small lib—— METERS l l 1 2 3 4 [I 2|3|4l5| ]:|2|3745 ICHNOFABRIC INDICES Great Basin unit Burgess Junction Gros Ventre Fm. Figure 258. Comparison of 5 meters of measured Gros Ventre Formation (a predominantly siliciclastic unit) with 5 meters of an approximately coeval carbonate-dominated section published in Droser, et al., (1988). CONCLUSIONS The ichnofossil Rusophycus is the product of a typical behavior of trilobites, but is only preserved under unique conditions. A unique confluence of conditions existed in the Middle Cambrian Gros Ventre environment and resulted in the preservation of a Rusophycus lagerstatte. These conditions include the background deposition of firm, cohesive muds possibly bound by bacterial mats. The muds had a firmness that allowed the burrows to maintain their shape until they were filled in and cast during the next phase of sand deposition. The medium-to-coarse—grained arkosic sand that cast the Rusophycus was probably derived from the nearby granitic shoreline during tidal channel ebb flow following storms. The absence of spreiten and presence of individual depositional events in sectioned Rusophycus support this conclusion. These results are in contrast to previous assertions that the Rusophycus ichnofossil was excavated by trilobites burrowing through sand to a muddy interface. The presence of Chondrites in Middle Cambrian strata of the Big Horn Mountains is in contrast to the narrower stratigraphic range advocated by other workers (e.g., Seilacher, 2007), but congruent with earlier reports (Hantzchel, 1975) The Gros Ventre Formation falls in the middle of the Cambrian Substrate Revolution. Bacterial mats were present before, during, and after the deposition of the Gros Ventre Formation. Bioturbation indices for the Gros Ventre Formation are comparable to those documented for the Middle Cambrian by 49 other workers (e.g., Droser, et al., 1988) and support the idea of a gradual increase of bioturbation intensity during the Middle Cambrian. There is still an abundance of work that can be done with this project. Some future directions for-work include correlation of sections elsewhere of the same age (Middle Cambrian) and similar facies to more widely document the Cambrian Substrate Revolution. Additionally, there is the prospect for analyses using quantitative methods and chemostratigraphy. The use of quantitative analyses could help better ascertain the number of ichnospecies present in addition to modeling diversity and faunal distributions in the Middle Cambrian. 50 Appendix 1, Bed-by-Bed Descriptions of Measured Localities All measurements are given from the lowest exposed portion of the section (Ocm) to the top of the exposed section. The measurements from the sampled exposures were recorded with the abbreviated outcrop name (e.g., BJ1, PR2) followed by a number that corresponds to the number of centimeters aboVe the base (Ocm) where the sample was collected. An asterisk (*) next to the sample number indicates an in situ ichnofossil sample was collected. Lithologic samples were collected at everv recorded Burgess Junction 1 (BJ1) mple . Description Droser- BottieL BJ10 green mudstone with Chondrites Miller- Small 4 Miller- Droser- I BJ1-+10" green mudstone with Chondrites 4 BJ1 +20 silty, tan to green muddy sandstone with Teichichnus on sole, medium sorting, Subangular grains BJ1-+35" green mudstone with Chondrites and Teichichnus at base BJ1+45 green to tan mudstone with Chondrites BJ1 +55 green to tan mudstone with Chondrites BJ1+65 green to tan mudstone with Chondrites BJ1 +75 green to tan mudstone with Chondrites BJ1 +85 green to tan silty mudstone with Chongites 1.5 BJ1 +95 green to tan silty mudstone with Chondrites 1.5 i . 