The Geological Society of America Field Guide 21 2011

Paleontology and stratigraphy of middle Eocene rock units in the Bridger and Uinta Basins, Wyoming and Utah

Paul C. Murphey* Department of Paleontology, San Diego Natural History Museum, 1788 El Prado, Balboa Park, San Diego, California 92101, USA

K.E. Beth Townsend* Arizona College of Osteopathic Medicine, Midwestern University, 19555 N. 59th Avenue, Glendale, Arizona 85308, USA

Anthony R. Friscia* Department of Integrative Biology and Physiology, University of California, Los Angeles, 621 Charles E. Young Drive So., Los Angeles, California 90095-1606, USA

Emmett Evanoff* Department of Earth Sciences, University of Northern Colorado, Greeley, Colorado 80639, USA

A grand scene burst open us. Fifteen hundred feet below us lay the beds of another great Tertiary lake. We stood upon the brink of a vast basin so desolate, wild, and broken, so lifeless and silent, that it seemed like the ruins of the world.

—Charles Betts’ description of the fi rst glimpse of the Uinta basin in his account of the 1870 Yale College Expedition led by paleontologist O.C. Marsh, published in Harper’s magazine, 1871, v. 43, p. 66.

ABSTRACT

The Bridger Formation is located in the Green River basin in southwest Wyo- ming, and the Uinta and Duchesne River formations are located in the Uinta basin in Utah. These three rock units and their diverse fossil assemblages have great scien- tifi c importance and are also of historic interest to vertebrate paleontologists. Nota- bly, they are also the stratotypes for the three middle Eocene North American Land Mammal “Ages,” the Bridgerian, Uintan, and Duchesnean, from oldest to youngest. The fossils and sediments of these formations provide a critically important record of biotic, environmental, and climatic history spanning ~10 million years (49–39 Ma). This article features a detailed fi eld excursion through portions of the Green River and Uinta basins that focuses on locations of geologic, paleontologic, and historical interest. In support of the fi eld excursion, we also provide a review of current knowl- edge of these formations with emphasis on lithostratigraphy, biochronology, deposi- tional and paleoenvironmental history, and the history of scientifi c exploration.

*[email protected]; [email protected]; [email protected]; [email protected].

Murphey, P.C., Townsend, K.E.B., Friscia, A.R., and Evanoff, E., 2011, Paleontology and stratigraphy of middle Eocene rock units in the Bridger and Uinta Basins, Wyoming and Utah, in Lee, J., and Evans, J.P., eds., Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain, and Terranes of the U.S. Cordillera: Geological Society of America Field Guide 21, p. 125–166, doi:10.1130/2011.0021(06). For permission to copy, contact [email protected]. ©2011 The Geological Society of America. All rights reserved.

125 126 Murphey et al.

