Proceedings of the 9th Conference on Fossil Resources Kemmerer, WY, April 2011 pp. 83–109 FIELD TRIP GUIDE PALEONTOLOGY AND STRATIGRAPHY OF THE MIDDLE EOCENE BRIDGER FORMATION, SOUTHERN GREEN RIVER BASIN, WYOMING PAUL C. MURPHEY1 and EMMETT EVANOFF2 1Department of Paleontology, San Diego Natural History Museum, 1788 El Prado, Balboa Park, San Diego, California, 92101, [email protected] 2Department of Earth Sciences, University of Northern Colorado, Greeley, Colorado, 80639, [email protected] ABSTRACT—The middle Eocene Bridger Formation is located in the Green River basin in southwest Wyoming. This richly fossiliferous rock unit has great scientific importance and is also of historic interest to vertebrate paleontologists. Notably, the Bridger Formation is the stratotype for the Bridgerian North American Land Mammal Age. The fossils and sediments of the Bridger provide an important record of biotic, environmental, and climatic history spanning approximately 3.5 million years (49.0 to 45.5 Ma). Additionally, the high paleontological sensitivity of the formation, in combination with ongoing energy devel- opment activity in the southern Green River Basin, makes the Bridger Formation a paleontological resource management priority for the Bureau of Land Management. This paper features a detailed field excursion through portions of the Bridger Formation that focuses on locations of geologic, paleontologic and historical interest. In support of the field excursion, we provide a review of current knowledge of the Bridger with emphasis on lithostratigraphy, biochronology, depositional and paleoenvironmental his- tory, and the history of scientific exploration. “A large part of the collection in this region was of the remains of small animals. The fossils were generally found in the buttes, and on account of their minuteness, their discovery was attended with much difficulty. Instead of riding along on the sure-footed mule and looking for a gigantic tell-tale vertebra or ribs, it was necessary to literally crawl over the country on hands and knees…Often a quarter of a mile of the most inviting country would be carefully gone over with no result, and then again someone would chance upon a butte which seemed almost made of fossils.” —A description of fossil collecting in the Bridger Formation written by an unnamed member of the 1871 Yale College Expedition, led by paleontologist O.C. Marsh. INTRODUCTION AND STRUCTURAL SETTING the southern margin of the basin, and an approximately north- south axis (Koenig, 1960; Roehler, 1992a). The greater Green Situated to the north of the Uinta Mountains in the southern River basin is divided into four smaller basins by three intraba- Green River basin, Wyoming, the Bridger Formation is of great sin arches. The largest of these arches, the north-south trending scientific importance and is of historic interest to vertebrate pa- Rock Springs uplift, divides the basin into roughly equal halves, leontologists. The Bridger Formation has been the focus of pa- with the Green River basin to the west, and the Great Divide, leontological investigations for the last 140 years. Because of Sand Wash, and Washakie basins to the east. The Bridger basin its economic importance and exquisitely preserved vertebrate, is located within the southern part of the Green River basin. The invertebrate and plant fossils, the Green River Formation is per- term Bridger basin (Hayden, 1871) traditionally refers to an area haps the most familiar of the rock units within the Green River located north of the Uinta Mountains and south of the Blacks basin. Despite the geological and paleontological importance Fork of the Green River in Uinta and Sweetwater counties, Wy- of this world-renowned lacustrine rock unit, this field excursion oming, and is a physiographic, not a structural basin (Figure 1). is focused on the stratigraphically adjacent and overlying flu- The greater Green River basin began forming during the Lar- vial and lacustrine Bridger Formation, best known for its middle amide orogeny, a period of tectonism in western North America Eocene vertebrate fossils. The Bridger Formation is the strato- that was initiated during the late Cretaceous and continued for type for the Bridgerian North American Land Mammal “Age” approximately 30 million years until the late Eocene. In addi- (NALMA) (Gunnell et al., 2009; Wood et al., 1941). The fos- tion to the uplifting of surrounding mountain ranges, Laramide sils and sediments of the Bridger provide a critically important tectonism resulted in rapid subsidence in basin depositional cen- record of biotic, environmental, and climatic history spanning ters, and lacustrine and fluvial deposition in the intermontane approximately 3.5 million years (49.0 to 45.5 Ma). basins was mostly continuous. Lacustrine deposition was char- The greater Green River basin occupies 32,187 km2 of acterized by a complex history of expansions and contractions in southwestern Wyoming and northwestern Colorado (Roehler, response to basin subsidence, climatic conditions, and volcanic 1992a). Structurally, it is a large asymmetrical syncline with activity (Murphey, 2001; Murphey and Evanoff, 2007; Roehler, mostly gently dipping flanks (3º to 5º) with steeper dips along 1992b). 