51 [12345112345] BJ1, continued Sample ' Description Droser- $3M BJ1 +1 05 green to tan silty mudstone with Chondrites 1.5 Miller- BJ1-+115 green to tan silty mudstone with Chondrites 1.5 BJ1+125* tan to green silty mudstone with Chondrites 1.5 BJ1+135* tan to green silty mudstone with abundant Chondrites 1.5 BJ1 +145 tan to green silty mudstone with abundant Chondrites 1.5 BJ1+155 tan to green silty mudstone with abundant Chondrites 1.5 BJ1 +165 tan to green silty mudstone with abundant Chondrites 1.5 Droser- Miller- BJ1-F175 ten to green mudstone, less silt 1.5 BJ1-+185 tan to green mudstone, less silt BJ1 +195 tan mudstone BJ1 +205* tan mudstone with sand- filled Teichichnus BJ1+215* tan mudstone with sand- filled Teichichnus 2.5 BJ1+225* tan mudstone with sand- filled Teichichnus 2.5 BJ1 4-230" tan sandy mudstone with sand-filled Teichichnus 2.5 BJ1 +235“ tan sandy mudstone with type burrows and scour marks on sole; graded bedding with subrounded gr IDS 1.5-1 52 l12345l12345] BJ1, continued Sample Description Droser- Battier Miller- Sma_it_ BJ1 +245 tan sandy siltstone; medium-well sorted with subrounded grains 1.5 Droser- Battier Miller- » Smail I BJ1+255 tan sandstone, fine to medium grained; medium-well sorted with subroundedflains BJ1 +305 tan mudstone 2.5 BJ1 +315" tan mudstone with sand- filled Rusophycus burrow 2.5 BJ1-+340 tan sandstone with rippled top surface, fine to medium grained; graded bedding with sgbrormded grams 3.5 BJ1 +325 brown mudstone BJ1 +350 tan mudstone 2.5 NA ”—8 BJ1-+360 tan sandstone, fine to medium grained; medium-well sorted with subrounded grains SJ 1 +370 tan sandstone with rippled top surface, fine to medium grained; graded bedding with sgbrounded grains BJ1+380 tan sandstone, fine to medium grained; medium-well sorted with subrourgled grains 1 *Weathered mudstone above, forming top of hill on which outcrop is exposed, which grades into a meadow where numerous Rusophycus were collected in float. 53 1234sh2345] Passage Resort 2 (PR2) ample Description (I) Droser- Miller- Droser- I Miller— I Battier Smail Battier Smail PR20 tan sandstone, fine to medium grained, medium sorting with subrounded grains 1 ‘l 1.5 PR2+10 tan sandstone, fine to medium grained, medium sorting with subangular grains PR2+20 tan sandstone, more massive, medium grained, medium sorting with subangular grains 25cm PR2+45 covered (shaley) tan sandstone, fine to medium grained, medium-well sorted with subangular grains PR2+55* tan mudstone with sandfilled Ruso h cus PR2+65 tan sandstone, fine to medium grained with mud/clay lenses, medium sorting with subrounded grains PR2+70 tan sandstone, fine to medium grained with mud/clay lenses, medium sorting with subrounded rains PR2+80 more massive tan sandstone, fine to medium grained with mud/clay lenses medium sorting with subangular grains 12345 12345 54 PR2, continued Sample Description Droser— Bottier Miller- Small PR2+90 more massive tan sandstone, fine to medium grained with mud/clay lenses, medium sorting with subangular grains Droser- Miller- 9 Bottle: Smail I PR2+1 00* green silty mudstone with Chondrites (basal Chondrites bed) 1.5 30cm covered (shaley with Chondrites ) PR2+1 30* green silty mudstone, approximately 100m thick with Chondrites \\ _s\ \\ PR2+140' green silty mudstone, thinner with Chondrites PR2+160* green silty mudstone, approximately 100m fithick with Chondrites PR2+1 60* green silty mudstone, thinner with Chondrites PR2+1 70* green silty mudstone, approximately 10cm thick with Chondrites PR2+1 75* green silty mudstone, approximately 5cm thick with Chondrites PR2+185* green silty mudstone, approximately 10cm thick with Chondrites PR2+1 95* green silty mudstone, thinner with Chondrites and Teichichnus 2.5 PR2+20? green silty mudstone, thin with Chondrites 2.5 40cm covered PR2+245“ green silty mudstone, thin with Chondrites 