INTRODUCTION AND STRUCTURAL SETTING The greater Green River basin occupies 32,187 km2 of south- western Wyoming and northwestern Colorado (Roehler, 1992a). Situated to the north and south of the Uinta Mountains in Structurally, it is a large asymmetrical syncline with mostly gen- Wyoming and Utah, respectively, the Bridger and Uinta basins tly dipping fl anks (3° to 5°) with steeper dips along the south- have great scientifi c importance and are of historic interest to ver- ern margin of the basin, and an approximately north-south axis tebrate paleontologists. The rock units and fossils of the Bridger (Koenig, 1960; Roehler, 1992a). The greater Green River basin basin (actually part of the southern Green River basin) and the is divided into four smaller basins by three intrabasin arches. The Uinta basin have been the focus of paleontological investiga- largest of these arches, the north-south–trending Rock Springs tions for the past 140 years. Perhaps the most familiar of the uplift, divides the basin into roughly equal halves, with the Green rock units within the Green River and Uinta basins is the Green River basin to the west, and the Great Divide, Sand Wash, and River Formation because of its economic importance and exqui- Washakie basins to the east. The Bridger basin is located within sitely preserved vertebrate, invertebrate and plant fossils. How- the southern part of the Green River basin. The term Bridger ever, despite the geological and paleontological importance of basin (Hayden, 1871) traditionally refers to an area located north this world renowned lacustrine rock unit, this fi eld excursion is of the Uinta Mountains and south of the Blacks Fork of the Green focused on three closely related stratigraphically adjacent and River in Uinta and Sweetwater counties, Wyoming, and is a phys- overlying fl uvial rock units that are best known for their assem- iographic, not a structural basin (Fig. 1). blages of middle Eocene vertebrate fossils. The Bridger, Uinta, The Uinta basin occupies 10,943 km2 of northeastern Utah. and Duchesne River formations are the stratotypes for the Bridg- Structurally, it is an asymmetrical, elongate east-west–trending erian, Uintan and Duchesnean North American Land Mammal synclinal basin bounded by the Uinta Mountains to the north, “Ages” (NALMAs) (Gunnell et al., 2009; Wood et al., 1941). the Douglas Creek Arch and Roan Plateau to the east, the Book The fossils and sediments of these formations provide a critically Cliffs/Tavaputs Plateau to the south, and the Wasatch Range important record of biotic, environmental, and climatic history to the west (Fig. 2). It was formed in the latest Cretaceous and spanning ~10 million years (49–39 Ma). Paleocene during the Laramide uplift of the Uinta Mountains.

Figure 1. Index map of the greater Green River basin showing the approximate lo- cation of the Bridger basin (type area of the Bridger Formation), major structural features, and surrounding uplifts (modi- fi ed from Murphey and Evanoff, 2007). Middle Eocene rock units in the Bridger and Uinta Basins 127

The Uinta basin is closely related structurally and sedimento- deposition was characterized by a complex history of expansions logically to the Piceance Creek basin in northwestern Colorado. and contractions in response to basin subsidence, climatic condi- The Uinta and Piceance Creek basins are separated by the Doug- tions, and volcanic activity (Murphey, 2001; Murphey and Eva- las Creek arch, a broad north-south–trending anticline that sepa- noff, 2007; Roehler, 1992b). rated the two sedimentary basins until the early-middle Eocene, when the two basins coalesced across the top of the arch to form PART I. BRIDGER BASIN FIELD TRIP one large sedimentary basin (Moncure and Surdam, 1980; John- son, 1985, 1989). Like the Uinta basin, the Piceance Creek basin With its abundant and diverse vertebrate fossils and exten- is highly asymmetrical (Johnson, 1985). Early Cenozoic strata sive exposures, the Bridger Formation provides an excellent in the Uinta basin dip gently from all directions to the northern opportunity to study middle Eocene continental environments of margin of the basin, where the strata are sharply upturned and North America. The dramatic and picturesque Bridger badlands faulted along the southern fl ank of the Uinta Mountains uplift are an 842 m (2763 ft) thick sequence dominated by green-brown (Johnson, 1985). and red mudstone and claystone, with interbedded scattered rib- The greater Green River, Uinta, and Piceance Creek basins bon and sheet sandstone, widespread beds of micritic, sparry, and began forming during the Laramide orogeny, a period of tec- silicifi ed limestone, and thin but widespread beds of ash-fall tuff tonism in western North America that was initiated during the (Evanoff et al., 1998; Murphey and Evanoff, 2007). late Cretaceous and continued for ~30 million years until the This fi eld trip offers participants the opportunity to exam- late Eocene. In addition to the uplifting of surrounding moun- ine paleontologically signifi cant strata of the Bridger Forma- tain ranges, Laramide tectonism resulted in rapid subsidence in tion in the southern Green River basin. The following sections basin depositional centers, and lacustrine and fl uvial deposition of the fi eld trip guide provide a summary of the Cenozoic geo- in these intermontane basins was mostly continuous. Lacustrine logic history of the Green River basin, as well as the history of

