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III-eh} 3.» .1153: .. .2 . 2:34:32. ‘ T 5...... .. a}. .. a... 1...! 35...! , a .X! t .. 35‘ a 5‘!“ S}: 7 'ti . 54‘ is: 2.... >1) .D It 3.31.859. L‘sl. 515..»qu 3......” 3...; ,Ibt... v f...‘ . 5 .- Willi: llll iii} iii llim This is to certify that the thesis entitled BRYOZOAN BIOGEOGRAPHY OF THE UPPER PLATTEVILLE GROUP AND THE LOWER DECORAH SHAIE OF IOWA, MINNESOTA, AND WISCONSIN AND EXAMINATION OF A POSSIBLE EXTINCTION ABOVE THE DEICH K-BENTONITE presented by Gregory J. Wasserman has been accepted towards fulfillment of the requirements for Masters degree in Geological Sciences Wm Major professor Date flcj. 22; /7[Z 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution LIBRARY MICMg-flfl State Unlverslty PLACE ll RETURN BOXto tomcat-IN. Multan your meat]. TO AVOID FINES mm on or Moro dd. duo. DATE DUE DATE DUE DATE DUE BRYOZOAN BIOGEOGRAPHY OF THE UPPER PLATTEVILLE GROUP AND THE LOWER DECORAH SHALE OF IOWA, MINNESOTA, AND WISCONSIN AND EXAMINATION OF A POSSIBLE EXTINCTION ABOVE THE DEICKE K-BENTONITE By Gregory J. Wasserman A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geological Sciences 1994 Mill and Othe; chanl reDla lhe bi ABSTRACT BRYOZOAN BIOGEOGRAPHY OF THE UPPER PLATTEVILLE GROUP AND THE LOWER DECORAH SHALE OF IOWA, MINNESOTA, AND WISCONSIN AND EXAMINATION OF A POSSIBLE EXTINCTION ABOVE THE DEICKE K-BENTONITE By Gregory J. Wasserman The Deicke K-bentonite is a thick, regionally widespread altered volcanic ash bed found throughout eastern North America in middle Ordovician (Caradocian) rocks. It has been suggested by previous authors that this bentonite represents the Blackriveran/l‘rentonian boundary and marks a major regional mass extinction horizon in the Upper Mississippi Valley. To test this hypothesis, samples from the Upper Mississippi Valley were collected and examined for their bryozoan content. Relatively few bryozoans disappeared above the Deicke K—bentonite and those that did go extinct were probably quite rare to begin with and may have gone extinct well before the eruption that extruded the Deicke. If fact, the bryozoans seem to survive not only the Deicke event, but the event which extruded the Millbrig K-bentonite, another widespread bed found in eastern North America and Baltica and that may represent the largest volcanic event during the Phanerozoic, and possibly two other volcanic events as well. Rather, the "extinction" seems to be due to a major biofacies change at the Blackfiveran/I‘rentonian boundary where shallow carbonate deposition is replaced by the deposition of siliciclastics. In fact. several bryozoan species disappear at the boundary only to reappear when carbonate deposition resumes. ac W1 su, nuts; mucl Whic; ACKNOWLEDGEMENTS First and foremost, I would like to thank Dr. Robert L. Anstey for all his help and advice while guiding me through the project, without which I would have never finished. I would also like to thank Dr. Duncan F. Sibley and Dr. Ralph A. Taggart for their helpful suggestions and for forcing me to analyse the project more critically. They always had an open door and were willing to listen to me. I am also grateful for the help Dr. Danita Brandt gave me in the field of taphonomy, a subject I knew little about before I started my thesis. I must also acknowledge the help Dr. Stig Bergstrbm and especially Steve Leslie gave me while I was writing out my rough drafts. They were often able to point out a reference to help answer any quenstions I may have had while refining my initial drafts. I must also thank Tom Weaver for allowing me to stay with him for a few months when I ran out of departmental support (actually, I was there for about two and a half months too long; I have to also thank his wife Dawn for putting up with me as I probably drove her nutsl). Finally, I am eternally indebted to my parents and grandparents for not only giving much needed financial support but for patience, understanding, and encouragement without which I couldn't have finished. Abs Aclt Intrt Resr Geo] Litht Bios Mate TABLE OF CONTENTS Page Abstract ................................................... ii Acknowledgments ............................................ iii Introduction ................................................ 1 Research History of Ordovician K-bentonites .......................... 3 Geologic Setting and History ..................................... S Lithostratigraphy ............................................. 1 1 Biostratigraphy .............................................. 29 MaterialsandMethods 34 Taphonomy ................................................ 38 Discussion ................................................. 43 Conclusions ................................................ 58 Appendix 1: Bryozoan Systematics ................................ 61 Appendix 2: Bryozoans of the Platteville Group ........................ 63 Appendix 3: Bryozoans of the Decorah Group ......................... 64 Appendix 4: Field Localities ..................................... 66 Appendix 5: Bryozoan Identification Key ............................ 67 Appendix 6: Measured Sections ................................... 75 Appendix 7: Other Localities where species have been reported .............. 81 Bibliography ................................................ 94 iv Figure \OOOQOtUtbbJN NHb-‘I-fit—il—II—th—IIII—Ir—it-i OOOOQO‘UI'AUJNHO LIST OF FIGURES Page Ordovician time scale ................................ 6 Position of tectonic plates during the Caradoc, 454 Ma ......... 9 Classification of rocks in the study area .................... 12 Carbonate mudstone of the Platteville Group ................ 13 Bryozoans present in lithologies of study area ............... 15 Altered layer from the Platteville Group .................... 16 Planar dolomite from the Platteville Group .................. 17 Stylolites in the Platteville Group ........................ 19 Polished sample from the Platteville Group ................. 21 The Decorah Shale ................................. 22 Distribution of Ordovician K—bentonites ................... 24 The Deicke K-bentonite from the St. Paul section ............. 25 The Millbrig K-bentonite from the St. Paul section ............ 27 Biostratigraphic zones ............................... 30 Echinoderms found in Platteville and post-Platteville rocks ....... 31 Field localities .................................... 35 Burrows in the Decorah Shale .......................... 42 First appearance of genera ............................ 52 Cluster analysis ................................... 54 Bryozoan generic ranges ............................. 56 Du record causes (Alva others scale, CUTIE! I. Introduction During the Phanerozoic, at least five major mass extinctions are recognized in the fossil record (reviewed by Raup and Sepkoski, 1982). Much current research deals with the causes and effects of these extinctions. Many hypotheses, including extraterrestrial impacts (Alvarez et al., 1980), sea level changes, and climatic changes (Brenchley, 1989), among others, have been postulated to explain these massive diversity declines. On a smaller scale, many minor mass extinctions have also been proposed and are the focus of much current research. One such event has been proposed to be contemporaneous with the Deicke K—bentonite, a thick, geographically widespread altered volcanic ash bed found in Ordovician aged rocks in Eastern North America (Sloan, 1987, 1992). Sloan (1987, 1992), examining rocks in the Upper Mississippi Valley, suggests that the observed diversity drop, which is highest at the species level, averaging 65% (and much lower at the generic level), was caused by the Deicke ash fall. He also stated (1987) parallel "extinction" events may have occurred at other ash fall horizons during this time interval, but that no extinction can be recognized above the Millbrig K-bentonite (1992). Tuckey and Anstey (1992) report a global decline in bryozoan diversity near this time interval as well, and DeMott (1987) records a major decline in trilobite diversity. Other groups also seem to have suffered major declines in overall diversity, including brachiopods (Rice, 1987), gastropods (Sloan, 1987), and echinoderrns (Kolata er al., 1987). However, the decline is variable for different taxa. For example, conodont decline is only 10%, while decline among echinoderrns was nearly 100%. Sloan (1987) also states that there is no evidence of any mobile benthic organisms burrowing out of the Deicke after examining over 100 exposures (1987). Sloan proposes that the affected organisms that disappeared were those with geographic ranges within the area of the ash fall. However, recent research casts doubt on whether large scale ash falls, such as the Deicke event, can cause mass extinctions. Working with huge Neogene eruptions, Erwin and Vogel (1992) found 1 DOE Sim geoc bent berm Nort Milli fall. Trent “PW! over]; of {at globa Decor had or divers Ofigim found 2 no evidence of any mass extinctions occurring at any of the largest ash fall horizons. Similarly, Huff er a1. (1992) found no major extinctions in microscopic and megascopic organisms across the "Big Bentonite" of Baltoscandia. They presented detailed geochemical evidence that indicates the ”Big Bentonite" is equivalent to the Millbrig K- bentonite in eastern North America, another thick, geographically extensive Ordovician bentonite found above the Deicke. They state that, while the Blackriveran faunas of eastern North America are replaced by Trentonian faunas in the interval between the Deicke and the Millbrig, there is no evidence to indicate that the changeover was caused by the Millbrig ash fall. The authors attributed this shift to a major environmental change during the Trentonian transgression. A major facies change does occur just above the Deicke in the upper Mississippi Valley (Sloan's study area), whereby the Platteville Formation is overlain by the Decorah Shale. The purpose of this study is to investigate the local causes of faunal change by examining the change in bryozoans, one of the groups experiencing global extinction at this time. The taphonomy of both the Platteville Formation and the Decorah Shale will be examined to determine the effects taphonomic processes may have had on biasing the fossil record in these strata and influencing the reality of observed diversity changes that may have occurred. The zoogeography of bryozoan generic originations will also be examined to determine the possible migration pattern of the fauna found in the field area. Kent! was e early clay l geoch He ah useful bentm (fl/em. II. Research History of Ordovician K-bentonites The first K-bentonite was described by Ulrich (1888) at a section in High Bridge, Kentucky. However, he did not recognize it as a bentonite, calling it a thick clay bed. It was either the Deicke or the Millbrig. It was not until more than three decades later, in the early 1920's, that the bentonites were recognized as such when Nelson (1921) described a clay bed in the Carters Formation in Tennessee that was 2 feet thick. He found it to be geochemically identical to Cretaceous and Tertiary bentonites in western North America. He also identified similar beds in Alabama and Kentucky, noting that they were potentially useful for correlation. Nelson (1922a,b) compiled further information on Ordovician bentonites in Tennessee and Kentucky and believed these bentonites would be useful in event-stratigraphic correlation. The bentonite he was working on was the Deicke. Sardeson (1924) reported bentonites in the lower part of the Decorah Shale of Minnesota and Wisconsin. He attempted to correlate them with Nelson's bentonite in Tennessee and Kentucky. Nelson (1925, 1926) published more information on two K—bentonites in Tennessee and Kentucky. Butts (1926) discovered six bentonite beds in Alabama, making correlation extremely difficult because no geochemical fingerprinting criteria were yet available to separate them. The term "metabentonite" was proposed by Ross (1928) who also showed that the chemistry of the different bentonites was variable. Sardeson (1928) followed up his earlier research by recording two bentonites in the Decorah of Minnesota as well as a questionable one at the top of the St. Peter Sandstone. The first regional correlation of a bentonite bed was proposed by Kay (1931), who described a Blackriveran aged bed in New York and Ontario and correlated it with one in the Decorah in the Upper Mississippi Valley. He also examined the occurrences of other bentonites in Missouri, Kentucky, and Tennessee and found their thicknesses to increase southward, perhaps indicating proximity to the source volcano. Later, Kay (1935) reviewed the distribution and occurrence of Ordovician bentonites throughout North America and emphasized their 3 inf Ha her bed ben 0rd bent Hufl Mill soutl Ordo oldes cone 4 usefulness in regional correlation. It was not until 26 years later that isotopic dating information became available in bentonite research (Adams et al., 1960). Mossler and Hays (1966) presented the most detailed study of the mineralogy and stratigraphy of eight bentonites in the Decorah and Galena formations, presenting supporting evidence that these beds were indeed volcanic ash beds. Nearly two decades followed until the next leap in bentonite research occurred when Huff (1983) used chemical fingerprinting to correlate Ordovician bentonites. Kolata er al. (1986) were able to trace several Middle Ordovician bentonites in the Upper Mississippi Valley for about 900 km using chemical fingerprinting. Huff and Kolata (1990) used chemical fingerprinting to correlate the Deicke and the Millbrig from the Upper Mississippi Valley to beds in Kentucky, Tennessee, and the southern Appalachians. It is important to note that the Deicke is not the oldest mid- Ordovician bentonite, as there are numerous other bentonites below it. It is however, the oldest major bentonite in the Ordovician of eastern North America and the first to be correlated over major geographic distances. dunn; an ab: millic these years, stages Black Shale Millbr SOmet Black] fauna] Overly 130mm tlentOn de‘v'elo 1989, DOSLde erTor in Was an; Pmona and incr ZirconS an Ofdo III. Geologic Setting and History The Platteville Formation and the Decorah Shale/Spechts Ferry Shale were deposited during the Caradocian, the second to the last series during the Ordovician (Fig. 1). Using an absolute time scale estimated by Sloan (1987), the Caradoc lasted approximately 18.6 million years, from 464.3 Ma to 445.7 Ma. (Fig. 1). Using the North American system, these units were deposited during the Mohawkian, a time period that lasted about 14 million years, from 463.5 Ma to 449.5 Ma. (Fig. l). The Mohawkian is further divided into three ' H:i—_.