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Turner et al.: The Morrison Formation Extinct Ecosystems Project

THE MORRISON FORMATION EXTINCT ECOSYSTEMS PROJECT

CHRISTINE E. TURNER + FRED PETERSON U.S. GEOLOGICAL SURVEY+ DENVER+ CO D.J. CHURE +U.S. NATIONAL PARK SERVICE+ JENSEN+ UT T.M. DEMKO+ EXXON PRODUCTION RESEARCH Co. +HOUSTON+ TX G.F. ENGELMANN+ UNIVERSITY OF + OMAHA

+ ABSTRACT on strata as old as the Entrada of Middle (middle Callovian) age. The Morrison Project is a multidisciplinary effort to interpret the ancient ecosystem that was A series of paleosols with locally abundant present in the Western Interior of the termite nests were found in the middle of the during deposition of the Upper Jurassic Morrison formation, just above the Salt Wash Member, and Formation. The project began in June of 1994 and appear to represent a hiatus or at least a brief period the first two of research (1994-95, 1995-96) of slowing in sedimentation rates. This is interpreted were devoted primarily to identifying problems and as a sequence boundary that probably has important gathering information that would form the basis for implications for regional correlations and later interpretations. Efforts during the fmal understanding varying sedimentation processes in (1996-97) were directed toward resolving remaining the depositional basin. problems or conflicting findings and synthesizing the various research endeavors into a reconstruction The precise stratigraphic position and of the ecosystem. nature of the upper contact was studied in considerable detail throughout the central and During the 1996 summer field season, the northern parts of the Western Interior and the Morrison Formation was studied in detail at selected contact appears to be a regional areas in the central and northern parts of the representing a time span of roughly 16 my (million Western Interior including eastern , northern years) or more. Some beds, previously assigned to , , and . Lower formations, were found to be Late Jurassic in age and thus are properly included in the Stratigraphically, significant progress was Morrison Formation. Work in the type area of the made. At National Monument, it was Morrison and Ralston Creek Formations just west of determined that the lower contact of the formation Denver demonstrates that both of these formations may be a simple depositional contact rather than an correlate with the entire Morrison Formation farther unconformity as previous workers had thought west in most parts of the or farther although there is good evidence that this contact is north in Wyoming. The charophytes and osctacqdes indeed unconformity farther south on the Colorado also support our correlations into Wyoming and Plateau where, in places, the Morrison rests directly Montana that were based largely on physical stratigraphy. They demonstrate that the upper

Published by Wyoming Scholars Repository, 1996 1 University of Wyoming National Park Service Research Center Annual Report, Vol. 20 [1996], Art. 4 21