2.5 2.5 30cm covered 55 1234sh2345] PR2, continued Sample I Description Droser- Battier. PR2+275* green silty mudstone, medium thickness (less than 10cm) 2.5 Miller- 2.5 Droser- , l I PR2+285* green silty mudstone, thin with black stain between beds and Teichichnus 2.5 2.5 PR2+295" green silty mudstone, medium thickness (less than 10cm) with Chondrites 2.5 2.5 PR2-+31 0* green silty mudstone, medium thickness with Chondrites 2.5 PR2+320* green silty mudstone with interbedded silt laminae, medium thickness with Chondrites 2.5 Miller- I 60cm covered Yr PR2+380* green siltstone, coarser grained than underlying beds, thicker bedding (approximately 20cm) with ChMes 2.5 \\ \\ PR2+400* green siltstone, coarser grained than underlying bedding, approximately 20cm thick with Chondrites 1.5 2.5 PR2+420* green siltstone, coarser grained than underlying beds, approximately 200m thick with Chondrites 2.5 2.5 \\ '\\ \ PR2-M40" green siltstone, coarser grained than underlying beds, approximately 20cm thick with Chondrites 1.5 2.5 56 1234sfi2345l PR2, continued Eample Description Droser- BottIeL PR2+445* green siltstone, coarser grained than underlying beds, approximately 15cm thick with Chondrites 1.5 2.5 Miller'- Droser- Bottle Miller— " Sm ' PR2+455* green siltstone, coarser grained than underlying beds, approximately 10cm thick with Chondrites 2.5 60cm covered PR2+515* fissile mudstone with scratch marks 2.5 PR2-+520 medium grained sandstone, medium sorting with subrounded grains PR2+535 medium grained sandstone, medium sorting with subrounded grains 1.5 1.5 PR2-+545 fissile mudstone PR2+555" medium grained sandstone with green clay rip-up clasts and large Rusophycus, medium sorting with subangular grains 0.5 *This interval begins very abundant Rusophycus , which are present in the sandier lenses. 1.5 PR2+565“ medium grained sandstone with Rusophycus, medium sorting with subrounded grains l \\\\ “— 57 L12345l12345:l_ PR2, continued Sample Description Droser- Bottjer PR2+570 tan siltstone 1 f_q MIller- mail Droser- I Miller- Bottier “mail 1.5 PR2+575' medium grained tan sandstone with Rusophycus, medium sorting with subrounded arains PR2+580* thinly bedded tan siltstone with scratch marks, medium—well sorted PR2+590* medium grained sandstone with type burrows on sale surface, medium sorting with subangular grains PR2+595 medium grained sandstone, medium sorting with subangular dram 0.5 PR2+600 medium grained sandstone, thicker than beds below with interspersed laminated mud drapes, medium sorting with subangular grains PR2+610 muddy siltstone, thin :bedding with mud draped surface PR2+620 medium grained sandstone with interspersed laminated mud drapes, medium sorting with subrounded qrains 58 12345.12345 PR2, continued Eample Description Droser- Bottier Miller- Smail Droser- I WET—I VBottier Smail PR2+630* thinner, mud-draped silty sandstone with Teichichnus, medium sorting with subrounded grains PR2+640* thicker, medium grained sandstone with rippled top surface, scratch marks, and tiny serially repeated Rusophycus, with graded bedding and subrounded grains 1.5 1.5 PR2+650 thinner, silty sandstone with mud lenses, medium sorting with subrounded rains PR2+660 thinner, silty sandstone with mud lenses, medium sorting with subrounded rains PR2+670‘ tan muddy siltstone with very abundant burrows PR2+680 tan muddy siltstone PR2+690* thicker, medium grained sandstone with interspersed mud drapes and tiny serially repeated Rusophycus, medium sorting with subrounded grains PR2+700 green silty sandstone with rippled top surface, graded bedding with subangular grains PR2+71 0 green muddy siltstone with muscovite flakes PR2+720* green muddy siltstone with muscovite flakes and Teichichnus 2.5 59 12345 12345 PR2, continued Sample Description Droser- BQIIIQL. 