-- stages, two of which are important to this study. The Platteville was deposited during the Blackriveran Stage according to Sloan (1987), while the Decorah Shale/Spechts Ferry 3 Shale was deposited during the Trentonian Stage. This would put both the Deicke and the Millbrig among Trentonian aged rocks, though in some older literature, the Deicke is sometimes put into the Blackriveran. Sloan feels the Deicke marks the Blackriveran/Trentonian boundary, which he puts at 454.2213 Ma., based upon a major faunal changeover. However, more recent data from Huff er al. (1992) put the age of the overlying Millbrig at 454 Ma. This paradox arises from the error involved in the techniques used. The error in each of the samples overlaps with the other, so both bentonites must be considered to be about the same age until more precise techniques are developed (a more recent published date puts the Deicke at 457 . lil Ma by Samson er al., 1989. However, Bergstrom [personal comm.] states that unpublished material by Tucker post-dates Samson et al.'s publication and puts the Deicke at 454 to 455 Ma). Much of the error involved with the older bulk sample techniques (where the entire bentonite sample was analyzed) was attributed to the presence of Precambrian aged zircons (Bergstrbm, personal comm.), perhaps derived from older material within the magma chamber walls and incorporated into the sample during the eruption. Newer techniques using single zircons have proven to be more accurate, though some zircons have a Precambrian core and an Ordovician exterior (according to Bergstrbm [personal comm.] this newer technique 5 ”VJ 438 504 An Ordovician Time Scale 438 United Kingdom United States r—Gamachian 0.5 My_ a Ashgillian 7.7 My Richmon‘aigr‘r‘ 3.9 My 3 E Maysvrnrafi 3.3 My 3 '4 445.7 445. 7 E = Edenian 3. 8 My 0 449. S :: Trentonian 4. 7 My .3 >4 Caradocian 18.6 My Tracks??? 0 My 3 E 5 ~— 4582—— g 4 N Ashbyan 5.3 My g F' 3; 464.3 463.5 g Llandeilian 4.5 My Chazyan 7.0 My 5 '3 468.8 470.5 252 El; Llanvirnian 9.5 My go! a c 478.3 E" E3 Areni 'an 12.2 M 433. 7 g] y Cassigisaknf. .7 My § 490.5 “Emilio“? 1 M’ g g Demingian 7.7 My 2 E Tremadocian 13.5 My 498.2 6 'E Gasconadian 5.8 My g 504 Figure l-Ordovician Time scale modified after Sloan (1987). 7 appears to be very promising, as the age of the Deicke may soon be distinguishable from the Millbrig, with an error of less than one million years). The Platteville in Minnesota (along with the Decorah Shale) was deposited in the Keweenawan rift valley/Midcontinent Geophysical Anomaly (Sloan, 1987). Deposition of both units was fairly continuous, albeit very slow, as no major regional unconforrnities exist within them. However, in Illinois, there is a break between the two, as the area was located at a high on the Wisconsin Arch. This produced minor unconforrnities due to minor sea level changes and pulses in uplift in the Great Lakes Tectonic Zone (Kolata et al., 1986; Sloan, 1987 ). Deposition of the Deicke occurred in subtidal to supratidal environments (Huff and Kolata, 1990). The extrusion of the Deicke coincides roughly with the beginning of the Taconic Orogeny (Huff and Kolata, 1990). At this point, the Transcontinental Arch in Minnesota was already well developed, and the Taconic Highlands on the East Coast began to form. This coincides with a well documented Trentonian transgression (Huff er al., 1992). Sloan (1987) states the first effects of this transgression were felt during the deposition of the marine St Peter Sandstone (marine reworking of Cambrian sandstones), a unit that was deposited over a karst surface developed "during the long Whiterockian unconformity". However, Witzke and Kolata (1988) detailed evidence suggesting the late Blackriveran was undergoing a regression in the Upper Mississippi Valley (see "I..ithostratigraphy") with the transgression starting at the Blackriverl'l‘renton boundary. Huff er al. ( 1992) and Leslie and Bergstrbm ( 1993) support this hypothesis, the latter using conodont biofacies. The highlands began to shed fine grained siliciclastics. which became the Decorah Shale/Spechts Ferry Shale, as well as shale units to the east, into the epicontinental sea (Huff and Kolata, 1990; Kolata er al., 1987). The highlands that shed the clay were formed during a major pulse in uplift. According to Droste and Shaver (1983), the Platteville was deposited in fairly shallow water. The Decorah Shale/Spechts Ferry Shale were deposited in deeper water than the Platteville due to a rise in sea level caused by the Trentonian transgression. Cisne er al. (1984) water I (Ieslie Shaver Bentor bentor al., 19 subset other 1 Amen beds i fossils randol and cc Du and £11 331 to: 8 (1984), however, presented evidence that the Decorah Shale was deposited in very shallow water (only a meter or so in depth). More recent studies using conodont biostratigraphy (Leslie and Bergstrom, 1993) are in agreement with the interpretations of Droste and Shaver (1983). Soon after the onset of Decorah deposition, the Millbrig (and ”Big Bentonite" [Huff er al., 1992]) was erupted and deposited in eastern North America (a thin bentonite has been discovered between the Deicke and Millbrig, as seen in Fig. 1 of Huff er al., 1992, but it has not been fully described yet). Numerous other bentonites were subsequently extruded, including the Elkport, Dickeyville, Calmer, Conover, and many other named and unnamed bentonites. At least 50 bentonites have been identified in North American Ordovician rocks (Bergstrbm, 1989), some of which might also correlate with beds in Baltoscandia. Based upon information gathered from acetate peels, most of the fossils in the upper Platteville appear to have been significantly reworked as they are randomly oriented, broken, and abraded. The carbonates are light colored, fine-grained, and contain little tenigenous mud or other siliciclastic material. During the Caradocian, the North American continent was in a tropical latitude (Fig. 2), and the Upper Mississippi Valley in particular (Scotese and McKerrow, 1991). Baltoscandia. on the other hand, was in a more temperate latitude (Fig. 2). However, the two continents appear to have been moving toward each other at this time, slowly closing the Iapetus Ocean. Also note that both plates extend to 30° S, and parts are at 15" S, the approximate location of the Upper Mississippi Valley. The study area, in southeastern Minnesota, was in proximity to the shoreline of the North American epicontinental sea during the Caradocian (Cisne er al., 1984). According to Sloan (1987), Decorah sedimentation was more rapid at St. Paul than near the Iowa- Minnesota state line. Sloan infers that the sediments were deposited 40% faster at St. Paul than at the Iowa-Minnesota state line because the Decorah thins from St. Paul to the south. The thinning trend may indicate St. Paul's proximity to the source of the Decorah relative to the rest of the field area. The Decorah sediments were eroded from exposed Proterozoic Baltoscandia Avalonia Figure 2-Position of tectonic plates during the eruption of the Deicke K-bentonite, 454 Ma. Modified from the program Terra Mobilis. shales in « interval b This may Wisconsil during the reworked 10 shales in central Minnesota (Sloan, 1987), probably Animikian sediments. Also, the interval between the Deicke and the Millbrig decreases to the south (Kolata et al., 1986). This may be due to a local high in topography associated with the southwestern flank of the Wisconsin Dome. Finally, there appears to be at least some erosion of Platteville rocks during the deposition of the Decorah Shale based upon the presence of weathered and reworked conodonts (Sloan, 1987). Grou'l Deco Valle regre. eVidc Arkal equiv repl‘e: IV. Lithostratigraphy The two major rock-stratigraphic units that are the focus of this study are the Platteville Group and the Decorah Shale. The Spechts Ferry Formation is the basal formation of the Decorah Subgroup in Iowa and Wisconsin (though in some literature, it is called the Decorah instead of the Spechts Ferry) (Fig. 3). The Deicke lies within the Carimona Member, a carbonate unit that makes up to the first six feet of the Decorah Shale, though at some localities it is absent. The Millbrig K-bentonite is found within the shalier portion of the Decorah Shale. The Platteville Formation is a thick sequence of clean carbonates. The upper Platteville is primarily a fine grained micrite with sparse fossils (Fig. 4) based upon acetate peels. Witzke (1987 ) reports the presence of hardgrounds in the eastern Iowa subsurface of this formation. This often is an indication of slow sedimentation at shallow depths (Palmer et al., 1988). It appears that the rate of deposition of the Platteville was inferred to be uniform throughout the field area (Sloan, 1987). According to Witzke and Kolata (1988), the upper Platteville was deposited in quite shallow water during a late Blackriveran regression based on the presence of "unfossiliferous mudflat facies" found in the Quimbys Mill Formation (upper Platteville) of Wisconsin (they also state that the lower Platteville was deposited in deeper water). They also cite the presence of a hardground (or ferruginized surface) at the top of the Platteville throughout much of the Upper Mississippi Valley and local erosion at the top of the Platteville at some localities. They feel that this regression is found throughout the eastern portion of North America citing several lines of evidence, including an unconfonnity at the Platteville equivalent Plattin Formation in Arkansas (Craig, 1988), birdseye carbonate fabrics in the Bromide Formation (also equivalent to the Platteville) capped by an unconformity in Oklahoma (Amsden and Barrick, 1988; Sweet, 1984), and geographically extensive supra- and intertidal deposition representing probable Platteville equivalents in Kentucky (Cressman, 1973), Ohio (Stith, 11 a l SOUTHWESTERN MINNESOTA NORTl-IEASTERN IOWA NORTHWESTERNMOST IOWA SOUTHWESTERN WISCONSIN 3 DUNLEITHFM. R. KIRKFIELDIAN KIMMSWICKSGP IONMB BEE” 35 3 w 333% @353 m. m“ g E c: m [5 g rum 2% §§U§ Ww‘mm §§ cmuom g“ a m ° “Rm... Wm QUIMBYS MILL FM- smcrrrs FERRY GUTI‘ENBURG m 32 5? BLACKRIVERAN PLA'I'I'EVILLE GP. PLATI'IN SUBGROUP BLACKRIVERAN PLATTEVILLE GP. MCGRBGOR FM. Figure 3-Classification of rocks in study area. The Ion Member is restricted to Iowa. r3 Figure 4—Carbonate mudstone of the Platteville Group. Magnification is 40X. 14 1979), Indiana (Droste and Shaver, 1983), and New York (Walker, 1973). The most common bryozoans found in acetate peels from rocks collected in the upper Platteville were the cryptostomes Srictopora (by far the most common bryozoan genus identified) and Escharopora. Only five other genera were identified from this interval, including the cryptostomes Sricroporella, Uln'chosrylus, and Arthroclema, and the trepostomes Bimuropora and Monorrypella (Fig. 5), though these are much rarer than Stictopora or Escharopora. Two of the genera, Srictopora and Escharopora, Ross (1970) found to be "most abundant in quiet shallow water bank facies" of middle Ordovician rocks of New York, with the genus Stictoporella being a "subdominant" form in this biofacies. However, as stated previously, it is quite possible that these rocks were deposited in much deeper water (curiously, Ross [1970] found that Mononypella was found as a subdominant genus in deeper water facies). The fossils in the acetate peels are usually very small and broken, indicating much reworking. According to Dunham's classification, most of the unit would be classified as a mudstone, though there are a few wackestones. There are, however, many very thin (usually >2 cm thick) lenses of somewhat finer grained, somewhat clay/silt rich (compared to the rest of the unit), brown (possibly oxidized iron minerals), altered material (Fig. 6). These thin lenses are more fossiliferous than the surrounding carbonate mudstone and could be classified as a packstone at the microscopic level. There are some thicker lenses that can be identified in hand sample composed of shelly material, though these appear more common in lower Platteville rocks (these layers could be classified as packstones and grainstones). These fossil accumulations may represent the distal portions of a storm deposit or may be formed by directional currents. The fossil material is usually very broken up in the carbonate mudstone. Dolomite is a common diagenetic constituent in the Platteville (Fig. 7) and is abundant in the altered layers. The dolomite can be found in both planar and non-planar forms, and both forms can be found in the same sample. Also, when planar dolomite is present, two distinct size classes may be present as well. At certain localities, dolomite is restricted to certain layers Ta Strctopo Stiaopo n Sricropo Stictopo ra I Escha no; Stictopo r AUhfocle U1 lichojr Monorryp Bi’flflropo ra Helopo ; Diplotry‘ A‘h’Oph ra_ Efidolm Here mm MONK-C1411; Ptilodicr Anhrorr). ”0010")? To ””10th S’iC‘OPOreI % Flgllfe S‘Br 15 Taxa Lower Platteville Mudetonee- Grainstones Upper Plattevllle Mudstones- Wackeetones Decorah Shale Packstones- Gratnstones Galena Group Dolomite- Qolomlhc Mudeton Stictopora sp. Srictopora exigua Sricropora lira Stictopora paupera? Escharopom sp. Stictoporella sp. Arthroclema sp. Ulrichostylus sp. Monotrypella sp. Bimuropora winchellr' Helopora 3p. Diplotrypa sp. Athrophragma :p. Eridonypa sp. Heterotrypa sp. Monticulr'pora 3p. Ptilodictya sp. Arthrosrylu: sp. Homorrypa sp. Tarphophragma sp. Stictoporellina sp. Prasopora 5p. X X X fi-QXXXX‘QXXXXXX X X X XXXXXX X X X QXXXXXXX XXXQ X XQXXXXX X 9% Figure 5-Bryozoans present in lithologies of study area. '31' :. -.; ’3" : . 'Q'.‘ .‘-:.::‘ao‘!hb— L , ( v - / 4,” I, oz.” ' ‘i ' V..