Morrison contact there is stratigraphically higher , termites, soil-dwelling bees, dermistid than many previous workers had thought. The beetles, caddisflies, ants, earthworms, nematodes, calcretes that occur either in the uppermost beds of and other organisms were common in the Morrison the Morrison and in the lowermost beds of Lower ecosystem. Assemblages of these traces along with Cretaceous formations were originally thought to studies of paleosols can be used to define the have formed entirely by underground processes. position of the ancient water table with respect to However, recent studies suggest that at least some of the land surface. This gives a rough idea of soil the calcretes were intermittently exposed at the moisture as well as whether or not permanent water surface. existed at shallow depths and was therefore available to the plant community. Sedimentologic studies of the paleosols indicate that the terrain was wetter nearer fluvial Some of the bivalves show annual growth channels and became drier farther away in the bands whereas others do not. Perhaps those lacking overbank floodplains between the fluvial channels. these types of bands lived where equable conditions persisted throughout the year. The annual banding Palynological studies (that is, studies supports the interpretation of seasonality in the primarily of spores, pollen, and dinoflagellates) ecosystem that is suggested by other studies. resulted in detailed dating of much of the Morrison Formation. The research showed that the bulk of the Studies of the vertebrates reveal that a wide formation is in age and the uppermost variety of animals including , mammals, beds of the formation are in age. The , sphenodonts, choristodeirans, , spores and pollen suggest that the lowest beds in the snakes, small crocodilians, , , lungfish, formation (Windy Hill Member) appear to be and inhabited the Morrison ecosystem. With Kimmeridgian in age although a conclusive age the recent discovery and recovery of a new member determination for these beds awaits age of the Family Allosauridae from Dinosaur National determinations from dinoflagellates. These detailed Monument, we now know of at least three genera of age determinations allow us to compare the these theropods that existed within the Morrison Morrison ecosystem with the global Late Jurassic animal community. environment. In contrast to previous investigations, the formation was found to contain a surprisingly Samples from carbonate rocks (mostly rich and diverse flora. The palynological ) were collected during the summer field research also revealed that the flora was much more season for analyses of stable isotopes and are diverse in the Late Jurassic Western Interior than undergoing laboratory preparation and analysis. previously had been thought. Stable isotopes yield information on the ancient climate, plant community structure, weather Research on charophytes (green algae) and patterns, storm tracks, temperature, trophic levels, ostracodes (crustaceans) resulted in age and eating habits of the vertebrate land animal. A determinations that correspond closely with those tentative conclusion based on preliminary data is obtained from the palynomorphs. Other conclusions that the amount of carbon dioxide in the atmosphere derived from these micro-organisms suggest that was appreciably higher than today. Because this is a standing bodies of water in which strata in the "greenhouse" gas, it may have been responsible for middle of the formation were deposited were the warmer global climate during the Jurassic Period somewhat brackish whereas the water bodies that as compared with today's climate, although we do were present during deposition of the lower and not yet know why the carbon dioxide levels were so upper beds largely consisted of fresh water. More high during the Late Jurassic. Detailed studies of salinity-tolerant genera existed in northern Colorado paleosols in the Fruita Paleontological Area (FPA) and northeastern Utah than farther south or north, of western Colorado and elsewhere in the Western which might also reflect the nature of water bodies Interior suggest that the Morrison climate was at in these areas. These organisms also suggest a least seasonally dry. Detailed sedimentologic studies gradual cooling of the climate during Morrison are underway in the FPA and should lead to an deposition. improved understanding of Morrison sedimentation in a relatively small area that could have important Studies of trace that can be related to implications for the overall project interpretations. specific organisms demonstrate that horseshoe crabs, Other studies of freshwater limestone beds indicate a

2 Turner et al.: The Morrison Formation Extinct Ecosystems Project 22

variety of life forms including charophytes, contains locally abundant and diverse fossil plant ostracodes, snails, fish, sponges, stromatolites, and and other animal communities that were fish lived in Morrison and will likewise shed contemporaries of the dinosaurs and that shed light light on this aspect of the Morrison ecosystem. onto the nature of the Late Jurassic ecosystem.

Research by 40Ar/39Ar isotopic dating 1/0• !OS" methodologies on altered volcanic ash deposits in the Morrison indicates that the formation was deposited during an approximately 8 million year so' time span from about 155 to 147 Ma (Ma = Milli­ annums = million years before the present). More samples were collected to better date the formation and to gain an understanding of the nature of the at the base and top of the Morrison. Tentative conclusions are that the time span represented by the K-1 unconformity at the top of the Morrison was approximately 16 my in duration but may be more or less depending on the locality. Geochemical studies of mineral grains recovered from Morrison beds (the altered volcanic ash deposits) indicate that the volcanoes that furnished the ash were rhyolitic in composition. This is consistent with the origin of the ash deposits in a volcanic belt above a subduction zone that extended along the west coast of during the Late Jurassic. The composition of certain mineral grains recovered from the bentonite beds -z----· and the known predominantly westerly to southwesterly winds over the Colorado Plateau region during the Late Jurassic suggest that most of J1 the volcanoes that furnished the Morrison ash were in California, western Nevada, and western .