1 PROCEEDINGS of the 9th CONFERENCE on FOSSIL RESOURCES, APRIL 2011 FIGURE 1. Index map of the Greater Green River Basin showing the approximate location of the Bridger basin (type area of the Bridger Formation), major structural features, and surrounding uplifts (modified from Murphey and Evanoff, 2007). With its abundant and diverse vertebrate fossils and exten- vulnerable to direct impacts as the result of surface disturbing sive exposures, the Bridger Formation provides an excellent actions and indirect impacts from increased public access to opportunity to study middle Eocene continental environments public lands. Fortunately, as will be discussed during the field of North America. The dramatic and picturesque Bridger bad- trip, well documented fossil distribution patterns in the upper lands are an 842 meter (2,763 feet) thick sequence dominated by Bridger Formation provide a reliable source of information upon green-brown and red mudstone and claystone, with interbedded which to base management decisions. scattered ribbon and sheet sandstone, widespread beds of mi- This field trip offers participants the opportunity to examine critic, sparry, and silicified limestone, and thin but widespread paleontologically significant strata of the Bridger Formation in beds of ash-fall tuff (Evanoff et al., 1998; Murphey and Evanoff, the southern Green River basin. The following sections of the 2007). field trip guide provide a summary of the Cenozoic geologic From a resource management perspective, the richly fossilif- history of the Green River basin, as well as the history of in- erous Bridger badlands present a challenge to land managers— vestigations, stratigraphy, depositional and paleoenvironmental in particular the Wyoming Bureau of Land Management. With history, and fossils of the Bridger Formation. This is followed a Potential Fossil Yield Classification (PFYC, BLM 2008-007) by a detailed road log. Discussions during the field trip will also ranking of 5 (very high paleontologic potential) (Murphey and focus on some of the resource management issues that relate to Daitch, 2007), the Bridger Formation contains fossils in both the exquisite fossils of this world renowned rock unit. surface accumulations and subsurface occurrences, and both are 2 PROCEEDINGSMURPHEY and of EVANOFF the 9th CONFERENCE—BRIDGER FORMATIONon FOSSIL RESOURCES, (FIELD TRIP APRIL GUIDE) 2011 PALEOGENE GEOLOGIC History 2001, Murphey and Evanoff, 2007; Surdam and Stanley, 1979). OF THE GREEN RIVER BASIN, WYOMING Mauger (1977) and Surdam and Stanley (1979) estimated that Lake Gosiute was ultimately extinguished by about 44 Ma. The greater Green River basin was filled with Paleocene and The Bridger, Green River, and Washakie formations are lo- Eocene fluvial and lacustrine sediments; sedimentation appears cally and unconformably overlain by the Oligocene Bishop to have been continuous in most of the basin during the Eo- Conglomerate and the middle-to-late–Miocene Browns Park cene. The oldest Cenozoic rock units in the greater Green River Formation. Since the Eocene, the greater Green River basin has basin—the Paleocene Fort Union Formation and the early Eo- been modified by erosion, regional uplift, and normal faulting, cene Wasatch Formation—are exposed mostly along its eastern but the basic structure of the basin remains the same as it was and western flanks. During the Paleocene and earliest Eocene, during deposition of the Wasatch, Green River, Washakie, and deposition in the greater Green River basin was predominantly Bridger formations. fluvial, with epiclastic sediments accumulating in river drain- ages and on adjacent floodplains. The onset of lacustrine de- History OF Paleontological Investigations position associated with the Green River lake system may have IN THE BRIDGER Formation commenced as early as the late Paleocene (Grande and Buch- heim, 1994). Lake sediments accumulated on broad floodplains John Colter, who traveled to the headwaters of the Green of low topographic relief, and the lake waters expanded and con- River in 1807, was probably among the first non-Native Ameri- tracted numerous times over the next approximately five million cans to visit the Green River basin (Chadey, 1973). Hundreds years in response to climatic changes, tectonic