“ '41:; ‘ u'f’ a" Figure 6-Altered layer from the Platteville Group. Magnification is 40X. The planar allochem in the lower center is a brachiopod as are the other identifiable allochems. Figure 7 -Planar dolomite from the Platteville Group. Magnification is 40X. The allochems have been replaced by coarser dolomite than the matrix. l8 (Badiozamani, 1973). Saddle, or baroque, dolomite is rarely present and was identified in only a few samples. This type of dolomite is formed from hot (60-150’ C) circulating fluids (Radke and Mathis, 1980). Individual (planar) dolomite crystals can often be found scattered within the thin altered lenses in nearly all samples, even in the cleanest carbonate layers where no other dolomite is present, though the layers are occasionally completely altered to dolomite. In addition to these layers, dolomite can also be found along existing fractures in the rock. or can pervasively invade the rock based on information from both polished samples and acetate peels. Pyrite is a common diagenetic mineral as well. Pyrite crystals are usually very small (less then 1 mm3, although some may be quite large, a few exceeding 1 cm3) and are usually identifiable in hand sample (as pyrite will not stick to an acetate peel). Pyrite may also act as a replacement material for fossils. Silica often replaces original skeletal material in certain horizons. There are two likely sources for the silica Some silica may have been derived from the dissolution alteration of original silica in skeletal material (Boggs, 1987), while other silica may have been derived from the bentonites themselves as observed in fossils found immediately below the Deicke K- bentonite, exposed in the upper Tyrone Limestone of Kentucky. The allochems most frequently replaced, based upon information from acetate peels, are echinoderrn fragments and the less common small rugose corals. There are a few Stylolites (Fig. 8) but they are uncommon throughout the sampled portion of the unit. It should be pointed out that much of what is called a "layer" is nothing more than a well lithified micrite bounded by two thin, brownish altered deposits. These layers may have acted as zones of focused flow due to higher abundance of fossil material (i.e. there was probably more porosity in these zones due to a greater amount of imbricated fossil material). This probably accounts for the material being altered as chemically charged circulating fluids may carry replacement minerals and oxygen. Similarly, circulating fluids may also lead to the dissolution of some carbonate material. Much of the fossil material in these layers is highly altered as well and difficult to identify in the acetate peels. The Platteville as a whole is visibly blotchy on a lI-(I ZIII'LO in!) .L.‘ |-2_-1 l9 Figure 8-Stylolites in carbonate mudstone of the Platteville Group. Magnification is 40X. The bryozoan in the upper right is the arthrostylid Ulrichostylus. 20 polished surface (Fig. 9) and this texture may represent bioturbation. In addition, there are often rarnifying "fingers" of the oxidized layers found within the less fossiliferous (though more extensive) micrite portions of this unit, perhaps caused by burrowing organisms. The Decorah Shale, the basal unit in the thick Galena Group, on the other hand, represents a marked contrast to the clean limestones of the Platteville Group. The mineralogy of the Decorah is predominately illite, with small amounts of montrnorillonite and kaolinite (W illman and Kolata, 197 8). These siliciclastics lie conformably on the Platteville in the northern part of the research area. However, in the southern part of the research area, an unconforrnity separates the Quimbys Mill Formation of the Platteville and the Spechts Ferry Formation. This hiatus in deposition is inferred to have lasted approximately 0.8 million years (Sloan, 1987 ) based upon missing conodont zones. The Decorah Shale increases in thickness to the northwest, possibly reflecting a source in the Transcontinental Arch (Witzke, 1980). Witzke and Kolata (1988), state that as the Trentonian transgression continued, the siliciclastic influx was choked, permitting carbonate deposition over the Decorah. Sloan (1987) estimates that the Decorah Shale was deposited 40% faster in St. Paul, Minnesota, than in Fillmore County (near the Iowa state line) based upon the differences in thickness (the rest of the Galena group, Sloan states, was deposited more or less at the same rate). It is composed of brownish to greenish-gray or olive-green shale (Karklins, 1987), interbedded with thin limy shell lag material. The unit is richly fossiliferous based upon field observations. The shale is fissile and blocky (Karklins, 1987). The limy interbeds collected are composed almost entirely of fossils (Fig. 10). Karklins (1987) reports that they are locally discontinuous. The fossils in the samples are often broken, and bivalved shells are usually disarticulated, many being stacked upon one another. These interbeds may be tempestites or caused by directional currents. The Spechts Ferry Formation (the bottom 10 feet or so of the Galena Group in Iowa) is the basal unit of the Decorah Subgroup in Iowa and Wisconsin. According to Karklins (1987 ), it is dark green to green-gray and often pyritic. It is interbedded with fine 21 Figure 9-Polished sample from the Platteville Group. Note blotchy texture. 22 Figure 10-The Decorah Shale. Magnification is 25X. The bryozoan is Sticropora, while the other allochems are brachiopods. Note sparite cement. 23 to medium grained argillaceous limestone. The Carimona Member is the lowest unit of the Decorah in parts of the field area. Based upon information from acetate peels, this carbonate unit generally contains more fossil debris than underlying upper Platteville rocks. It is also coarser grained than underlying Platteville rocks, being wackestones to packstones (though there are some mudstones), using information from point count data. The Deicke K-bentonite is one of 50 bentonites documented in eastern North America (Bergstrbm, 1989) and is also one of the thickest and most extensive. It can be traced from 3 Minnesota to Alabama, and is also found in New York (Fig. 11). Using geochemical -. fingerprinting, Huff and Kolata (1990) determined that the T-3, or Pencil Cave, K- bentonite in southeastern North America (to Alabama), is equivalent to the Deicke. In the ‘3 study area, it is located in the Carimona member of the Decorah Shale (Fig. 2). The Deicke E is thickest in southeastern North America (Huff and Kolata, 1990), and thins towards Minnesota. However, the Deicke in the study area is thinner in Iowa than it is in central Minnesota (a few cm vs. >10 cm). This is probably due to a local topographic high and subsequent erosion of the Deicke, as the Deicke is missing at Guttenburg, Iowa. Compared with the thickness of the Deicke to the south, where it is up to 2 m thick, it is much thinner in the study area. In exposures visited for this study, the Deicke weathers to a pinkish color. Fresh samples are usually a gray-white color. It usually weathers back to form an re-entrant in an exposed rockface as it is very soft and is easily eroded (compared to the bounding limestones) (Fig. 12). Mineralogically, the Deicke is poor in biotite and quartz and contains labradorite (Ab45.50), authigenic albite, K-feldspar, and Fe-Ti oxides (Huff er al., 1992). The absolute age of the Deicke is still debated. Sloan (1987) reports it as 454 Ma, using previously determined radiometric data (Kunk and Sutter, 1984; Kolata er al., 1986; Samson, 1986). More recent evidence, however, puts the age of the Millbrig/"Big Bentonite" at 454 Ma (Huff er al., 1992). Until newer evidence is published, both the Deicke and the Millbrig are considered to be within the same age bracket (as the error involved with chemical fingerprinting overlaps), though the Deicke must be 24 Figure 11-Distribution of Ordovician K-bentonites (modified after Kolata er al. 1986). 25 Figure 12-The Deicke K-bentonite from the St. Paul section. The Deicke is located at the lowest re-entrant, best seen in the lower left of the photograph. 26 considered to be slightly older. It appears that the four major bentonites exposed in the Upper Mississippi Valley were erupted within one million years of one another (Sloan, 1987), so this overlap is acceptable (also, the most recent evidence, though unpublished, puts the Deicke at 454 to 455 Ma). The volume of the Deicke in eastern North America is estimated at 330 km3 (Sloan, 1987). However, Dokken (1987) estimated that the Deicke's original thickness was was compacted 3.4 times its present thickness based on the deformed trace fossils Chondrites and Planolr'tes compacted as a result of overburden. Thus, the pre-compaction volume of the Deicke is as much as 1122 km3 (Sloan, 1992, states that the amount of ash extruded was about 1083 mm. As yet, the Deicke has not been traced to the Baltoscandian plate. Bergstrom (1989) had suggested that a thick bentonite under the "Big Bentonite” might be a likely candidate to correlate with the Deicke, but later stated (personal comm.) that no equivalent could be established through chemical fingerprinting as yet. This suggests a different source than the Millbrig (see below). If the Deicke is ever correlated to Baltoscandia, a considerable amount of volume could be added, potentially making it the largest volcanic event in the Phanerozoic. The Millbrig K-bentonite is regionally as extensive as the Deicke in eastern North America. Huff et al. (1992) have determined that the "Big Bentonite" of Baltoscandia is equivalent to the Millbrig. In North America, the Millbrig is equivalent to the T4, or Mud Cave, K-bentonite in southeastern North America. In the study area, it is usually found in the lowest 3 feet of the Decorah Shale (though this can be quite variable). To locate it, one must trench into the shale, as the shale usually weathers and covers the bentonite (Fig. 13). It is usually whitish-grey in color. When exposed, it usually weathers in the same manner as the Deicke. It is thinner than the Deicke in the study area, but locally can be thicker, especially in southeastern North America. In a similar thickness pattern to the Deicke, it thickens to the south and east. Mineralogically, the Millbrig is rich in biotite and quartz, has a small amount of andesine (Ab55-6o), and contains authigenic albite and K-feldspar (Huff er al., 1992). Huff er al. (1992) have determined that the Millbrig is about 454 Ma 27 Figure 13-The Millbrig K-bentonite from the St. Paul section. 28 using U-Pb ages from zircons and 40Ar/39Ar dating of biotite and sanidine. The volume of the Millbrig/"Big Bentonite" complex has been estimated at 340 km3 dense rock equivalent of ash (Huff er al., 1992). If the amount that may have fallen into the Iapetus Ocean is added to this figure, an additional 800 km3 can be added, for a total of 1140 km3 dense rock equivalent of ash. This would make it one of the largest eruptions in the Phanerozoic, larger than even the massive Toba eruption (only 800 km3 dense rock equivalent). No mass kill beds were observed below either of the bentonites at any of the field localities nor F were any observed below either bentonite at localities in Kentucky, where both bentonites are more than twice as thick as those exposed in the study area. Both bentonites were deposited instantaneously in a geologic sense. Sloan (1992), citing in press data from Haynes, suggests that the Millbrig was extruded in at least four eruptions, though if Bergstrbm (personal comm.) suggests that it was erupted continuously, though in pulses over several days. However, the effects on the biota, if any, should be the same. Just where the source area was for these eruptions is still in question. Huff and Kolata (1990) believe the source of the Deicke was somewhere southeast of the Georgia-Tennessee state line. Samson er al. (1989), using geochemical methods, felt the source was in either the United Kingdom or New Brunswick. As for the Millbrig, Huff et al. (1992) believe the source to be somewhere in the closing Iapetus Ocean between the North American and Baltoscandian plates (Fig. 2). They base this conclusion upon the felsic character and thickness of the bentonite as well as isopachous maps of the thickness of the Millbrig in North America and the "Big Bentonite" in Baltoscandia. V. Biostratigraphy The crux of the Deicke ”extinction" problem lies with interpretation of the biostratigraphy. Several authors have noted a major faunal change at the Platteville/Decorah boundary. Regionally, the Deicke and Millbrig/"Big Bentonite" lie within the lower part of the Phragmodus undatus Midcontinent conodont zone and the Climatograptus bicornis graptolite zone in North America (Huff er al., 1992) (Fig. 14). P They are also in the upper part of the Amorphognarhus rvaerensis Atlantic conodont zone and the upper part of the Diplograpars multidens graptolite zone. Along with chronostratigraphic age dates of the bentonites (see "Lithostratigraphy"), biostratigraphic techniques, specifically based on conodonts, allow fairly precise correlation of [‘r“ contemporaneous units. According to Sloan (1987), Sardeson (1926) was the first author to note a faunal change above the Deicke. DeMott (1987) noted that 9 of 10 trilobites disappeared above the Deicke, for a 90% faunal loss. Only 10% of the conodonts disappeared (2 of 21), while 7 of 18 brachiopods disappeared (Rice, 1987). Sloan (1987 ) noted a diversity decline of 80% in gastropods (48 of 60 species). Citing Kolata er al. (1987), Sloan (1987) states that local faunal loss among echinoderms was 100%. However, a review of the data (Sloan, 1987, p. 180-181) will show that this is not the case (Fig. 15). Some echinoderrns cross the boundary and several that disappear at the horizon reappear higher in the section. As yet, no evidence has been compiled for bryozoans. Karklins (1987) reports that cryptostome bryozoans of the Platteville in Minnesota have not been recently studied, though Bork and Perry (1967, 1968a,b) compiled much data on bryozoans at this horizon (though working only on the Mifflin and Quimbys Mill Formations in the Platteville as well as the Guttenburg and Ion Formations in the Galena Group) two decades previously, working exclusively with trepostome bryozoans (no cryptostomes were examined in their report). Tuckey and Anstey (1992) have noted a global decline in bryozoan diversity at 29 30 Midcontinent North American Baltoscandic Belodina compressa Conodont Graptolite Graptolite K-bentonites Zones Zones Zones Climacograpnrs Phragmodus americanus _______ Di lo r tus undatus p 3 ap Climacograptus mu ltidens \\\\\\\\\\\ bicornr's \ Millbrig Deicke Figure l4-Biostatigraphic zones that the Deicke and Millbrig K-bentonites fall within. Based on Huff er al. (1992). A-Z'Ml- _.._1 _. 31 Taxa Platteville Decorah Post-Decorah Crinoidea Abludo lygtocrinus char tom Cincinnaticrinid — Cupulocrinus jewettr Porocn'nus , —_ pentagomus Cyclocystoidea C clo stides ysp. 22$ C. halli Cystoidea Glyptocystites sp. -—-— Figure 15: Echinoderms found in Platteville and post- Platteville rocks. Modified from Kolata et al., 1987. 