Figure 1.-The Morrison depositional basin in the + BACKGROUND Western Interior of the United States and southern Canada. Also shown are National Park Service units The Morrison Formation 1s distributed that contain significant exposures of the Morrison throughout much of the Western Interior Formation. physiographic province of the United States and is one of science's best windows into the world of ' ARCH (Arches National Park), dinosaurs and nonmarine ecosystems. BIHO (Bighorn National Recreation Area), Because of its varied environments, rich fossil BLCA (Black Canyon of the Gunnison deposits, extensive rock exposures, and broad National Monument), geographic distribution, the formation offers an CARE (Capitol Reef National Park), outstanding opportunity for a multi-park, COLO (Colorado National Monument), interdisciplinary approach to the evolution of CURE (Curecanti National Recreation environments, habitats, communities, and climate Area), through some 8 million years of Earth history. The DINO (Dinosaur National Monument), formation covers some 700,000 square miles in the GLAC (Glacier National Park), Western Interior (Figs. 1, 2) and is world renowned GLCA (Glen Canyon National Recreation for its rich concentration of dinosaur remains, with Area), more than 100 dinosaur quarries identified to date HOVE (Hovenweep National Monument), and new ones discovered yearly. The formation also YELL (Yellowstone National Park).

Published by Wyoming Scholars Repository, 1996 3 University of Wyoming National Park Service Research Center Annual Report, Vol. 20 [1996], Art. 4 23

The outline of the Morrison depositional basin is The Morrison Formation Extinct modified from: Ecosystems Project, hereafter called the Morrison Project, began on June 1 of 1994. The project is a multidisciplinary and multiinstitutional endeavor designed to determine the nature, distribution, and Af:[ ,w. JTRATICRAPHIC VIII r 10 evolution of the ancient ecosystems that existed in r...... LATE the Western Interior of the United States during the CI:ETAC[OUS _"'"f" ~-- ·1 lA•I< Late Jurassic Epoch when the Morrison Formation (r. .o c~,. . , ~, .- . ,. · h'- PJJ, J,~ !~ "4rln•~ trrr -r-z u711~~,1.,_ , , /Of>- · and related rocks were deposited.

AfJ; •• The information obtained from the research can be used to suggest appropriate resource ItO management actions. The project will also provide ( . ,J., .., - Mo ~,t ~ ilf an improved understanding of the geological and Fer,.,#ti. ,. ~ EAIUY At''' " paleontological history of NPS units and better ~ C~ETACEOI.IS 110 information for interpretive programs and ~ publications. ~ "' /30 ?l----_; An expanded summary of progress, products, and interpretations by the individual investigators are available upon request from C.E. t'lo- Turner. Because the research is continuing, any interpretations must be considered tentative and

.... LATE subject to revision after additional research has been .$ 71/I?ASS/C accomplished .

+ CONCLUSIONS

The Morrison Project is a multidisciplinary effort designed to investigate the nature and Figure 2.-The age of the Morrison Formation in evolution of environments, habitats, communities, northeastern Utah. Compiled from Kowallis and and climate in and near many National Park Service others (1991) and B.J. Kowallis (written units in the Western Interior of the United States communication, 1994). through some 8 million years of Earth history when the Morrison Formation of Late Jurassic age was deposited. The project was devoted primarily to The Morrison Formation has significant gathering information that would form the basis for exposures in many units within the Western Interior later interpretations. Hence, scientific interpretations of the United States. These include Arches National concerning the ancient Morrison ecosystem are Park (ARCH), Bighorn National Recreation Area tentative in this report because of the recency of the (BIHO), Black Canyon of the Gunnison National research. The Morrison Formation was studied Monument (BLCA), Capitol Reef National Park mostly in Utah, Colorado, Wyoming, and Montana (CARE), Colorado National Monument (COLO), but also to some extent in Arizona and Curecanti National Recreation Area (CURE), to provide a more accurate regional framework and Dinosaur National Monument (DINO), Glacier to collect samples that might require long National Park (GLAC), Glen Canyon National preparation time in the laboratory. Included here are Recreation Area (GLCA), Hovenweep National brief discussions of interpretations according to Monument (HOVE), and Yellowstone National Park major subspecialty categories. The last names of the (YELL). In addition, the formation is not exposed at researchers most involved in the specific topics are the surface but is present beneath approximately enclosed in brackets. 10 other National Park Service units in the Western Interior.