30cm covered Miller- _§maiL. PR2+750* green to brown mudstone with sand filled Rusophycus 3.5 PR2+760 green to brown mudstone with sand lenses 3.5 PR2+770* medium grained sandstone with interspersed mud drapes and traces, medium well sorted with subrounded grains 1.5 PR2+780* thin muddy siltstone with poorly preserved traces 2.5 PR2-”90* thin muddy siltstone with poorly preserved traces PR2+800* thinly bedded medium grained sandstone with multiple tiny Rusophycus, medium- well sorted with sgbrounded grains 1.5 PR2+805 thinly bedded medium grained sandstone with interspersed mud drapes, medim-well sorted with subrounded grains 1.5 PR2+810 grey to brown siltstone with mud drapes very productive—numerous Rusophycus , scratch marks, and burrows 1.5 Droser- Miller- — A\\ PR2-+820 grey to brown silty mudstone @2345112'345] PR2, continued Sample Description Droser- Battier Miller- Smail PR2+830* thinly bedded medium grained sandstone with interspersed mud drapes and Rusophycus, medium-well sorted with subrounded grains 1.5 Droser— Miller- I » PR2+840* silty mudstone with pooly preserved burrows 1.5 1.5 PR2+880* medium grained sandstone with rippled top and burrows on the sole surface, graded bedding with subrounded grains PR2+890 coarser silty mudstone 2.5 1.5 PR2-+900 medium grained rippled sandstone approximately 5cm thick, graded bedding with subrounded grains 1.5 TOP OF SECTION 61 1234sh2345] Passage Resort #1 (PR1) Sample Description Droser- Botti r Miller- Sm i Droser- _ Battier Miller- I PR1 0 pink to green medium grained consolidated sandstone, medium sorting, subangular grains PR1 +15 pink to green medium grained consolidated sandstone, medium sorting, subangular grains PR1 +25 pink to green medium grained consolidated sandstone with visible surface burrows, medium sorting with subangular grains PR1 +35 1.5 pink to green medium grained consolidated sandstone, more arkosic (potassium feldspar) PR1 +45* buff sandstone, more nodular with interspersed muds and Teichichnus, medium sorting with subangular grains 1.5 PR1+55* pink to green medium grained consolidated sandstone with surface burrows, medium sorting with aubangular grains 1.5 PR1 +80 massive (25cm thick) pink to green sandstone, medium sorting with subrounded grains 20cm covered 62 l12345h2345| PR1, continued ample Description Droser- Battier Miller- Smail Droser- Bottier I Miller— I Smail PR1+100 pink to green medium grained sandstone with mud drapes, medium- well sorted with subrounded grains PR1 +115” tan medium grained crumbly sandstone, medium—well sorted with subrounded grains PR1+130 tan medium grained crumbly sandstone with Teichichnus, medium- well sorted with subrounded grains PR1 +1 40 tan medium grained crumbly sandstone, medium-well sorted with subrounded grains PR1+150 tan medium grained crumbly sandstone, medium-well sorted with subrounded grains PR1 +1 60 tan medium grained crumbly sandstone, medium-well sorted with subrounded grains PR1 +1 70 tan medium grained crumbly sandstone, medium—well sorted with subrounded grains PR1+185 tan medium grained crumbly sandstone, medium-well sorted with subrounded grains PR1+195* basal Chondrites bed, green muddy siltstone PR1 +415 , Isubrounded grains Massive (50 cm) tan sandstone, burrows on sole, well sorted with 63 12345312345 PR1, continued Begin unsampled measurements 40cm green muddy siltstone with Chondrites 40cm pink to green medium glained sandstone 40cm fissile green silty mudstone, coarsening ugwards 100cm green muddy siltstone with Chondrites 110cm fissile green muddy siltstone with Chondrites 25cm green medium grained consolidated sandstone with Chondrites 20cm fissile mudstone 40cm massive medium grained arkosic sandstone with burrows on sole surface 100cm fissile mudstone with sandy lenses