32 about this time interval. Cisne er al. (1984) has noted a lateral facies relationship in which bryozoans are found in greater abundance in "onshore" siliciclastic units. There seems to be much controversy as to the timing of faunal repopulation into the region after the decline. Sloan (1987, 1992) maintains that the repopulation after the Deicke event was gradual, while Huff er al. (1992) believe the repopulation was nearly instantaneous after the ash fall. The uppermost Platteville limestone is a very fossil poor unit based upon data gathered fi'om acetate peels. However, echinoderrn fragments, mostly in the form of crinoid buttons, seem to be the most plentiful, followed by brachiopod fragments (however, brachiopods seem to be somewhat more plentiful in the altered layers). Other phyla are present in much lower numbers, including bryozoans, cephalopods, rugose coral, tabulate corals, mollusks (which are almost always replaced by spar), trilobites, conodonts (though they are extremely difficult to find in an acetate peel), algae, and peculiar conical fragments which may represent several animals, though the most likely candidates are annelid worm skeletons or Comulites (which have been reported from this interval). Occasionally, crinoid fragments and rugose corals are silicified. The reason for this is unclear, but may have something to do with the microstructure of the skeleton. Most of the fossils are concentrated in thin, brown lenses. These lenses are occasionally reworked when they are very thin and often disturbed by bioturbation regardless of thickness. Escape burrows, interpreted in this study as those burrows which cut across the layer from top to bottom in acetate peels, are not uncommon. Dokken (1987) identified 20 ichnogenera (23 ichnospecies) of trace fossils in the Platteville, including Chondrites and Planolr'tes burrows as the dominant forms. He states that the assemblage is characteristic of Seilacher's (l964a,b, 1967) Cruziana ichnofacies. This assemblage is indicative of a shallow marine habitat. Furthermore, this assemblage is characteristic of a shallow marine environment which Dokken puts at a depth of 10 to 20 m and that the conditions were uniform throughout the region. 33 In contrast, the fauna of the Decorah Shale is dominated by brachiopods, judging by information gathered from acetate peels. They are overwhelmingly the most plentiful organisms, though other phyla are represented. Bryozoans are also very plentiful, while trilobites, conodonts, crinoids, and the conical organisms similar to that found in the Platteville (Cornulites?) are present as well. The fossils are also found in thin, very fossil rich lenses (packstones and grain stones), though the lenses in this unit are usually much thicker than those found in the Platteville. Fossils may also be collected in the float itself (though no fossils were collected from float for this study). The thicker lenses show almost no bioturbation or escape burrows. The Deicke K-bentonite contains no fossil material. However, the bentonite does contain numerous trace fossils. Dokken (1987) reports seven ichnogenera of trace fossils within the Deicke, including Arenicolites, Bifungites, Chondrites, lingulichnites, Thalassinoides, Planolr’res, and "Problematica type 1". However,Sloan (1987 ) states that he has not found any escape burrows in the Deicke. Thus, while there are no escape burrows, the Deicke appears to have been heavily bioturbated and has the same assemblage of trace fossils as that found in the Carimona Member (Dokken, 1987). V1. Materials and Methods The first step was to select six localities at which both the Deicke and Millbrig K- bentonites were reported (Fig. 16). The localities were chosen from published sources (Huff er al., 1986). At each locality, a stratigraphic section was measured (Appendix 6). Samples were then taken, numbered, marked with an "up" arrow and north arrow. Each was recorded as to the position in the measured section the sample came from, as well as the thickness of the sample. It should be noted that several good localities could not be reached due to road construction. Other localities were passed by for safety reasons, while others had become grown over since they were first reported The samples were then taken back to the lab for preparation. Each sample was cut twice (along the length and width of the rock). The slabs were then smoothed off on an 80 grit diamond wheel to remove any grooves or edges left from the saw blade. It was smoothed even further with a 180 grit diamond wheel to remove the grooves left from the 80 grit wheel. (The 180 grit wheel is incapable of smoothing off edges or saw blade grooves.) Next, the samples were given a pre-polish using 600 grit silica-carbide paper. Afterwards, the rocks were polished with 6 pm diamond paste. Once the samples were polished, they were then etched in 0.8% formic acid for approximately 8 to 10 minutes. The samples were washed in a sonic cleaner before etching and rinsed in tap water and dried with compressed air after etching. Acetate peels were then made by completely covering the polished surface in acetone and then immediately pressing it into acetate paper. This was done under a ventilation hood while wearing latex rubber gloves. The samples were then dried overnight. Once completely dried, the acetate paper was peeled away from the sample and placed between two pieces of preformed window glass (to prevent the peel from curling). Often, air bubbles in the peel were present, and the sample had to be repolished, re-etched, and a new peel made. (This process was often repeated several times with certain samples.) Bubbles in peels are 34 35 l. McGregor South Section 2. Preston West Section 3. Mabel Section 4. Spring Grove West Section 5. St. Paul Section 6. Lancaster West Section Figure 16-Field localities in the study area. 36 caused by several factors. First, and most importantly, the presence of different mineralogies in a given sample caused many bubbles. For example, dolomite, when mixed together with a calcite matrix, would dissolve much more slowly than the calcite, causing the dolomite surfaces to be raised higher than the calcite surfaces. Clay minerals also responded in a similar manner to the dolomite, especially since the two are often found together. Cracks on the surface of the sample would allow the acetone to drain from parts of the polished surface more quickly, allowing for uneven amounts of acetone over the entire polished surface. The presence of pyrite also influenced whether bubbles would form on a peel, though the reason for this is unclear. Differences in porosity in a sample may also cause bubbles to form as acetone may drain into more porous parts of a sample, again producing an uneven amount of acetone on the surface as a whole. (Differing amounts of porosity in a given sample may also affect the etching process as well. If a particular part of a sample is more porous than the rest, more acid may invade this section, allowing more of the rock to dissolve away. This may loosen or raise grains in that particular part of the sample, affecting the quality of the acetate peel.) Once the peels were made, they were point counted using a 36-point ocular grid. A total of 360 points were counted for each slide to determine the rock type according to Dunham's classification. All slides were re-examined petrographically for the presence of bryozoans. Because the samples were cut randomly, the level of identification was down to genus only (although a few common species could be identified; see Fig. 5). Lists of bryozoans were compiled noting their position in the stratigraphic reports. Lists of bryozoans in the study area were also made using a comprehensive global data base compiled by Tuckey (1988) based on all known previous literature. Bryozoans were tracked from their continent and time of first occurrence to the continent and time of last occurrence form the Arenigian (Ordovician) to the Pridolian (Silurian). Species from the study area were also recorded in a similar manner, though the time ranges involved were much shorter. The data base is limited by the paucity of information in certain areas, such 37 as South America, Africa, and western North America, not to mention oceanic islands. Much more detailed lists have been published for North America, where bryozoan genera and species have been reported in a state by state manner (including areas in Canada) from the Arenigian to the Caradocian in the Ordovician. Locality 6, the Lancaster Section, was not used in this study. After the samples were examined in greater detail in the lab, it turned out that the material from this section was primarily sucrosic dolomite and no identifiable fossil material was preserved. Also, this was the only section in which fewer than thirty samples were taken. While the Deicke was exposed here, the exposed section was rather thin compared to the others in the study area, so fewer samples were taken. Various computer programs were used in the creation of lists, drafting of diagrams, and computing of statistics. The program Excel was used to compile lists. Statistics were done using Systat II. World-wide maps were created using the program Terra Mobilis and modified with Super Paint. This program is based upon the Scotese and McKerrow model of tectonic plate reconstruction. Regional maps were constructed using the program Map Art and Claris CAD. VII. Taphonomy The taphonomy of the fossils in the upper Platteville Formation represents a marked contrast with that of the Decorah/Spechts Ferry Shale, reflecting very different depositional settings. Based on data gathered from acetate peels, the ostracods are the only organisms ever found articulated in numbers, while bivalves, brachiopods, trilobites, and crinoids are nearly always found disarticulated, with a high degree of breakage and corrosion. Much of ’1‘ the skeletal debris observed in acetate peels from the Platteville is very small. For example, . most of the bryozoan debris in this formation were small cryptostomes; no complete massive or domal trepostome colonies were identified in this interval. Crinoid columnals are rarely found as anything other than single isolated "buttons", although some small k; "stacks" of columnals can be identified. As for single unit skeletons, gastropods are the only elements that may found with little fragmentation (there are some, however, that are highly fragmented), although there are exceptions. A few very small rugose corals (<5 cm) are found as whole elements, as were a small number of cephalopods. Tabulate corals and non-cryptostome bryozoans always show a high degree of fragmentation and corrosion (this made identification of non-cryptostome bryozoans extremely difficult). All elements were randomly oriented in the carbonate mudstone/wackestone portion of the unit. The unit has a blotchy appearance, possibly due to a high degree of dioturbation. Using Droser and Bottjer’s (1986) ichnofabric classification, most of the upper Platteville has an ichnofabric index of 6 as the muddy carbonate sediment has been "completely homogenized", though a few beds have an ichnofabric index of 5. Dodd and Stanton (1981) suggest that indistinct burrows such as those found in the Platteville are indicative of a soft and fluid substrate. Thus, the Platteville may have been partially thixotropic if this model is correct though an alternate hypothesis is that there was a firm substrate with little bioturbation. The thin brown altered layers, however, do not fit this model. They are more fossil rich than the rest of the unit and contain some fine-grained siliciclastic material, 38 39 as they are surrounded by fine-grained carbonate mud These layers were not completely reworked like the rest of the formation because they contained more fossil material, perhaps making it difficult for the burrowing organisms to overturn the material and preventing extensive bioturbation. As stated previously, these lenses may represent the distal portions of a storm deposit or may have formed by directional currents. Thus, they were probably deposited more rapidly than the rest of the upper Platteville. No organisms are found in life position in this unit, with the possible exception of some of the ostracods. Using available information from polished samples and acetate peels, both the sedimentation rate and the energy of the environment can be determined by using Brett and Baird's (1986) biostratinomic features of different skeletal elements. The fragile, ramose organisms (bryozoans and corals) show tremendous fragmentation and corrosion. The multi-element skeletons (crinoids and trilobites) are almost always disarticulated and are never sorted. Bivalved organisms (with the possible exception of some ostracods) are disarticulated and often fragmented and corroded. Thus, the environment, according to Brett and Baird's (1986) model, was low in energy and had a low to intermediate sedimentation rate. The trace fossil Chondrites adds further support to this hypothesis, with one excellent example of this trace fossil found in samples collected for this study from the upper Platteville. This type of trace fossil is found in quiet, fairly shallow water environments. As previously stated, the ichnogenera assemblage reflect a Cruziana ichnofacies, indicative of shallow water conditions and, according to Shroud and Levin (1976), working on Chondrites from the contemporaneous Plattin Limestone of Missouri, lower rates of sedimentation. The only exceptions to this hypothesis are the thin, brown layers, which were conceivably deposited in a very short period of time. The fossils of the Decorah Shale, on the other hand, has a very different taphonomic history. The fauna living in the Decorah environment was much more diverse than that of the Platteville based on data collected from acetate peels and collected samples. Brachiopods dominated the fauna, though bryozoans and trilobites appear to be fairly 40 common. According to Sloan (1987), crinoids were an important element as well, though crinoid skeletal fragments were not abundant in collected samples. However, stacked crinoid columnals could be collected from the float. Other organisms were present in collected samples, including ostracods, corals, bivalves, conodonts, cephalopods, conical organisms (Cornulites?), other echinoderrns, and gastropods (reviewed by Sloan, 1987). The fossils in this unit are much larger and show less fragmentation than those of the Platteville and excellent specimens can be collected from the float at some of the localities. n The fossils found in the float do not always show a high degree of disarticulation or abrasion, and it is possible to collect fairly complete fossils from the float. Similar to the Platteville, the Decorah contains numerous shelly lags (Fig. 10). Again, these shelly accumulations may have been deposited as a result of directional currents or I; tempestites. They are much less bioturbated and there are almost no escape burrows evident in hand samples or in polished samples except in very thin lags. The Decorah was deposited in deeper water than the Platteville due to the Trentonian transgression. There is some controversy as to