4 Turner et al.: The Morrison Formation Extinct Ecosystems Project 24

STRATIGRAPHY AND SEDIMENTOLOGY calcretization associated with early diagenesis during deposition of overlying Lower Cretaceous deposits. Regional stratigraphic and sedimentologic These two paleosols are interpreted to mark the tops studies [Turner and Peterson] demonstrate that strata of two aggradational sequences that comprise deposited in relatively dry environments are fairly Morrison deposition. common in the Morrison Formation in the southern and central parts of the Western Interior. These The calcretes in the uppermost part of the include extensive evaporite deposits (chiefly Morrison Formation and in the lowermost part of gypsum) in the lowermost part of the formation in the Lower Cretaceous are Utah and Colorado and locally in eastern Wyoming; being studied petrographically and can be classified large eolian field deposits (erg deposits) at or as mostly alpha type calcretes [Skipp]. That is, they near the Four Comers area; small eolian sandstone consist largely of a dense micrite to microcrystalline beds scattered through Arizona, Utah, Colorado, groundmass that contains peloids, nodules, and extending as far north as northern Wyoming; circumgranular cracks, floating grains (both detrital and the rather extensive alkaline, saline deposits and authigenic}, coated grains, coated fractures, and of ancient Lake T' oo' dichi' in the eastern part of the complex arrays of micro- to macro-fractures. In Colorado Plateau. Studies of the paleosols in the addition to the microcrystalline textures, some overbank floodplain areas [Demko] also suggest dry coarse recrystallized textures were observed in climates. Farther north in west-central Montana, samples collected in Salt Valley anticline near extensive beds in the upper part of the Arches National Park, Utah. Very few biogenic Morrison suggest deposition there was in a more textures were observed. There are suggestions of humid climate than existed farther south. subaerial exposure on several of the surfaces within some of the calcretes, suggesting that the calcretes Fluvial and overbank architectural styles formed at shallow depths in the sediment package [Demko, Newell] are suggestive of seasonally wet­ and that they may have formed locally at the surface dry precipitation-discharge regimes. Floodplain or were exposed briefly by subaerial erosion paleosols in the Morrison Formation are associated processes. Additional studies are underway that will with each of the fluvial deposits. Thin floodplain characterize the calcretes further and, hopefully, will paleosols in the Morrison Formation, like the provide more information pertaining to the channel and overbank architectural styles, are paleoclimatic conditions that governed their indicative of at least periodic dryness. formation.

Two sequence-bounding paleosols (or paleosol complexes) have been identified in the Morrison Formation and represent significant The terrestrial facies of the Morrison is diastems in the part of the formation where they fossiliferous in all areas where the formation is occur. The first is immediately above the Salt Wash sufficiently exposed to allow a reasonable chance of Member (within the lower part of the Brushy Basin encountering the fossils [Engelmann]. However, Member). This paleosol complex marks a change in there is a variation in the geographic and fluvial style from low- to high-sinuosity planform stratigraphic distribution of some types of fossils. patterns. Some contemporaneous erosion is Both of these observations have important associated with this horizon in the Dinosaur implications for resource management in the parks National Monument area. The climatic implication and monuments that contain outcrops of the of this paleosol is no different from those of the Morrison Formation. In addition, Morrison faunas other calcic paleosols in the Salt Wash and Brushy are diverse, including fossils of vertebrates, Basin Members and is only differentiated by its invertebrates, and traces of both, and plant fossils of greater development and regional extent. The second various types. In some of the study areas, sequence-bounding paleosol complex is at the top of invertebrate trace fossils, including the burrows of the Brushy Basin Member, at its boundary with crayfish, termites, ants, beetles, and other insects, overlying Lower Cretaceous deposits. This paleosol were found to be abundant, particularly within the represents a long period of exposure and upper Salt Wash and lower Brushy Basin Members. pedogenesis ( > 5 my) and possibly the overprinting This horizon may be explained by modeling of different climatic regimes (wetter upon drier). Morrison sedimentation in that area as accumulation This interval is also typically overprinted by later