and abundant traces, coarsening ugwards 20cm massive medium grained arkosic sandstone with numerous mud lenses 20cm cover (suspect productive layer over sandstone bed) silty mudstone coarsening upwards with sandlenses 64 PR1, continued unsampled measurements 25cm massive medium grained sandstone with rippled top surface, rip-up clasts, and mud drapes 10cm covered 60cm fissile silty mudstones with abundant ichnofossils, coarsening upwards 10cm medium grained sandstone with rippled top surface and ichnofossils on sole surface productive layer—silty mudstone with abundant ichnofossils 75cm fissile silty mudstones with abundant ichnofossils, coarsening upwards 10cm medium grained sandstone with rippled top surface productive layer—silty mudstone with abundant ichnofossils TOP OF SECTION 65 Bear Lodge 2 (3L2) Base of Section nodular sandstone, well- sorted, medium grained, highly “packed” top and bottom surfaces 20cm covered 10cm platy sandstone with mud drapes 20cm massive nodular sandstone covered 60cm begin Chondrites green silty mudstones, coarsen upwards to muddy siltstones 60cm green muddy siltstones with abundant Chondrites , more consolidated 35cm thinner green muddy siltstones with fewer Chondrites 30cm thinner green muddy silstones with fewer Chondrites At this point, there is a 20 meter long pediment in the outcrop. This seems to mark the end of the Chondrites beds and where the sand/silt/mud oscillations begin. No Rusophycus were collected in the bottom part of the outcrop, but are weathered out in abundance in the top section (described jelow) 40cm covered Bear Lodge 2 (3L2), continued unsampled measurements 10cm medium grained sandstone with muddy rim clasts 50cm covered 10cm medium grained sandstone with muddy rip-up clasts (sample collected) 20cm covered 20cm productive silty mudstones 10cm cove red 10cm medium grained sandstone with rippled top surface 20cm covered 10cm medium grained sandstone with rippled top surface Top of Section 67 APPENDIX 2 DESCRIPTIONS OF LOCALITIES STUDIED . Burgess Junction 1 (BJ1) This locality is an exposure of the Gros Ventre Formation on US Highway 14 at its intersection with US. Highway 14-Alternate Route (Figure 4). Approximately four meters of section are exposed with predominant lithologies of shale, silt, and sandstone (Figures 20a and 20b), which coarsen upwards with sandstones becoming more arkosic towards the top of the section. Ichnofossils collected at this locality include Chondrites (found in the Chondrites marker bed at the base of the section), Teichichnus, Rusophycus, and Skolithos. No body fossils were found at this locality. Figure 26. Burgess Junction #1 (Gros Ventre Formation). 68 Passage Resort 1 (PR1) Located in a drainage gully near park access road number 155 (Figure 4), the base of the section has well-sorted sandstones that grade into muddier green shale with abundant Chondrites—very few Rusophycus are found and they are poorly preserved (Figures 20a and 20b). In the third meter of the section, the Rusophycus become very abundant, as more sand and silt beds enter the section. Further upsection, the mudstones become more silty which seem to be silt beds draped in mud with a few sandy lenses. The thicker sand beds have graded bedding with mud lenses and muddy rip-up clasts within the bedding as well as mud drapes around the beds. The thick sand beds are almost always topped with a densely bioturbated silty layer grading into silty mudstones and topped by succeeding sand, silt, and mudstone beds repeating themselves all the way to the top of the section. .5314 K, _ f 7 Figure 27. Massive Sandstone Bed at PR1 69 Figure 28. Passage Resort #1 (Gros Ventre Formation) 70 v _, . "i a - a. Figure 29. PR1, (Gros Ventre Formation) pink to green sandstones at base of section. Hammer (22 inches) for scale. 