how deep the water was during the deposition of the Decorah. Cisne er al. (1984), suggest that the water was shallow to very shallow (only a couple of meters), while Droste and Shaver (1983), Witzke and Kolata (1988), and Leslie and Bergstrtlm (1993), the latter group using conodont biostatigraphy, feel the Decorah was deposited in deep water, or deeper water than that experienced during the deposition of the Platteville, due to a well documented Trentonian transgression. Bergstrtlm and Leslie's data (1993) record a Phragmodus—dominated fauna in the lower Decorah, suggestive of a deeper water environment than the shallow water Platteville. Further, carbon isotope data is consistent with a deepening phase during the Decorah. Hatch et al. (1987), record a major global positive in 813C. They concluded that isotopically light carbon is being stored in the sedimentary record and depleting 12C in the oceanic reservoir, consistent with a decrease in ventilation in the ocean and a general worldwide deepening of the ocean during this time interval. The energy in the environment of deposition appeared to have been 41 higher than that of the Platteville, as iron ooids have been discovered in subsurface drill cores of the Decorah Shale in Iowa (Witzke, 1987). It is possible that the source for the iron is a bentonite as Sturesson (1992) reports a similar source for iron ooids found in the middle Ordovician (Llanvirn-Llandeilo) in Sweden. The unit is not as heavily bioturbated as the Platteville nor does it have a blotchy appearance like the Platteville. Burrows found in polished sections are distinct and contrast with the rest of the matrix in many instances. The bifurcating burrows are both horizontal and vertical, often forming a "T" in cross section (Fig. 17). According to Dodd and Stanton (1981), this is often indicative of a firm substrate as the burrows would otherwise become deformed. In conclusion, the Decorah Shale was deposited in deeper water than the upper Platteville carbonates. The degree of fragmentation is much less than that of the Platteville and the fossils are much larger in size. Articulated brachiopods are common and larger crinoid columnals than those found in the Platteville could be collected from float. Large fragments of bryozoan colonies are present in float, as well as entire hemispherical colonies. Iron ooids have been identified in horizons in this unit. Thus, the environment of deposition was most likely higher in energy than the Platteville, with an intermediate sedimentation rate. 42 Figure 17-Burrows in polished sample from the Decorah Shale. VIII. Discussion There appears to be no mass extinction of bryozoans above or near the Deicke K- bentonite. All genera cross the Deicke, based upon data gathered from the literature. Of the 35 species present in Platteville rocks in the study area, five species have disappeared by the onset of Decorah deposition, and, judging by data gathered from examination of acetate peel, these were probably very rare (as none of the five were identified in any of the acetate peels; see Appendix 6) and may very well have disappeared before the eruption of the Deicke. This represents an apparent faunal loss of 14.3% of the bryozoans, or 1 in 7 species. This is well within the range of normal background extinctions. The apparent extinction may appear to be more severe if the number of species that do not cross over into the Decorah but are found higher in the section are added. By adding these surviving bryozoans that are not found in the Decorah (but are found higher in section), the number that appear to go extinct doubles, jumping to ten species or an extinction of 28.6%. This is nearly a third of the species and is more significant. However, five of the species found in the Platteville but not in the Decorah later reappear when carbonate deposition resumes higher in the section, though not nessecarily in the study area (Appendix 7). This constitutes a biofacies change in which several species were probably unable to survive in the shaly tenigenous clastics. They were probably excluded from the area until more suitable conditions resumed. Of the five species that actually became extinct, none appear to be common elements in any environment in any location worldwide during the Caradoc (Appendix 7) based upon previous literature. One, Amplexopora grandis, is restricted to the Platteville of Minnesota. It can be argued that its disappearance was caused by a biofacies change as easily as a massive volcanic eruption. The other four species that disappeared are also very restricted in geographic extent. Stictopora pediculata is found only in Eastern Ontario (in addition to being restricted to Minnesota Platteville rocks) in rocks that are roughly the lateral equivalent of the lower Platteville. These rocks, from the 43 44 Lowville Formation, are also a carbonate unit. Papillalunaria perampla is found only in the Platteville of Minnesota and the New Market Formation of Virginia (though this formation is Llandeilo in age). Again, both units are limestones. Stictopora grandis is a bit more extensive, being found in the Platteville of Minnesota and Northern Illinois, as well as the Plattin of Missouri (the lateral equivalent of the Platteville) and the Tulip Creek Formation in Oklahoma (Llandeilo in age). All of the units are carbonates, except the Tulip Creek, which includes both shale and limestone. The last species to disappear with the onset of f siliciclastic sedimentation, Leptonypa hexagonalis, is found exclusively in the Platteville in t North America, including Minnesota, Wisconsin, and Northern Illinois, and in the "C2" (Kuckers) of Baltica (upper Llandeilo-lower Caradoc in age). This species appears to have .‘h to survived in both shales and carbonates, diverging from the previous pattern of being restricted to largely carbonate units. However, this genus represents a unique case as it often encrusts motile organisms (though in Northwest Illinois, it encrusts Hyolithes baconi, a small, conical, problematic organism that has been interpreted to have a sessile mode of life. It is difficult to envision the bryozoan encnrsting this particular organism when it was alive as the encrustation is around the entire skeleton, not just its upper surface.) Thus, the disappearance of the five species could be explained as a biofacies change. None of the five species appears to be very common elements in any environment or community. Two of the genera, Leptotrypa and Papillalunan’a, were not identified in rocks below the Deicke. Though many fragments of Stictopora were identified (this genus was the most common bryozoan identified from samples collected from the upper Platteville), none could be positively identified as S. grandis or S. pediculata. Rather, the only species that were identified belonged to S. exigua or S. lira. Four of the species are restricted to North America. Further, as stated previously, Cisne er al. (1984) found a lateral facies relationship in which bryozoan abundance was greater in the Decorah of St. Paul, Minnesota and lower in the contemporaneous carbonates of Guttenberg, Iowa. Thus, there appears to be a strong biofacies control on bryozoan distribution as bryozoan 45 diversity (and abundance) did increase in the field area, as observed in this study as well, when crossing the Platteville/Decorah boundary (Appendix 6). Similarly, there may be a major sampling bias between the carbonate and siliciclastic units. As stated previously, the upper Platteville Group is a generally fossil poor unit, with only a few bryozoan genera identified from acetate peels (Fig. 5), with Stictopora and Escharopora being the most common. The Decorah Shale is much more fossiliferous and diverse fossils from various group may be collected at many localities, with about twice as many bryozoan genera identified from acetate peels from a much larger number of fragments. The potential for preservation of rarer elements is probably higher in the Decorah Shale than the carbonate units and more species can thus be retrieved. There is potential for over-splitting of species in this unit as well due to higher preservation and collection of different parts of a single colony. For example, Cuffey (1967) was able to show that three different genera, Tabulipora, Stenodiscus, and Stencpora (from a unit in the Permian), were probably a single genus but different parts of the same colony or different stages in the astogeny of a single colony. Thus, it is possible that some of the species have been over-split and are parts of a single colony. Because the bryozoan fragments were identified from a single, sometimes oblique, section, it is possible that diversity was somewhat higher than reported in Fig. 5 as many closely related bryozoan genera look the same or very similar in a single cross-section. Also, the preservation of many of the bryozoan fragments identified in the acetate peels from the upper Platteville is extremely poor. There is no evidence to suggest that any of these bryozoans that disappeared are found directly under the Deicke or even close under it. In fact, Willrnan and Kolata (197 8) report that Leptorrypa hexagonalis is found in the Mifflin Formation of the lower Platteville of Northwest Illinois (as stated previously, this genus was not identified in any of the acetate peels). It appears that this species may have disappeared long before the eruption of the Deicke. It may be possible that the other four species may have disappeared before the eruption of the Deicke as well. Bork and Perry (1967) do not report any of the trepostomes that disappeared in the 46 Quimbys Mill Formation just below the Deicke from localities in northwestern Illinois, Iowa, and Wisconsin. Thus, the extinction hypothesized by Sloan (1987) is not observable among bryozoans, and may likely be a biofacies change with the permanent disappearance of some Species due to normal background extinction processes. This should provide a good test for the hypothesis of volcanically induced extinctions, as most bryozoans are small, sessile, filter feeding organisms (some encrust on organisms which may be motile), and should be susceptible to extensive ash falls (though bryozoans seem to F" tolerate muddy habitats better than say, corals). However, it seems that the bryozoans make it past the Deicke event without suffering a major extinction. In fact, it appears that many of the Decorah species survived three other widespread bentonites (as well as one small recently discovered K-bentonite between the Deicke and Millbrig), the Millbrig, the Elkport, and the Dickeyville, in order of stratigraphic appearance, found in the Decorah (Sloan [1992] notes that no extinction takes place above the Millbrig, a conclusion verified by Huff er al. [1992] and Scharpf [1990]). According to Sloan (1987), this cluster of bentonites was erupted in less than a one million year time span. Furthermore, radioactive dates for the Deicke and the Millbrig overlap each other. Several species are restricted to the Decorah Shale (as are a few subspecies) but the more common elements are found in younger formations above the Decorah (Appendix 7) based upon previous literature. Using stratigraphic evidence provided by Rice (1987) presented in Sloan's (1987) volume, there is no mass extinction or abrupt, sharp, massive decline in bryozoan diversity within this interval (Sloan, 1987, p. 145-146). Rather, there appears to be a gradual extinction and origination of species during this period. This is further evidence that a volcanically induced extinction event is suspect and agrees with Erwin and Vogel's (1992) findings at Neogene volcanic horizons. This conclusion raises some interesting questions about other, similar causes for mass extinction. Sloan (1987) states that it is the volcanic dust and ash that causes the "extinction" event. Alvarez er al. (1980) invoke a similar cause for extinction at the K-T boundary (some authors, Officer at al., [1987] for example, also 47 invoke a volcanic mechanism for this particular extinction). Instead of volcanic ash as the killing agent, they present evidence of a huge asteroid or comet striking the earth, marked in the geologic record by an iridium layer. They state that the asteroid impact ejected large quantities of material into the atmosphere and ”some dust-sized material ejected from the crater reached the stratosphere and spread over the globe”. The authors hypothesized that the dust obscured the sun, preventing light from reaching the earth's surface for a period of "several years", causing the cessation of photosynthesis and subsequent collapse in food chains which ultimately resulted in the end-Cretaceous mass extinction. Kauffman (1986) does note a significant decline in bryozoans (though cheilostomes and tubuliporates in this case) at this horizon, with about 60 species disappearing at the boundary. He evokes the impact theory proposed by Alvarez er al. as the killing mechanism. Curiously, he attributes the apparent stepwise extinction of various organisms to multiple impacts, though showing bryozoans disappearing rather abruptly, not stepwise. Taylor and Larwood (1988) also report a major decline in bryozoans at this interval, with almost 50% of the bryozoan genera disappearing at the end of the Cretaceous. They note that Brood (1972) found an extremely low level of species extinction, but attribute this apparent paradox to "differences in taxonomic opinion". They also note that Hékansson and Thomsen (1979) find a disparity in the disappearance between cheilostomes and tubuliporates. The tubuliporates decline by only 25%, while the cheilostomes decline by more than 80%. They felt that there may have been a facies control as the Danian (earliest Tertiary) faunas seem to have had a lack of hard substrata for encrustation. While there are many problems with the extraterrestrial impact hypothesis, too numerous to cover in this study, the most obvious is causal mechanism. It is apparent that no mass extinction occurred among bryozoans above the Deicke K-bentonite and the larger Millbrig K—bentonite/"Big Bentonite" horizon (Huff er al., 1992), and it appears that no mass extinctions occuned above the other two large bentonites in the Decorah Shale, the Elkport and the Dickeyville. Many of these volcanic events, most notably the Deicke and especially the Millbrig/"Big Bentonite", ejected nearly 48 as much or more ash and dust than the end-Cretaceous impact event, judging by the thickness and extent of the Ordovician bentonites. The ash, if violently erupted into the atmosphere, would surely act in the same manner as the impact event proposed by Alvarez er al. (1980), obscuring the sunlight for a period of time (it is important to