Published by Wyoming Scholars Repository, 1996 5 University of Wyoming National Park Service Research Center Annual Report, Vol. 20 [1996], Art. 4 25 in a tectonically-controlled depositional basin with The shells are well sorted by size; however, climatic and ecological controls. fragmentation and abrasion are minimal. These features indicate a transported assemblage that bas Based on comparative studies of the not been moved very far from the original life mammalian faunas of the Morrison, [Englemann] habitat (probably less than a few kilometers, at there is some geographic variation. Mammal faunas most). The shell form of the bivalves indicate they from the Fruita Paleontological Area near Colorado inhabited a fast velocity, small fluvial system. The National Monument and from Dinosaur National abundance of the specimens suggests that a nearby Monument lack docodonts, which is typical for upstream optimum habitat for unionid bivalves Morrison faunas west of the . This provided shells from the dead and decayed members lack of docodonts contrasts with the presence of of the population. Only a few articulated specimens them from even small samples of Morrison have been found and they are generally not in life mammalian fossils east of the Rockies. This may be position, suggesting they were transported, buried, explained by the presence of a divide separating the and unable to reestablish life functions. The Morrison into eastern and western geographic disarticulated valves, which dominate the bivalve subareas, an interpretation that may be suggested by assemblages, require days to weeks of postmortem other stratigraphic evidence. decay of the ligament to disjoin the valves, suggesting that the fauna represents a transported Illegal collecting of fossils within the Park death assemblage. The unique dispersal mechanism Service units studied does not appear to have been employed by unionacean bivalves permits intensive, in contrast to the evidence of extensive documentation of paleodrainage confluence patterns, collecting on State and Federal lands adjacent to the which will enhance understanding the Park Service lands [Engelmann]. This makes the paleogeography of the Morrison basin. interpretation of observations on fossil occurrences somewhat problematic. In particular, the apparent Paleoecological interpretations comparing scarcity of silicified wood in the vicinity of Arches the ostracode and charophyte genera with their National Park and Colorado National Monument modem counterparts suggest mostly nonmarine relative to occurrences at Dinosaur National environments, although the salinity could have been Monument may be the result of intensive collecting slightly higher in several beds, perhaps to about 16 by rock hounds. parts per thousand in some cases [Schudack] . An analysis of the spatial distribution of both ostracodes Preliminary work indicates that the and charophytes suggests close but complex dominant group of theropod dinosaurs from the biogeographic relationships between North America Morrison Formation belong to the Family and Europe during the Late Jurassic. Allosauridae [Chure]. A new and relatively primitive allosaurid from the Salt Wash Member The wide variety of palynomorphs suggests that was recently excavated at Dinosaur National that much of the Morrison depositional basin Monument ts currently being studied. contained a highly diverse mesic flora during much Reexamination of the original material of or most of Morrison time [Litwin]. Saurophaginax (originally Saurophagus) from the uppermost Morrison ·of western has The integration of ichnological (i.e., trace established that it is genetically separate from fossil) information [Hasiotis] combined with . Thus, at least three genera from Family paleontological, sedimentological, paleopedological, Allosauridae (the new Salt Wash primitive and geochemical data, produces a more holistic allosaurid, Allosaurus, and Saurophaginax) existed reconstruction of the paleoecological settings during during the Late Jurassic in the Western Interior of Morrison times. Though some contradictions do the United States. exist between different sources of data, the results are still meaningful. In general, the traces reflect the Taphonomic analysis on the bivalve sedimentological pattern of a drier climatic setting in assemblage preserved in the Quarry Sandstone Bed the lower Morrison to a slightly more seasonally wet at Dinosaur National Monument demonstrates that climatic setting at the end of Morrison deposition. the shells occur in current-stable orientations, ne interpretations deduced from the trace fossils locally with imbricated stacking, in troughs of mid­ corroborates many of the interpretations from the channel bars within the ancient stream bed [Good]. molluscs, pollen, and plants that suggest there were