71 Passage Resort 2 (PR2) Located at the intersection of US Highway 14 with park access road number 155 (Figure 4), this section is nearly identical to the exposure of Passage Resort 1, as the two localities are within close proximity to each other. The exposure at the Passage Resort 2 locality has the lowest exposed portion of the stratigraphic column, approximately 1.5 meters below the base of Passage Resort 2 (Figures 20a and 20b). Also present at this locality is a massive bed of sandstone (approximately 55 cm) that is not present at any of the other localities. This bed, however, has simply been interpreted by this author as an infilled channel. The exposure was sampled at a 5-10 centimeter interval. This outcrop is an exposure in a gully in an alpine meadow. The meadow is steeply sloped with a stream running through it, cutting into weak shaley beds. The exposure is highly weathered in some spots, causing lots of cover in the exposure. In some places, you can walk a meter or more laterally across the more resistant sandstone beds. The highly weathered slopes around the exposure are littered with Rusophycus, Teichichnus, and other ichnofossils. Almost every single piece of float has something of interest. 72 Figure 30. assage Resort #2 (PR2) 73 Figure 31. PR2 (Gros Ventre Formation), Rippled Sandstone Bed. Hammer (22 inches) for scale. 74 Bear Lodge 2 (BL2) This exposure of the middle Cambrian Gros Ventre Formation is located in a meadow across US Alternate Route 14 from the Bear Lodge in Burgess Junction (Figure 4) in a drainage gully leading to the same stream exposed at the base of the Passage Resort localities. This exposure is nearly identical to the Passage Resort exposures, but is much more weathered and has a shelf in the middle of the exposure, creating a sizable ledge in the middle of the exposure . The Bear Lodge 1 exposure is more weathered than the Passage Resort Exposures, so it was not sampled, only measured (Figures 20a and 20b), described, photographed, and surface collected for ichnofossils. Figure 32. Bear Lodge #2 (Gros Ventre Formation). 75 North Tongue 1 (NT1) This exposure of the upper Flathead Formation is located approximately one half mile north of the intersection with US Alternative Route 14 along National Forest Road #15 (Figure 4) on a switchback portion of the road that leads to the turnoff for the Twin Buttes, an exposure of the Ordovician Big Horn Formation, which is in clear view from the North Tongue exposures. The three North Tongue sections are all part of the same exposure, but have been separated due to location within the stratigraphic column or position within the switchback on the road. The Flathead is exposed in a ravine at the base of the switchback and contains thin to medium-thickness sandstone and mudstone beds. There is a minimal amount of bioturbation present on the sole and top surfaces of the sandstone beds. The majority of the traces were Skolithos burrows, but some Chondrites were also found. A series of trilobite hash beds were found, and help confirm the stratigraphic positioning with the upper Flathead, as they are a considered a key marker bed (Koucky and Cygan, 1963). Numerous fragments of the sedimentary structure Kinneyia collected in float, interpreted as the product of bacterial mats. 76 77 Figure 34. North Tongue #1 (Flathead Formation). 78 North Tongue 2 This exposure of the Upper Cambrian Gallatin Formation is located next to North Tongue 1 (Figure 4) along the middle and upper section of the switchback. Stratigraphically, it is positioned anomalously on the upper Flathead Formation. There is a fault present in the exposure, which pinches out all the shaley Gros Ventre Formation (Figure 5). Present at this locality were flat pebble conglomerates. Flat pebble conglomerates have been interpreted as evidence of the presence of bacterial mats in the Upper Cambrian (Sepkoski, 1982). Figure 35a. Flat pebble conglomerate from NT2 (Gallatine Limestone). Side and top view. Top of scale bar is 4 cm. 79 Figure 35b.. Flat pebble conglomerate from NT2 (Gallatine Limestone). Top view. Scale bar is 10cm. '~ .