note that most of the ejecta from impact craters noted in the geologic record is not spread out over large geographic distances; rather, it appears that most is restricted to only a few crater diameters of the impact site itself, with small amounts being spread worldwide, a conclusion reached by mm, [1982], studying impact craters on both the earth and the moon). Yet no extinction took place at any of these volcanic ash horizons. This may mean that the mechanism proposed by Alvarez er al. needs to be reexamined and that, if an asteroid does indeed cause a major mass extinction, the causal mechanism may not be ash or aerosols injected into the atmosphere. Some authors evoke a multiple asteroid impact to explain the apparent gradual extinction of organisms leading up to the terminal Cretaceous event in which several asteroids struck the planet and killed off different organisms in "waves". However, this is suspect as well since bryozoans seem to survive four volcanic events within about a one million year time span (Sloan, 1987) in middle Ordovician rocks of the Upper Mississippi Valley (in fact, it appears that bryozoans were diversifying throughout this interval and were enjoying their greatest diversity during this time). It is possible that an impact event may have had substantially different effects than an eruption, and it is also possible that two different volcanic events may have had very different effects as well (the basaltic eruptions of the Deccan Traps during the late Cretaceous versus the potassic eruption of the Deicke for example), but, as yet, these potentially different effects have not been addressed. Rather, the mechanism evoked for causing a mass extinction in all of these cases is dust and aerosols ejected into the atmosphere, a mechanism that does not work for post-Deicke bryozoans or most post-Millbrig organisms (Huff et al., 1992). While the bryozoans do not suffer a great mass extinction above the Deicke K-bentonite, many of the species in the field area do disappear by the start of the Ashgillian (Appendix 49 7), appearing to suffer a significant decline during the latter stages of the Caradoc, a phenomenon noted by Anstey and Tuckey (1992), as well as an end-Caradoc local extinction in the Cincinnati area, with the loss of 50% of all bryozoans (Anstey and Rabbio, 1989). Using information gathered from thin sections, the bryozoan fauna of the upper Platteville is dominated by cryptostomes such as S tictopora and Escharopora, with Srictapora being by far the most common bryozoan fragment identified (as stated .3... previously). There were relatively few trepostomes identified from acetate peels made from these rocks, the only exception being the trepostomes Bimuropora winchelli and Monotrypella, though both were much rarer than either S tictopora or Escharopora (in terms of the absolute number found in a given sample). However, the lower Platteville rocks are :r' much more diverse, and contain several more trepostomes. The number of bryozoans found within the Carimona member increases dramatically, though the diversity does not increase. The diversity increases in the Decorah in terms of both absolute numbers of organisms found and number of genera. Cryptostomes are still the dominant order in all of these rocks, but the number of trepostomes increases dramatically in the Decorah rocks (when compared to rocks below the Deicke). This suggests that these faunas belong to the Red River-Stony Mt. bryozoan province designated for Late Ordovician bryozoans by Anstey (1986) (or represent the early development of that province). This difference is probably due to a biofacies change rather than repopulation due to a mass extinction as nearly all the species found in the Platteville are also found in the Decorah, and, in the case of the trepostomes, in greater abundance. It is interesting that, of the five species that disappear during Platteville deposition (Fig. 5), three were not identified in the peels and the other two, from the genus Stictopora, could not be differentiated from other species from the genus as the material was so fragmentary (though they may be restricted to the lower members of the Platteville based upon previous literature). This must mean that they were relatively rare components of the fauna or more common in older Platteville rocks and 50 disappeared long before the eruption of the Deicke. One of the more puzzling aspects of the Platteville is its sparse bryozoan fossil debris found in the acetate peels yet seemingly diverse fauna as reported in the literature. There are two possibilities. First, the sparse material found in the sampled sections may be allochthonous, and derived from a more diverse locality. This may be partly true based on petrographic and taphonomic evidence as much of the material appears to have been reworked (see ”Taphonomy"), with some material possibly being washed in from other localities. There may have been some small, local bryozoan mud mounds within the region or material was transported in from deeper water areas, perhaps the Michigan basin or Illinois basin. A second possibility is that the fossil bearing units are below the sections sampled. According to Willrnan and Kolata (1978), the most fossiliferous unit within the Platteville of Northern Illinois is the Mifflin, the thinnest formation within the Platteville and the second unit from its base, well below most of the sections sampled in the study area (as stated previously, this is where Leptonypa hexagonalis was reported by the same authors). Bork and Perry (1967, l968a,b) report only 13 trepostomes from the Platteville (7 from the Mifflin in the lower Platteville and 6 in the Quimbys Mill in the upper Platteville; those species found in the Mifflin are not found in the Quimbys Mill). None of the bryozoans that disappear during the deposition of the Platteville are found in either of these formations. Similarly, using information gathered from acetate peels, the most diverse part of the unit collected was the lower Platteville. It has already been noted by Cisne er al. (1984) that the Decorah bryozoans seem to be more common in siliciclastics than in the carbonates, an observation that is likewise readily apparent in the field when going from the upper Platteville to the Decorah, as many bryozoans could be collected from the Decorah while the sampled portion of the upper Platteville was nearly devoid of macrofossil material. The number of both genera and species rises dramatically in the Decorah compared to the Platteville (Appendix 2,3). However, when carbonate deposition resumes above the Decorah, in the Guttenberg and Ion Formations (for example) of the 51 Galena Group, the number of genera and species drops back down to levels more reminiscent of the Platteville (based upon previous literature and acetate peels). This is probably a biofacies phenomenon, perhaps due to tolerance of carbonate deposition and substrate stability. Biogeographically, using data gathered from the literature, the genera found in the Platteville and Decorah first appear from several different plates, though most of the species originate on the North American plate, some being endemic to the North American plate (Fig. 18). Of the 54 genera of bryozoans reported in the Platteville and Decorah, 17 first appear in North America, roughly 32% of the genera, and only three of which remained endemic to the continent (Fig. 18). These include some of the oldest known bryozoans from the Arenig of Utah. Only one genus first appears in China, namely Ptilodicrya (Fig. 18). Similarly, only one genus first appears in the Southern Europe plate (Fig. 18). This genus, Sagenella, first appears during the Arenig. Seven genera appear first in Siberia (Fig. 18). The largest percentage of the genera found in the study area come from Baltica, where 22 first appear (about 41% of the total) and are later found in North America (Fig. 18). Six genera first appear on two different plates at about the same time (Fig. 18). Three appear on both the North American plate and the Baltoscandian plate (Peronopora and Byrhopora during the Caradoc and Corynotrypa during the Llandeilo), two on the North American plate and the Siberian plate (Eurydictya during the Caradoc and Ceramoporella during the Llandeilo), and one appears on Baltoscandia and Siberia (Homorrypa during the Llanvirn) during the same time period. Considering plate reconstructions, based upon the Scotese and McKerrow model, those found on North America and Baltoscandia and Siberia and Baltoscandia at the same time are acceptable if sample biases are taken into account (Fig. 11). Since North America and Baltoscandia were moving towards each other, an origination and subsequent migration of a particular genus on one plate to the other could be accomplished but not preserved in the fossil record due to some sort of sampling bias, obscuring the origins of the taxa (remembering the vast majority, nearly 52 Figure 18-First appearance of genera. 53 75%, of bryozoans documented in the study area originate in either North America or Baltoscandia). Similarly, Siberia and Baltoscandia were not that far apart during the Llanvirn, so a similar scenario as that described above may have occurred. Finally, this scenario can also explain the two genera that seem to originate in North America and Siberia during the same time interval as plate reconstructions place them close to one another (Fig. 2). No genera from the field area appear to have originated on any of the plates that composed the Gondwana land mass or the Avalon plate as much of the Ordovician rocks on any of these land masses remains largely unstudied, though some faunas are known from North Africa and Australia. Most come from either North America or Baltoscandia. This makes sense as the two continents have been interpreted as moving toward one another throughout the Ordovician. Several also appear first in Siberia, a continent that was close to both North America and Baltoscandia throughout much of the Ordovician. South Europe does not contribute much to the fauna nor does China. China was, however, composed of at least two major plates during the Ordovician, so better resolution is needed. Similarly, no information exists for the Kazakhstan plate, so further research in this area is needed as well. The data from Gondwana is also quite incomplete. If a cluster analysis is run on the genera present on the various plates from the field area, North America, Baltica, and Siberia will form one cluster, with the shortest distance being between North America and Baltica, the plates of Gondwana (including Australia, Tasmania, and South America), England (on the Avalonian plate according to the Scotese and McKerrow plate reconstruction model) and China forming another cluster, and South Europe is isolated (Tuckey, 1990, 1988) (Fig. 19). Bryozoan faunas from Antarctica, Africa, and South America are essentially unknown though some very incomplete information exists for South America. The clusters formed make sense considering the plate reconstructions based upon the Scotese and McKerrow model. As explained previously, North America, Baltica, and Siberia were all quite close to one another, with North America and Baltica moving together. Tuckey (1990) reports a Blackriveran 54 0.0000 mm 1.0000 North America Baltica I_| Siberia South Europe | Bow Australia Tasmania J South America Gondwana Figure l9-Cluster analysis of bryozoan genera found in the field area using Euclidean distance method. Data for analysis taken from Tuckey, 1990. 55 migration event in Baltica, North America, and Siberia for all bryozoans, and this hold true for the bryozoans found in the field area. He also reports that Southern England and Wales have a cosmopolitan fauna with affinities to North America and Baltica during the Caradocian, though this does not seem to be true for the fauna present in the field area during the Blackriveran and Trentonian. In the second cluster, the plates composing Gondwana cluster together with China. China was situated very close to the western edge of the Gondwana land mass. England was also very close to Gondwana. South Europe appears between the two clusters, though it seems to have close affinities to the Gondwana cluster. It is possible that England (and the rest of the Avalonian plate) and South Europe, situated between Gondwana and North America and Baltica, were receiving genera from both areas (though England has some genera that are unique to the Avalonian land mass). More research needs to be done and data collected to provide a better resolution. For example, very little data exists for Ordovician bryozoans in South America, Africa, and Antactica. If more data is compiled, it could be used as a test to the SWEAT (Southflestern US. Eastern AnIarctica) hypothesis. If this hypothesis is correct, the South American continent would probably cluster with North America, Baltica, and Siberia, as the hypothesis puts this continent next to North America during the Ordovician (Dalziel er al., 1994). If family diversity is examined, Baltica has all of the same families (19 total families) present in the study area. The diversity is much lower on all of the other plates. Siberia and South Europe contain eleven families each, Gondwana has nine of the families, England seven, and China six. Four of the families are restricted to North America and Baltica (based upon the bryozoans found in the study area). The geologic range of most of the genera is quite long (Fig. 20). Only two genera fail to cross over into the Ashgill, Diasroporina and Srictoporellina. Diasroporina, however, may belong to the genus Sagenella. Up to 14 genera (Arthrostylus has been questionably found in material from Wenlock-Ludlow aged rocks) disappear before the beginning of the Silurian. A decline of similar magnitude occuned during the Ludlow (Silurian). 56 Figure 20-Bryozoan generic ranges. 