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 NEBRASKA+ OMAHA + ABSTRACT on strata as old as the Entrada Sandstone of Middle Jurassic (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 United States 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 years 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 year 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 Late Jurassic 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 unconformity areas in the central and northern parts of the representing a time span of roughly 16 my (million Western Interior including eastern Utah, northern years) or more. Some beds, previously assigned to Colorado, Wyoming, and Montana. Lower Cretaceous 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 Dinosaur 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 Colorado Plateau 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 crayfish, 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 Kimmeridgian in age and the uppermost variety of animals including dinosaurs, mammals, beds of the formation are Tithonian in age. The lizards, sphenodonts, choristodeirans, salamanders, spores and pollen suggest that the lowest beds in the snakes, small crocodilians, turtles, fish, lungfish, formation (Windy Hill Member) appear to be and frogs 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 fossil flora. The palynological limestone) 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 fossils 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 lakes 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 unconformities 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 bentonite 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 North America 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