6 Turner et al.: The Morrison Formation Extinct Ecosystems Project 26

periods of seasonally ample rainfall during numerical ages in terms of millions of years before deposition of portions of the Morrison Formation. the present (Ma). The Brushy Basin Member at the The trace fossils, along with the carbonate paleosols top of the formation gives single-crystal, laser­ and geochemical analysis, support a seasonally wet­ fusion and step-heating, plateau 40Ar/39Ar ages on dry climatic setting for most of the Morrison, but sanidine that range between 147.8 Ma at the top of differs in that the traces (especially the rhizoliths the member and 150.2 Ma at the base of the and crayfish) show slightly higher precipitation than member. The Tidwell Member at the base of the what might be expected. The diversity of formation contains one datable ash bed about 3 m invertebrate and vertebrate body fossils and trace (10 ft) above the J-5 unconformity that has been fossils (e.g. invertebrates) suggests that the overall found in at least two widely separated localities. sedimentologic and paleoenvironmental landscapes This bed has been dated using 40Ar/39Ar dating of had to sustain climates analogous or roughly sanidine and gives ages ranging between 154.75 and analogous to today's savannah of the southern 154.87 Ma. The Morrison Formation, therefore, African continent. The larger vertebrates probably ranges in age from about 147 to 155 Ma. By migrated latitudinally between conterminous comparison with recent standard geologic time landscapes shaped by different depositional systems. scales that attempt to calibrate the relative time scale based upon fossils with isotopic dates, the Morrison STABLE ISOTOPE GEOCHEMISTRY Formation and related beds are entirely Late Jurassic in age. Many fossil materials retain their carbon and oxygen stable isotopic compositions through The field work for the Morrison Extinct geologic time. A suite of materials from the Ecosystem Project has ended and efforts currently Morrison Formation (fossil soil nodules, plant underway are devoted to further laboratory materials, eggshells, teeth, lake sediments) have determinations and examinations and to synthesizing been analyzed [Ekart and Cerling] to determine the results of the research effort. A workshop among patterns in the distribution of isotopes. Focus during the project participants was held in April, 1996, in the last year was on carbonate nodules from Denver for the primary purpose of evaluating the paleosols (fossil soils) in the Brushy Basin Member progress thus far and to focus the remaining field of the Morrison Formation. The isotopic data are and laboratory studies toward specific or remaining evidence of a rain shadow operating in much of the problems to ensure that the project goals are Western Interior during the Late Jurassic, which is attained. Participants were asked to prepare consistent with paleogeographic reconstructions that presentations of their data so that we could place the Morrison landscape east of a collectively evaluate where the group was reaching Mesocordilleran mountain range. agreement (convergence) and where remaining controversies (points of divergence) remained. This AGE DETERMINATIONS report is largely a summary of research thus far accomplished, with the current but preliminary Research on the relative age of the status of knowledge and interpretations that have Morrison Formation based on studies of been generated up to the date of this report by the palynomorphs (spores, pollen, and dinoflagellates) project participants. [Litwin] and calcareous microfossils (charophytes and ostracodes) [Schudack] has established a much more refined relative age for the formation. It is + LITERATURE CITED now clear that all of the Morrison Formation and related beds are entirely Late Jurassic in age and Kowallis, B.J., Christiansen, E.H., and Deino, were deposited during the latest Oxfordian(?), A.L., 1991, Age of the Brushy Basin Kimmeridgian, and early Tithonian Ages. None of Member of the Morrison Formation, the microfossil evidence corroborates previously Colorado Plateau, western USA: published accounts proposing an Cretaceous Research, v. 12, p. 483-493. age for some of these beds.

Isotopic dating of altered volcanic ash beds in the The Morrison Formation in Utah and Colorado [Kowallis and Christiansen] yielded

Published by Wyoming Scholars Repository, 1996 7 University of Wyoming National Park Service Research Center Annual Report, Vol. 20 [1996], Art. 4 27

Peterson, J.A., 1972, Jurassic System; in, Mallory, Poulton, T.P., Christopher, J.E., Hayes, B.J.R., W.W. (ed.), Geologic Atlas of the Rocky Losert, J., Tittemore, and J., Gilchrist, Mountain Region: Rocky Mountain R.D., 1994, Jurassic and lowermost Association of Geologists, p. 177-189 (map Cretaceous strata of the western Canada on p. 185). sedimentary basin; in, Mossop, G., and Shetsen, 1., (compilers), Geological Atlas of the western Canada sedimentary basin: Canadian Society of Petroleum Geologists and Alberta Research Council, p. 297-316 (map on p. 312).

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