57 Species distributions are far less cosmopolitan. About two-thirds of the 135 described species are endemic to the North American plate (87 of 135) (again, using information gathered from previous literature). The rest can be found on either the Baltoscandian plate or the Siberian plate. Several species have, in fact, migrated from these plates or appear in the fossil record at about the same time on two of the plates. This suggests that the species has a somewhat longer geologic history (or the organism was misidentified). N 0 species from the study area are found on any other plates. Again, this is probably due to the arrangement of the plates during the Ordovician when Baltica and North America were F moving toward each other, slowly closing the Iapetus Ocean, and Siberia was located .. above both plates (based on the Scotese and McKerrow model). This offers another test for the SWEAT hypothesis as the model places South America next to North America. If E the SWEAT model is correct, many bryozoan species that migrated to North America from Baltica and vice versa should also be found in South America as well. The origin of the bryozoans in the study area is variable. Many genera and even several species clearly migrated from Baltica and Siberia. Based upon paleobiogeographic reconstructions, it appears that the Siberian taxa must have come from the north through Canada and New England. Some of the Baltic taxa may have taken the same route, while others may have migrated from the south (the Taconic mountains prevented any taxa from migrating directly from the eastern sea board). Of the native North American taxa, a few taxa ultimately migrated from Utah and Nevada (where some of the oldest known North American bryozoans are found; the oldest known North American bryozoan is found in Tremadocian rocks of Oklahoma). Some of the taxa originated in Oklahoma and migrated from there, while others migrated from the westem edge of the Taconic mountains (Sloan, 1992, reports that much of the Trenton fauna migrated from the latter two areas as well). IX. Conclusions There is no conclusive evidence to suggest that there was a major mass extinction of bryozoans above the Deicke K—bentonite as stated by Sloan (1987,1992). While his observation of a significant faunal changeover from Blackriveran rocks to Trentonian rocks is correct, the faunal change is more likely due to a biofacies change. There is a major lithofacies change just above the Deicke in the field area, where the carbonates of the Platteville Group are replaced by siliciclastic sediments of the Decorah Shale, and it is this change that likely controls the faunal change. The appearance of the bentonite is merely coincidental with the lithofacies change. Less than 15% of the bryozoan species disappear at the Platteville/Decorah boundary. Five species disappear at the boundary but later reappear when carbonate sedimentation resumes, being excluded from the area during shale deposition. Of the five species that become extinct, none appear to be very common species worldwide or even on the North American plate. None were identified in acetate peels. Three of these species are restricted to carbonate units. Of the other two species that disappear, one is found in a limestone/shale bearing unit (besides the Platteville) and the other, found in only one other unit, though a shale, is an encrusting bryozoan documented in one of the lower units in the Platteville of Illinois well below the Deicke. Thus, a more likely explanation for the faunal changeover at the Blackriveran/Trentonian boundary is a biofacies change in which many of the organisms were excluded from the environment as a result of a major change in sediment types. Similarly, there appears to be no extinction at any of the other three large bentonites found in the study area This conflicts with not only Sloan's hypothesis, but other proposed volcanically induced extinctions in the geologic record such as the well-known end-Cretaceous extinction (Officer er al., 1987 ), but is in agreement with previous work by Erwin and Vogel (1992) working on Neogene volcanics and Huff er al. (1992) working on the Millbrig K-bentonite. This raises questions as to other extinction mechanisms evoking ash and other aerosols being ejected into the 58 59 atmosphere and causing extinction by blocking the sunlight, preventing photosynthesis, and inducing a collapse in the food chain. Alvarez er al. (1980) proposed that an asteroid or comet struck the earth at the end of the Cretaceous and induced a mass extinction by this method, marked by an iridium rich-layer at the K-T boundary. While there is good evidence to support an asteroid impact at the end of the Cretaceous, the lack of extinction at bentonite horizons throughout the geologic record, most notably the Toba, Deicke, and especially the Millbrig, where at least as much material, if not more in the case of the Millbrig and possibly the Deicke, was ejected into the atmosphere as that proposed for the impact theory at the end of the Cretaceous, would suggest a re-evaluation of the actual extinction mechanism of an asteroid impact since the proposed mechanisms for the two processes are the same (many mass extinctions, including the Late Permian event, seem to be linked with multiple flood basalts, i.e. massive eruptions that last over an extended period of time). A detailed study of other known impact horizons must be undertaken to verify any impact induced extinction utilizing ejected dust as the mechanism for extinction. By examining the taphonomy of the fossils from the upper Platteville Group and the lower Decorah Shale, the depositional environment can be reconstructed. The carbonates of the upper Platteville Group are mudstones and mud-rich wackestones with thin, shale- rich, oxidized layers which are interpreted to be the distal portions of storm lags or the result of directional currents. The rocks are light bluish-gray in color and are very blotchy, perhaps due to bioturbation. The carbonate sediments appear to be completely reworked and may have been partially thixotropic based upon the indistinct burrows. Most fossils are small, broken, and randomly oriented. The Platteville carbonates were deposited in a shallow, quiet epeiric sea. However, an alternate hypothesis is that the upper Platteville was deposited in much deeper water and had a much firmer substrate. This contrasts with the depositional setting for the Decorah Shale, which were deposited in deeper water due to a major Trentonian transgression. There are many more and larger fossils in this unit. This unit is also bioturbated, but much less than the Platteville, as the sediment does not 60 appear to be completely reworked. The burrows are more distinct, often forming a "T" structure, and do not show signs of much compaction. Thus, the sediment surface was probably firm, allowing for a greater variety of organisms to colonize the area. There are many thin shelly layers (though much thicker than the average thickness of those found in the Platteville) in the Decorah, though they are much less bioturbated than those in the Platteville, and contain few escape burrows. Similarly, there are no ”fingers" of sediment from these layers intruding into the underlying shale layers, unlike what can be seen in the Platteville. Many of the genera and species from the study area appear to have migrated from other plates, most importantly Siberia and Baltica. Baltica was moving toward North America during this time interval, while Siberia was located above both plates based upon the Scotese and McKerrow plate reconstruction model. Since there are many species found on North America and Baltica, a good test for the SWEAT hypothesis would be to examine the bryozoan fauna on the South American plate as it would be found between the two other plates. Another area that is lacking in information is bryozoan data from the western portion of North America. The oldest known bryozoans are from Tremadocian rocks from Oklahoma (as well as China and the Russian Arctic and the Leningrad, or St. Petersburg, region), though almost nothing else from this region is known or studied. This would suggest that much evolutionary and biogeographic information is missing on bryozoans. Part of the problem is the lack of exposed rocks of this age in the region. However, any information would aid tremendously in understanding the origins, evolution, and migration of bryozoans. APPENDICES Appendix 1 Systematics of bryozoans in the Platteville Group and the Decorah Shale Phylum Bryozoa Class Stenolaemata Order Trepostomata Suborder Amplexoporina Family Amplexoporidae Amplexopora decipiens(?), grandis, minnesotensis, subaequata(?) Anaphragma crenulata Cyphotrypa divaricata, informis Leptotrypa hexagonalis Monotrypa magna Monon'ypella normalis, aequalis—grandr's Orbignyella werherbyi, sublamellosa Family Uncertain Spatiopora tuberculosa, lineata-incepta Suborder Halloporina Family Batostomellidae Bythopora alcicornis, herricki, subgracilis En'donypa mutabilis—major Family Birnuroporidae Bimuropora winchelli, winchelli-spinulosum Family Halloporidae Parvohallopora dumalis, subnodosa, undulata, arguta Diplotrypa catenulata Tarphophragma angularis, incontroversa, multitabulata Family Heterotrypidae Dekayia sp. Heterotrypa pauca, praenuntia, praemmtia-echinara, praenuntia-multipora, praenuna'a- naevigera, praenuntia-simplex, trentonensis Stigmatella claviformis Family Mesotrypidae Mesotrypa infida, spinosa(?) Family Monticuliporidae Aractoporella insuera, ramosa, typicalis-praecipra Homotrypa curvata, dickeyvillensis, exilis, exilr's var. A, intercalaris(?), minnesotensis, minnesotensis-montrfera, pauperata, pauperi, separata, subramosa, tuberculata Monticulipora arborea, grandis, incompta Peronopora parasitica (may be Monticuhpora parasitica) Prasopora conoidea, contigua, lenticularrls, simulatrix, simulatrix-orientalis Family Trematoporidae Batostomafem’le, fertile-circulate, humile, magnoporum, montuosum, ovata, varium Hemiphragma irrasum, ottawaense Nicholsonella Iaminata, ponderosa Trematopora sp. Order Tubuliporata Suborder Paleotubuliporina Family Corynotrypidae 61 62 Corynotrypa delicatula, inflata ”Stomatopora" sp. Family Crownoporidae Clonopora mundulum(?) Diastoporina flabellata (may be reassigned to Sagenella) Sagenella minnesotensis Family Unknown "Proboscina" tumulosa Order Cryptostomata Suborder Phylloporinina Family Phylloporinida Phylloporina halli, reticulum, sublaxa Sardesonina corticosa Suborder Ptilodictyina Family Escharoporidae Escharopora angularis, confluens, limitaris(?), ramosa, subrecta Graptodictya bifurcata, elegantula, reversa, simplex Family Ptilodictyidae Ptilodicrya sp. Family Rhinodictyidae Athrophragma foliata Eurydictya multipora Phyllodictya frondosa(?), varia Stictopora exigua, fidelis, grandis, lira, minima, mutabilis, mutabilis-major, nmtabilis-semilis, neglecta, nicholsoni, paupera, pediculata Trigonodictya acuta, concilatrix, elegans, fimbriata, occidentalis, pumila Family Stictoporellidae Pseudostictoporella angularis, dumosa, frondifera Stictoporellina cribosa Suborder Rhabdomesina Family Arthrostylidae Arthroclema armatum, comutum, pulchellum, striatum Arthrostylus conjunctus, obliquos(?) Helopora altemata Ulrichostylus divaricatus Order Cystoporata Suborder Ceramoporina Family Ceramoporidae Accmthoceramoporella granulosa-minor Ceramophylla alternata, frondosa, trentonensis(?) Ceramoporella inclusa Papillalunaria perampla Suborder Fistuliporina Family Anolotichiidae Anolotichia impolita Bythotrypa laxata Appendix 2 Bryozoans of the Platteville Group of Iowa, Minnesota, and Wisconsin Amplexopora decipiens(?), grandis Batostoma fertile, fertile-circulate, ovata Byrhopora subgracilis Bython‘ypa laxata Corynon'ypa delicatula Cyphotrypa informis Diplotrypa catenulara Escharopora angularis, limitaris(?), ramosa, subrecta Eridorrypa subtilis Heteronypa praenuntia-simplex Homorrypa curvata, exilis, exilis var. A, minnesotensis Leptorrypa hexagonalis Monotrypa magna Monorrypella aequlis-grandis Monticulipora grandis Nicholsonella ponderosa Orbignyella wetherbyi, sublamellosa Papillalwraria perampla Parvohallopora arguta Phylloporina sublaxa Sardesonina corticosa Spatiopora sp. Stictopora grandis, mutabilis, mutabilis-senilis, nicholsoni, pediculata Stigmatella claviformis Stomatopora sp. Tarphophragma angularis(?), incontroversa Trematopora sp. 63 Appendix 3 Bryozoans of the Decorah/Spechts Ferry Shale of Iowa, Minnesota, and Wisconsin Acanthoceramoporella granulosa-minor Amplexopora decipiens(?), minnesotensis, subaequata(?) Bimuropora winchelli, winchelli-spinulosum Anaphragma crenulara Anolotichia impolita Ardrroclema armatum, comutum, pulchellum, striatum Arthrostylus conjuncrus, obliquos(?) Atactoporella insueta, ramosa, typicalis-praecipta Athrophragma fioliara Batostoma fertile, fertile-circulare, humile, magnoporum, montuosum, ovata, varium Bythopora alcicornis, herricki, subgracilis Bythotrypa laxata Ceramophylla alternarum, frondosa, trentonensis(?) Cermoporella inclusa Clonopora mundulum(?) Corynotrypa delicatula, inflara Cyphotrypa divaricata, informis Dekayia sp. Diastoporinaflabellata (may be Sagenella) Efidonypa mutabilis-minor Escharopora angularis, confluens, limitaris(?), ramosa, subrecta Eurydictya multipora Graptodictya bifurcata, elegantula, reversa, subrecta Helopora altemata Hemiphragma irrasum, ottawaense Heteronypa pauca, praenuntia, praenrmtia-echinata, praemartia-multipora, praenuntia- naivigera, praenuntia-simplex, trentonensis Homotrypa dickeyvillensis, exilis, intercalaris(?), minnesotensis, minnesotensis-monufera, pauperata, pauperi, separata, subramosa, tuberculara Homotrypella instabilis, mulliporata, ovata(?) Mesotrypa infida, spinosa(?) Monotrypella normalis Monticulipora arborea, grandis, incompta Nicholsonella Iaminata, ponderosa(?) Parvohallopora dumalis, subnodosa, undulata Peronopora parasitica (may be Monticulipora parasitica) Phyllodictya frondosa( ?), varia Phylloporina halli, reticulata, sublaxa Prasopora conoidea, contigua, lenticularis, simulatrix, simulatrix-orientalis "Proboscina" tumulosa Pseudostictoporella angularis, angularis-intermedia, dumosa, fi'ondrfera, rigida Ptilodictya sp. Sagenella minnesotensis Sardesonina corticosa Spatiopora tuberculosa, lineata-incepta Stictopora exigua, fidelis, lira, minima, mutabilis, mutabilis-major, mutabilis-senilis, WV a 64 65 Stictoperellina cribosa Stigmatella clavrformis "Stomatopora" sp. Tarphophragma angularis, incontroversa, multitabulata Trigonodictya acuta, concilarrix, elegans, fimbriata, occidentalis, pumila Ulrichostylus divaricara Appendix 4 Field Localities 1. McGregor South Section-quarry on the east side of State Highway 340, 1.2 km south of intersection of highways 340 and 18 in McGregor, Clayton Co., IA (SW 1/4 SW 1/4 SEl/4 Sec. 27, T 95 N. R 3 W; Prairie du Chien lS-minute Quad). N . Preston, MN, West Sections-roadcuts and quarry on highway 12, 9.4 and 10.4 km, respectively, southeast of Preston, Fillmore Co., MN (NE 1/4 Sec. 7, T 102 N, R 9 W; Lanesboro 7 .5-minute Quad). 3. Mabel Northwest Quarry-quarry on north side of county road, 0.4 km west of Minnesota State Route 43 and 2 km north of Mabel, Fillmore Co., lvm (SE 1/4 SW 1/4 SEl/4 Sec. 15, T 101 N, R 8 W; Mabel 7.5-minute Quad). 4. Spring Grove Wast Section-quarry on the north side of State Highway 44, 5 km west of Spring Grove, Houston Co., MN (SE 1/4 SW1/4 SE1/4 Sec. 17, T 101 N, R 7 W, Spring Grove 7.5-minute Quad). 