Introduction to the Bridger Formation, southern Green River Basin, southwestern Wyoming

   




 

During the latest Cretaceous Period and the Paleocene and Eocene epochs of the Tertiary Period (~ 75 to 40 Ma), the Rocky Mountains of western North America were uplifted. Geologists have named this period of mountain building the Laramide Orogeny, and it resulted in the formation of mountains and intervening basins in which thick sequences of sediment accumulated. In the central Rocky Mountains, the primary sources of these basin sediments included erosion from neighboring mountain ranges, ash and volcaniclastic debris from volcanoes, and organically- and evaporitically- precipitated sediments which formed in lakes and ponds and on playas. Over millions of years, the remains of the animals and plants that lived in this region were deposited in basin sediments, and some were preserved as fossils. These fossils are scientifically important because they represent an important time in the evolution of animals and plants following the extinction of the dinosaurs 65 million years ago (Ma). During the Eocene Epoch (55 to 34 Ma), most of the modern mammalian orders evolved, and more archaic groups went extinct.

View looking south at exposures of the Twin Buttes and Turtle Bluff members (subdivisions C - E) of the upper Bridger Formation with Gilbert Peak and the Uinta Mountains in the background.

Dorsal view of the skull of the fossil crocodile Leidyosuchus (CM 11435) collected from Twin Buttes, Sweetwater County, Wyoming.
Between approximately 49.5 and 46 Ma, during the middle Eocene Epoch, the Bridger Formation was deposited in what is now known as the Green River Basin in southwestern Wyoming. Sediments which comprise the Bridger Formation were deposited in a variety of depositional environments including river channels, floodplains, lakes, ponds and alluvial fans which formed at the bases of the northern foothills of the Uinta Mountains. As indicated by both fossils and the sediments in which they were preserved, middle Eocene environments of Wyoming were vastly different than the dramatic and picturesque high desert badlands that exist there today.
Paleontologists study ancient organisms and the environments in which they lived by interpreting many different types of evidence present in rocks and fossils. Although paleontologists have been studying fossils from the Bridger Formation for more than 135 years, concerted efforts to document the stratigraphy of the formation and provide a detailed stratigraphic framework for the Bridger were not initiated until the 1990's. Researchers and students from the University of Colorado Museum, along with colleagues from other institutions, began a program of detailed stratigraphic fossil collection, stratigraphy and sedimentology, and geologic mapping. This research was focused largely on the upper part of the Bridger Formation, which is subdivided into two members: the Twin Buttes and Turtle Bluff members. Our ongoing research projects seek to interpret the detailed depositional history, paleoenvironments, taphonomy, paleoecology, biostratigraphy, and geochronology of this scientifically and historically important rock unit and its world-renowed fossils.

Map of the Greater Green River Basin in southwest Wyoming showing major structural features and the location of the Bridger Basin Project study area.

Satellite photograph of southwestern Wyoming, northwestern Colorado, northeastern Utah, and southeastern Idaho, showing the location of the Bridger Basin Project study area in red.

This website is intended to provide an overview of the Bridger Formation, including previous investigations, its paleontology, geology, stratigraphy, some of the results of previous research conducted by the author and colleagues, and current projects. It also includes a bibliography of scientific literature related to the geology and paleontology of the Bridger Formation. The website for the Bridger Basin Project was authored and is maintained by Dr. Paul C. Murphey, Paleontology Principal Investigator at SWCA Environmental Consultantsand a Research Associate in the Department of Paleontology, San Diego Natural History Museum.

Financial or logistical support for the Bridger Basin Project, part of which comprised the author's doctoral dissertation, was provided by the University of Colorado Museum Walker Van Riper Fund and William H. Burt Fund, the Wyoming Geological Association J. David Love Wyoming Field Geology Fellowship, the Colorado Scientific Society Steven J. Oriel Memorial Fund, the Paleontological Society, the University of Colorado Deans Small Grant Program, the University of Colorado Department of Geological Sciences Mentoring Program, the USDI Bureau of Land Management, and the U.S Geological Survey.

The advice, assistance and support of the many friends, colleagues and students who have contributed to this research is gratefully acknowledged. Please direct questions and other inquiries to pmurphey@rockymountainpaleontology.com, or pmurphey@sdnhm.org.

 

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