Figure 2. Index map of the Uinta Basin showing adjacent struc- tural and physiographic features. The gray region represents the areal extent of Tertiary deposits in both the Uinta basin, Utah, and Piceance Creek basin (not labeled), western Colorado. The Douglas Creek Arch separates the Uinta basin from the Piceance Creek basin. 128 Murphey et al.

investigations, stratigraphy, depositional and paleoenvironmental the Absaroka Volcanic Field in what is now northwestern Wyo- history, and fossils of the Bridger Formation. This is followed by ming. These sediments were washed into the basin in rivers and a detailed road log. streams. Some volcaniclastic sediments were transported into the basin via eolian processes and deposited as ash fall in lakes and Paleogene Geologic History of the Green River on fl oodplains. A large infl ux of fl uvially transported volcanicla- Basin, Wyoming stic sediment is believed to have led to the fi nal middle Eocene fi lling of Lake Gosiute (Mauger, 1977; Murphey, 2001; Murphey The greater Green River basin was fi lled with Paleocene and Evanoff, 2007; Surdam and Stanley, 1979). Mauger (1977) and Eocene fl uvial and lacustrine sediments, and, during the and Surdam and Stanley (1979) estimated that Lake Gosiute was Eocene, sedimentation appears to have been continuous in most ultimately extinguished by ca. 44 Ma. of the basin. The oldest Cenozoic rock units in the greater Green The Bridger, Green River, and Washakie formations are River basin, the Paleocene Fort Union Formation and the early locally and unconformably overlain by the Oligocene Bishop Eocene Wasatch Formation, are exposed mostly along its eastern Conglomerate and the middle-to-late–Miocene Browns Park and western fl anks. During the Paleocene and earliest Eocene, Formation. Since the Eocene, the greater Green River basin has deposition in the greater Green River basin was predominantly been modifi ed by erosion, regional uplift, and normal faulting, fl uvial, with epiclastic sediments accumulating in river drain- but the basic structure of the basin remains the same as it was ages and on adjacent fl oodplains. The onset of lacustrine deposi- during deposition of the Wasatch, Green River, Washakie, and tion associated with the Green River lake system may have com- Bridger formations. menced as early as the late Paleocene (Grande and Buchheim, 1994). Lake sediments accumulated on broad fl oodplains of low History of Paleontological Investigations in topographic relief, and the lake waters expanded and contracted the Bridger Formation numerous times over the next approximately fi ve million years in response to climatic changes, tectonic infl uences, and epi- John Colter, who traveled to the headwaters of the Green sodic volcanic activity. River in 1807, was probably among the fi rst non–Native Ameri- Occupying the center of the basin in the shape of a large, cans to visit the Green River basin (Chadey, 1973). Hundreds irregular lens (Bradley, 1964; Roehler, 1992b, 1993), the Green of subsequent trappers and explorers traversed the basin during River Formation is the result of at least fi ve million years of the fi rst half of the nineteenth century, and a number of records lacustrine deposition lasting from ca. 53.5 to 48.5 Ma (Smith of these early explorations make reference to fossils and coal et al., 2003), although lacustrine deposition may have persisted (Roehler, 1992a). The earliest scientifi c observations on the later in the southernmost part of the basin along the Uinta Moun- geology of the Green River basin were made by Army Lt. John tain front (Murphey and Evanoff, 2007). The Green River For- C. Fremont. After entering the basin through South Pass at the mation was deposited in a vast ancient lake system that existed southern end of the Wind River Mountain, Fremont (1845) from the late Paleocene to the middle Eocene in what is now described varicolored rocks (now known as Eocene-age Wasatch Colorado, Utah, and Wyoming. The smallest and oldest of these Formation) along the Big Sandy and New Fork rivers. He also lakes, Fossil Lake, was deposited in Fossil basin, which is located collected fossil shells from near Cumberland Gap (Veatch, in the Wyoming overthrust belt just to the west of the Green River 1907). The earliest vertebrate fossils reported from the Green basin in southwestern Wyoming. Lake Gosiute was deposited in River