5. St. Paul (Summit Avenue) Section-exposure in bluff of Mississippi River at end of Summit Avenue in St. Paul, Ramsey Co., MN (El/2 NE1/4 SW 1/4 Sec. 5, T 28 N, R 23 W, St. Paul West 7.5—minute Quad). 6. Lancaster West Section-roadcut on County Road A, 6 km west of intersection of A with US. Highway 61 in Lancaster, Grant Co., WI (SWl/4 NE1I4 SW 1/4 Sec. 31, T 5 N, R 4 W; Hurricane 7.5—minute Quad). 66 Appendix 5 A Key to the Trepostome Bryozoans of the Decorah and Platteville Formations Group I: Mesozooecia and acanthostyles present A. Diaphragms and cystiphragms present; hemiphragms absent 1. Cystiphragms not strongly overlapping (or numerous). Zooecia have obliquely curved diaphragms crowded in mature zones, with acanthopores and closely tabulate mesopores. Mesotrypa 2. Cystiphragms strongly curved, forming overlapping series a. Cystiphragms restricted to mature zone, mesozooecia restricted to maculae Homotrypa b. Cystiphragms throughout zooecia (1) Few mesozooecia i. Thin walled, polygonal zooecia. Small acanthostyles with granulose walls. Mesozooecia contain diaphragms. Exozone not easily definable. Monticulipora (2) Mesozooecia common i. Bifoliate growth form a. Monticules generally flush with zoarial surface; cystiphragms radially arranged on sides of zooecia nearest monticules; walls laminated. Medial laminae. Peronopora ii. Non-bifoliate growth form a. Zooecia strongly petaloid, indented by numerous acanthostyles; acanthostyles not restricted to zooecia] junctions. Atacroporella b. Cystiphragms go into endozone. Thin walled, polygonal zooecia. Small acanthostyles with laminated walls. Well developed exozone. Homotrypella 67 68 B. Diaphragms and hemiphragms present, cystiphragms absent. 1. Hemiphragms in mature region. Hemiphragma C. Diaphragms present, cystiphragms and hemiphragms absent 1. Zooecial tubes oblique to zoarial surface in mature zone. a. Diaphragms numerous, present throughout zooecia; dark divisional line well- developed in walls. Eridotrypa b. Diaphragms few to nearly absent, mesozooecia few. Bythopora 2. Mesozooecia not abundant enough to isolate adjacent zooecia. a. Diaphragms present throughout zooecia (1) Diaphragms locally curvate, mesozooecia locally moniliforrn; acanthostyles generally having a large central lumen; dark divisional line in granular wall. Batostoma b. Diaphragms not present throughout zooecia (1) Diaphragms confined to lower part of exozones. i. Crenulated walls in endozone. Thin zooecial wall. Stigmatella ii. Thicker zooecia walls in exozones. Walls not strongly crenulated in endozone. Large acanthostles with wide central lumen. Dekayia (2) Diaphragms absent in endozone. i. Diaphragms nearly absent Mesopores rare. uptown ii. Diaphragms present throughout exozone; mesozooecia closely tabulate; arnalgarnate wall. Heterotrypa 3. Mesopores can isolate zooecia in inner exozone a. Irregular branches, some flattened. Mesozooecia are replaced by thick wall material in outer exozone. Diaphragms can be found in endozone. Walls are very granular. Nicholsonella 69 D. Diaphragms absent, closed mesozooecia and acanthostyles present 1. Beaded mesozooecia, may contain peristome. Solid interspaces contain small acanthostyles and distinctly moniliforrn granular mesopores. Trematopora 2. No beaded mesozooecia; walls crenulate in endozones; lack diaphragms. Anaphragma Group II: Mesozooecia present, acanthostyles absent A. Diaphragms and cystiphragms present, hemiphragms absent. 1. Cystiphragms strongly curved to hemispherical. Mature zone only, immature zone inconspicuous, may have inconspicuous acanthostyle-like structures; cystiphragms radially arranged on sides of zoecia away from monticules; hemispherical growth form. Prasopora B. Diaphragms present, cystiphragms and hemiphragms absent. 1. Rounded zooecia, complete diaphragms; may have a few centrally perforate diaphragms. Ramose growth form. Parvohallopora 2. Mesozooecia in exozones. acanthostyles absent. Ramose growth form. Wall structure has dark line down center. Diaphragms common. T arphophragma Group III: Mesozooecia absent, acanthostyles present A. Diaphragms present, cystiphragms and hemiphragms absent. 1. Walls contain dark divisional line. a. Apertures perpendicular to zoarial surface; zooecia straight walled, having a constant diameter in mature zone. Acanthostyles laminated, numerous, not restricted to zooecial angles. Amplexopora 2. Walls lack dark divisional line. a. Apertures not petaloid. Elongate monticules. Zooecia have large acanthostyles. Attached to nautiloid shells. Spatiopora b. Thin walled zooecia; diaphragms in recurrent mature zone. Hemispherical growth form. Cyphonypa B. Diaphragms and cystiphragms present, hemiphragms absent 1. Zooecia angular, with sharply defined wall; diaphragms often curved. Orbignyella 70 Group IV: Mesozooecia and acanthostyles absent (may have dark granules present in walls) A. Diaphragms present, cystiphragms and hemiphragms absent. 1. Diaphragms few, widely spaced; zooecial wall crenulate. Monorrypa 2. Diaphragms numerous, closely spaced; zooecial walls straight. Zooecial apertures polygonal, non-rhomboidal. Monomella 71 A Key to the Tubuliporates, Cryptostomes and Cystoporates of the Platteville and Decorah Formations Group I: Lunarium Present A. Communication pores common. 1. Acanthostyles and diaphragms abundant. Acanthoceramoporella B. Communication pores rare to absent. 1. Diaphragms absent. a. Exilazooecia abundant, communication pores few and are found in the inner exozone. Lunaria not radially arranged. Hollow center. Encrusting thin layers. Walls laminated. Ceramophylla 2. Diaphragms present. a. Diaphragms commonly at same level in adjacent zooecia. (1) Diaphragms abundant, thin and tabular. Exilazooecia rare. Monticules with few exilazooecia interspersed among autozooecia; monticular autozooecia much larger than interrnonticular autozooecia. Autozooecia generally angular or subangular in cross-section, a few subcircular. Papillalunma (2) Diaphragms convex or planar, on per autozooecia, and are not abundant. Ceramoporella b. Diaphragms not commonly at same level in adjacent zooecia. (1) Lunarium elevated, few mesozooecia. Acathostyles and stereom absent. Large granular autozooecia. Monticules poorly defined or absent. Walls granular. Encrusting or massive growth form. Anolotichia (2) Monticules inconspicuous, depressed to slighly elevated. Lunaria moderately large in outer endozone, large at zoarial surgace. Vesicular tissue abundant, partially isolating autozooecia. Walls thin throughout, straight to crenulated. Zoarium encrusting, massive or hemispherical. Bythotrypa Group II: Lunarium absent A. Bilarninate colony form. 1. Median rods present in median lamina a. Median rods circular. 72 (1) Mesothecae straight to sinuous. Mural styles indistinct. Monticules commonly vesicular in inner exozones, laminar in outer zones Acanthostyles lacking. Dark lines in walls. Vesicular (cyst-like) tissue in endozones. Trigonodictya b. Median rods elliptical, subelliptical and/or subcircular. (1) Mural styles absent or indistinct i. Monticules common, mesothecae straight to sinuous. Median rods subcircular. Acanthostyles not present in rows, zooecia not well aligned. Endozones have cyst-like vesicles. Arhrophragma (2) Mural styles common. i. Mesothecae slightly sinuous. A. Median rods elliptical to subcircular. Monticules rare to common, generally scattered in zoaria. Mural styles arranged singly or in discontinuous rows. Acanthostyles common. Explanate growth form. Elongate zooecia Phyllodictya ii. Mesothecae straight. A. Subelliptical median rods present, monticules common. Then ribbon growth form. Acanthostyles form rows separating aligned zooecia. Stictopora B. Explanate (flat sheet) growth form. Acanthopores abundant but not in strong rows. Median rods elliptical. Mural styles common in autozooecial boundaries and locally in walls. Monticules common and may be arranged in a rhombic pattern. Eurydictya 2. Median rods not abundant. a. Exilazooecia sparse or absent in zoarial midregions. (1) Zooecia oval. i. Autozooecial ranges in endozone alternating, farming 80—90' angle with themselves in exozones. Mesothecae sinuous, may zig-zag. Branching colony in which the edge has no zooecia. Graptodictya (2) Zooecia not oval. i. Zoarium unbranched autozooecial ranges in endozones aligned or alternating, forming 50-80° angle with mesothecae in exozones. Laminae on opposite sides of mesothecae intertongue along broadly 73 serrated zone in zoarial margin. Rhombic zooecia. Escharopora ii. Autozooecial boundaries generally not visible. Mesothecae straight, rarely zig-zagging. Straight rows of rectangular zooecia. Ptilodictya b. Exilazooecia common, regularly arranged in pairs or singly between successive autozooecia, or in groups along zoarial margins. (1) Zoariuru unbranched, autozooecia in straight ranges alternating in endozones. In exozones, autozooecia form 50-80" angles with mesothecae; contiguous or separated by exilazooecia locally. Discontinuous mesothecae. Monticules rare to absent; when present, flat to slightly raised. Exilazooecia subelliptical to irregularly polygonal, regularly arranged in pairs or singly in successive autozooecia. Stictoporella (2) Cribrate growth form. Exilazooecia irregularly polygonal to subcircular; arranged singly or in scattered groups in zoarium.. Monticules common to absent, generally raised and irregularly shaped. Stictoporellina B. Non-bilaminate colony form. 1. Encrusting colonies of adnate (encrusting on one side) simple zooids. a. Runner growth form of uniserial zooecia. ( l) Subtubular zooecia branching at characteristic angles to form indefinite polygons. "Stomatopora" (2) Proximal part of zooecium constricted for union with preceeding one. Corynonypa b. Non-uniserial zooecia (1) Multiserial expansions (tubes abutting to form fan) with surface of tubes marked by strong transverse wrinkles. Sagenella (2) Ribbon-er branched colony with tubes showing some arrangement, single zooecium may be enlarged to form ovicell. "Proboscina" 2. Colonies comoposed of tubular, closely adpressed zooids. a. Fenestrate colony form. (1) Ridge down center of each branch. i. No zooecia on back side of colony. 4 to 6 zooecia on each side of 74 ridge. Sardesonina (2) Lacks ridge down center of each branch. i. Mesozooecia absent. No zooecia on back side of colony; back side has faint striations. Reticulate branches with 2 or more rows of short tubed granular zooecia with diaphragms; apertures separated by irregularly placed acanthopores. Chasmatopora ii. Mesozooecia with closely spaced diaphragms and acanthostyles, 4 to 8 rows of aperatures on the front side of colony, none on the back side. Diaphragms widely spaced or lacking. Phylloporina b. Nonfenestrate colony form. (1) Zooecia absent on backside of colony. i. Zooecia radiate from a central axis. Narrow endozone, thick exozone. Typically, cross-section has only 3 zooecia. Arthrostylus (2) Zooecial opening all around colony. i. "Articulated" colony. A. Only 6 zooecia around branch. Few diaphragms. Thick walls and endozones. Dark styles. Sinuous, discontinuous dark lines. Arthroclema ii. Rigid, non-"articulated" colony. A. Diaphragms scattered in elongate zooecia; zooecia display eight-fold symmetry. Thick endozones. Zooids in very straight rows. Dark lines do not fan out to edge. Ulrichosrylus B. Diaphragms thickened, irregularly spaced. Acanthostyles large and well developed. Colony has more zooecia than Arthroclema. Non-branching. Helopora 3. Cylindrical branches. a. Zooids split from each other, appearing to unravel from each other. Branches very thin. Clonopora l l L | l l I Z / / / / / l / / / / l Appendix 6: Measured Sections Legend Calcareous Shale Limestone Dolomite Dolomitic Limestone Wavy Bedded Limestone Bentonite 75 m=Mudstone w=Wackestone p=Packstone g=Grainstone d=Dolomite dm=Dolomitic Mudstone dw=Dolomitic Wackestone dp=Dolomitic Packstone 76 1 meter mm: mm. m m wmwm ._. rmnm} xwrlsl} e e GEQEQE . . segues: . assesses: n o o 0 3:22:96: .m o o o 3.928»: W o o 333% m a. e e e e e e e co Semafig my a o o o o quagutS pm 0 a. 3333* M o . Semeéteé? .- e E39253. o o o o o §8e§§§e§~i 0 e e e e e e e Seusguum 000000 -e 0 one o .e’ecaeaegutm. 0:34:35 333 77 Preston Section 1 meter 3383i 0 o Eagetem o a. 5:43.83 . . engage: o .- oeaeneuagm o §~bu§ut5 . 3QE§< eEuEESuv. . o o §uue§§e§8m a eueqeuguum e e e 00 e e e a: 03.250834 Egan—Sm 338% 380 «co—«O 78 Mabel Section 1 meter _ J m mnmwmmmmmmnmmmn mm mm _ P_ _ _ . h _ — __ — _ _ _ _ q _ — — u d _ _ d _ _ a o 33954855 0 e e e usbnguahb o Sexueuctv. o o o nauaeaeufiueuaui o o o o o eueqeuefiam e e e e e e e e e e ee 0 e e Ecuficum. 22m c.8800 . nevus—.8"— £3833 79 Spring Grove Section .m P w P 8% 8 7 _ _ m _L|_ — _w mw :“ i “I u“ E22334. * “x: o o o gas—33335 o o o o a 393:5 o 0 «6.5835 c o 0 53908.82 0 o o o o o o o o o o o o 52535.5 0 o o o §~€S~ut~b o Benefit 0 m§~§oét< o Safiueét< o o o o o o o o gauuafiefifi. o o o o o o o o o o o cavucficuflfigauum o on o o o o o o o o o Eamoguam o 00 o o o o o o oQ on 00 00 on 0 o o 00 Eamguflm. .m m i...\1 m m 1 H14. \_\\__._ _w __ __*__._ _ ___ I; I I. .L .l . . II. II. I .I __:q _+;_.: i r.__. _ _1 _____.._ 1. ill] 2% $889 «fine»: Sampson. gags; 80 St. Paul Section w mdm d «__w $511.2. _ _m A.“ in m t O O O O '0 '0. g n W M x . \ J] // / / / f / / East ékbssx Eeggm 33383 3§§€< swansfigé ggzuum Ea§§a Sufism 0:335 Appendix 7 Other localities where species have been reported 81 Dl Johvi tofieldaren Caradoc Lhnvirn-Llandeilo Llandeilo Caradoc Caradoc Arthroclema cornutwn Arthroclema tofieldarea to area Cundoc Candoc Lhnvim-Umdeilo Llandeilo America NorthArner'iee North America North America North America NorthAmerien America ennessee New Centnl New out Centnl Central New ort Central Central New York Central New York Iowa Central New York St Lawrence Caradoc monruoswn to field are. Batostama ovata to field m Batostoma varium Caradoc Caradoc Dl Johvi D2 Keila tofieldaren 7 treMOnensr's Caradoc Caedoc Central New York Minnesota ennessee Northwestern Illinois divan‘cara to field are. cannulata mutabilis-minor ramosa Amsterdam C3 Idavere Carters ottawaense Car-doc Lhnvirn—Llandeilo to field are: Caradoc exilr‘s var. A tofieldaren to are: to arm Cundoc C3 ldever'e tuberculata ? interralaris Lower Viola instabilis C2 Kickers Northwestern Illinois C2 Kuckers Platteville Trenton Red Rover-Set New Market Clc Tnllin Caradoc Caradoc tofieldu'en Caradoc St. Central New York Central Tennessee to field area to field area corticosa Estonia Central New York Kansas Caradoc Caradoc to field area Llandeilo-Caradoc to field tea Llanv'n'n-Llandeilo Llandeilo Clc Tallin BIBLIOGRAPHY Bibliography Adams, J.A.S., Osmand, J.K., Edwards, 6., and Henle, W., 1960, Absolute dating of the Middle Ordovician: Nature, v. 188, p. 636-638. 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