‘u- ‘ .i i \ ((35% "N. . ‘ " . .. I. .~ it kw" .11." . " Figure 36. Slab of flat pebble conglomerate collected in float Hammer (22 inches) for scale. at NT2. 80 Figure 37. Arrow pointing to exposure at North Tongue #2 Locality (NT2). Gravel road to right of arrow for scale. North Tongue 3 This exposure of the Upper Cambrian Gallatin Formation is adjacent to North Tongue 2 on the opposite side of the switchback in the upslope direction (Figure 1, locality map). Samples of carbonate Kinneyia and stromatolite-Iike structures were collected in float from this highly weathered locality, providing further evidence, which is interpreted as having been produced by bacterial mats. Figure 38. Flat pebble conglomerate from NT3 (Gallatin Formation). Scale bar is 2 cm. 82 Steamboat Point This is the only locality studied outside of Burgess Junction. It located east of Burgess Junction along US Highway 14 (Figure 4). There is a trailhead and parking lot at the base of the exposure for the Steamboat Point trail. This large section exposes the Middle Cambrian Flathead Sandstone through the Ordovician Big Horn Dolomite, which is exposed at the top of the exposure and bears a similarity to the hull of a steamboat. At the base of the trail leading to the Steamboat Point ledges, which are massive beds of Bighorn Dolomite, the Flathead Sandstone is exposed. In the Flathead, there are two prominent ledges of cross-bedded sandstone approximately seven meters apart. The top ledge could possibly be the Flathead/Gros Ventre contact, abovewhich the slope drastically lessens. The exposure of the Gros Ventre Shale is extremely weathered, covered with vegetation, and not well exposed. Pine trees seem to prefer growing in the sandstone and carbonate beds, but not in the silty/muddy Gros Ventre Formation. This area is grassed over, but no trees are growing. The Gallatin Limestone crops out above the weathered Gros Ventre with the: massive Ordovician Bighorn Dolomite exposed above the Gallatin Limestone. The Gallatin Limestone is a fine-grained limestone with four distinct flat pebble conglomerate beds (each bed is approximately 40 cm thick). Carbonate Kinneyia slabs were found in float near the basal portion of the section. Kinneyia was also found in 83 float from the Flathead Formation, though preserved in sandstone. Unfortunately, neither occurrence could be documented in situ. Figure 39. Steamboat Point (Middle Cambrian Flathead Formation through Ordovician Bighorn Formation). 84 APPENDIX 3 Global Positioning System (GPS) Coordinates for Studied Exposures Burgess Junction #1 (BJ1) N 44° 46’ 10.6” W 107° 31’ 3.5” Elevation 2445 meters North Tongue #1 (NT1) N 44° 47’ 24.3” W 107° 32’ 28.1” Elevation 2444 meters North Tongue #2 (NT2) N 44° 47’ 33.3” W 107° 32’ 15.2” Elevation 2455 meters Passage Resort #1 (PR1) N 44° 45’ 34.5” W 107° 30’ 51.4” Elevation 2479 meters Passage Resort #2 (PR2) N 44° 45’ 48.1” W 107° 30’ 51.2” Elevation 2505 meters Steamboat Point N 44° 48’ 28.07” W 107° 21’ 35.76” Elevation 2246 meters 85 REFERENCES Baldwin, C.T.,1977. lntemal structures of trilobite trace fossils indicative of an open surface furrow origin. Paleogeography, Paleoclimatology, Paleoecology, vol. 21, pp. 273-84. Bergstrcm, J.,1973. Organization, life and sytematics of trilobites. Fossils and Strata, 2:69p. Birkenmajer, K. and Bruton, D.L., 1971. 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