basin were fi shes discovered in the Green River Forma- the greater Green River basin, which includes the Green River tion. In 1856, Dr. John Evans collected a specimen of a fossil and Washakie basins in southwestern Wyoming, and the Sand fi sh from an unknown Green River Formation locality west of Wash basin in northwestern Colorado. Fossil Lake and Lake Green River City. He sent this specimen to paleontologist Joseph Gosiute may never have been physically connected (Surdam and Leidy in Philadelphia for study, and Leidy named it Clupea Stanley, 1980). Lake Uinta was deposited in the Uinta basin in humilis (later renamed Knightia humilis) (West, 1990). Hayden northeastern Utah and the Piceance Creek basin in northwest- (1871) described the discovery of a locality he referred to as ern Colorado. Lithologically, the Green River Formation in the the “petrifi ed fi sh cut” along the main line of the Union Pacifi c greater Green River basin is a complex sequence of limestone, Railroad ~2 miles west of Green River. Employees of the rail- shale, and sandstone beds with a maximum thickness of ~838 m road had initially discovered the locality and later turned many (2750 ft) (Roehler, 1993). It was deposited lateral to and above specimens over to Hayden. Paleontologist Edward Drinker Cope the predominantly fl uvial Wasatch Formation, and lateral to and described the fi sh fossils from the petrifi ed fi sh cut in Hayden’s below the fl uvial and lacustrine Bridger and Washakie forma- (1871) expedition report. tions. The Laney Member is the uppermost member of the Green The initial discovery of mammalian fossils in the Green River River Formation in Wyoming and represents the fi nal expansion basin was probably made by a long-time local resident. Trapper of Lake Gosiute. Jack Robinson (also called Robertson) found what he described Most volcaniclastic sediments deposited in the Green River as a “petrifi ed grizzly bear” sometime in the late 1860s in what basin during the middle Eocene were apparently transported from is now called the Bridger Formation but which had initially been Middle Eocene rock units in the Bridger and Uinta Basins 129 named the “Bridger Group” by Ferdinand V. Hayden in 1869. to 1873. Leidy’s only excursion to the West took place in 1872, This story was related to Joseph Leidy by Judge William Carter when he visited the Bridger badlands guided by the Carter broth- of Fort Bridger as an explanation for the name “Grizzly Buttes,” ers of Fort Bridger. Cope’s only visit to the Bridger badlands an area 10–15 miles southeast of Fort Bridger where fossils were occurred in 1872, while Cope was attached to the Hayden sur- particularly common (the name Grizzly Buttes has since disap- vey as the paleontologist. This visit infuriated Marsh, who, at the peared from the local geographic vocabulary). time, considered the Green River basin and Bridger Formation Several government geological and topographical surveys his exclusive fossil-collecting territory. By the late 1870s, Cope with specifi c but overlapping territories were operating in the and Marsh had left the Green River basin for good, although both southern Green River basin between 1867 and 1879. Hayden and men independently and at different times retained the services of his party collected along the Henrys Fork valley and further north paid fossil collector Sam Smith (West, 1990). in the vicinity of Church Buttes in 1870 as part of the 1867– Other early fossil collectors who visited the Green River 1878 U.S. Geological and Geographical Survey of the Territories basin in 1877 and 1878 included Henry Fairfi eld Osborn, Wil- (Hayden, 1873). Fossils collected by Hayden’s group were sent liam Berryman Scott, and Francis Speir for Princeton University. to Joseph Leidy in Philadelphia for study and were described in Scott returned to the area with Speir in 1886. Jacob Wortman and his 1873 monograph on fossil vertebrates. Later paleontological James W. Gidley collected for the American Museum of Natu- studies for the Hayden Survey were carried out by E.D. Cope. ral History (AMNH) in 1893. The early fossil-collecting expe- Under the direction of John Wesley Powell, the U.S. Geologi- ditions to the Green River basin resulted in large collections of cal and Geographical Survey of the Territories, Second Division fossils primarily from the Bridger Formation at the Philadelphia (1875–1876), worked along the Henrys Fork River in 1869, and Academy of Natural Sciences (Leidy), Yale University (Marsh), in a corridor 10–20 miles wide on either side of the Green River the AMNH (which purchased Cope’s collection just before the in 1871 (Powell, 1876). The U.S. Geological Survey of the For- turn of the century), and Princeton University (Osborn, Scott, and tieth Parallel (1867–1872), directed by Clarence King, worked in Speir). Unfortunately, these early collectors paid little attention the Green River basin in 1871 and 1872. The fossils collected by to the stratigraphic provenance of the fossils they collected. Their the King Survey were sent to Othniel Charles Marsh for descrip- collections do, nevertheless, contain the holotypes of most pres- tion. Most of the fossils collected during these surveys were dis- ently recognized Bridgerian mammal taxa. covered in the Bridger Formation. In 1902, H.F. Osborn, who was then the U.S. Geological Many of the early scientifi c expeditions to the Green River Survey paleontologist, initiated the fi rst program of stratigraphic basin were based out of Fort Bridger that was originally set up fossil collection to take place in the Green River basin and one of as a trading post in 1843 by trapper and guide Jim Bridger and the fi rst in North America. Osborn charged Walter Granger and his partner Louis Vasquez. The fort became an army post after William Diller Matthew of the AMNH with the task of carrying the 1857 Mormon War. Judge Carter and Dr. J. Van A. Carter, out the study. Matthew was also directed to fi nd a uintathere to later residents of Fort Bridger, maintained an active correspon- display at the AMNH. The AMNH party, led by Granger, worked dence with Joseph Leidy in Philadelphia during the late 1860s in the Bridger basin from 1902 to 1906 (Matthew, 1909). The and early 1870s. This correspondence included mailing fossils to second halves of the 1903 and 1905 fi eld seasons were devoted Leidy, which were described in subsequent publications (Leidy, to mapping and describing the stratigraphy of the Bridger For- 1869, 1871, 1872a, 1872b, 1873). Leidy, who is often regarded as mation, while the remainder of the time was spent searching the the father of North American vertebrate paleontology (Lanham, badlands for fossils. The efforts of the AMNH parties over these 1973), named the fi rst Bridger Formation fossil to be formally four years resulted in an excellent fossil collection that was, for described, the omomyid primate Omomys carteri, after Dr. Carter its time, very well documented stratigraphically. (Leidy, 1869). Omomys carteri was also the fi rst-described fossil These AMNH expeditions also resulted in the fi rst paper to primate from North America. be published on the geology of the Bridger Formation, which Early reports of fossils from the Green River basin did not was authored by William J. Sinclair (1906), who had joined the go unnoticed by rival paleontologists O.C. Marsh and E.D. Cope. AMNH fi eld party for the summer of 1905. In Matthew’s classic The incidents that set the stage for the long and bitter confl ict 1909 monograph, The Carnivora and Insectivora of the Bridger between these two men began in the Green River basin while Basin, Middle Eocene, the geology of the Bridger Formation was they were prospecting in the Bridger Formation in 1872. Some- described briefl y, and a system of stratigraphic subdivisions for times referred to as the “bone wars,” the dispute between Marsh the formation was introduced. These subdivisions, Bridger A–E, and Cope lasted for more than 30 years and included efforts by were based on areally extensive limestone beds, which Matthew each man to destroy the scientifi c reputation and integrity of the called “white layers.” other. This confl ict soured Leidy’s interest in paleontology and Following the early fossil-collecting expeditions of the nine- led to his eventual abandonment of the discipline after 1872. teenth century and initial scientifi c fi eld studies conducted by Professor Marsh was the fi rst professional paleontologist to AMNH crews in the early twentieth century, the Bridger Forma- collect fossils from the Bridger Formation; he brought crews with tion in the Green River basin has remained the focus of almost him from Yale College for four consecutive summers from 1870 continuous paleontologic inquiry because of its abundant and 130 Murphey et al. diverse vertebrate fossils, although Matthew’s (1909) original topic dating of four ash-fall tuffs; and geologic mapping of more stratigraphy was only recently refi ned. West (1990) wrote an than 600 mi2 of the southern Green River basin at the scale of excellent historical summary of vertebrate paleontological work 1:24,000. Geologic maps and publications relating to the Bridger in the Green River basin from 1840 to 1910. Formation are available at http://www.rockymountainpaleontol- H.F. Osborn (1929) devoted considerable discussion to the ogy.com/bridger. Bridger Formation and its fossils in his monograph, The Titano- theres of Ancient Wyoming, Dakota, and Nebraska. Horace Elmer Stratigraphy and Depositional Environments of Wood (1934) divided the Bridger Formation into two members. the Bridger Formation The Blacks Fork Member corresponds to Matthew’s Bridger A and B, and the Twin Buttes Member corresponds to Matthew’s The Bridger Formation was named the “Bridger Group” Bridger C and D, with the Sage Creek White Layer marking their by Hayden (1869). The fi rst stratigraphic framework for the boundary. Contrary to rules of stratigraphic nomenclature, these Bridger Formation was established by W.D. Matthew (1909) of members were defi ned on perceived faunal differences rather than the AMNH in the southern Green River basin where the forma- lithologic differences. The informal usage of the terms “Blacks- tion is thickest and best exposed. Matthew’s (1909) stratigraphic forkian” and “Twinbuttean” as land mammal subages derives subdivisions of the Bridger Formation were based primarily on from the names of the two Bridger members. Under the direc- fi ve areally extensive limestone beds. These he named the Cot- tion of J.W. Gidley, followed by C. Lewis Gazin, the Smithson- tonwood, Sage Creek, Burnt Fork, Lonetree, and upper white ian Institution began an active collecting program in the Bridger layers, and some were used to subdivide the formation into fi ve Formation beginning in 1930. Gazin was active in the Green units: Bridger A, B, C, D, and E, from lowest to highest. Mat- River basin from 1941 to 1968. This period of activity resulted thew’s intent was to make it possible to stratigraphically locate in a relatively large and well-documented collection that was the the numerous known fossil localities in the formation. Because subject of numerous publications by Gazin focused primarily on they are the most fossiliferous, the Bridger B, C, and D were the systematic paleontology of Bridgerian mammal fossils (e.g., further divided into fi ve subunits corresponding to basal, lower, 1934, 1946, 1949, 1957, 1958, 1965, 1968, and 1976). middle, upper, and top levels (e.g., B1, B2, B3, B4, B5). Because Paul O. McGrew and Raymond Sullivan worked on the stra- Matthew (1909) did not defi ne the upper and lower boundaries of tigraphy and paleontology of the Bridger A in the late 1960s and these subunits with stratigraphic markers or measured sections, published the results of their work in 1970. Robert M. West began correlations between them and the later subdivisions proposed an active collecting program for the Milwaukee Public Museum by Evanoff et al. (1998), Murphey (2001), and Murphey and Eva- in 1970 and worked in the basin until the late 1970s. West’s work, noff (2007) are uncertain. The history of stratigraphic nomencla- which also resulted in a large number of paleontological publica- ture for the Bridger Formation is provided in Figure 3. tions, included the use of screen-washing techniques to collect In his 1909 monograph, Matthew (1909, p. 296) gave a brief microvertebrates, a portion of the fauna that had not been previ- description of his proposed fi ve members and his white layers. ously well sampled. Like Wood (1934) and Koenig (1960), West “Horizon A” was 200 ft thick, composed primarily of calcareous (1976) noted diffi culties with the correlation of Matthew’s white shales alternating with tuffs, and with rare fossils. “Horizon B” layers across the basin and suggested that a bipartite division of was 450 ft thick, consisting of two benches separated by the Cot- the Bridger into upper (Twin Buttes) and lower (Blacks Fork) tonwood white layer and containing abundant and varied fossils. members was most appropriate. West and Hutchison (1981) He went on to note that the largest number of complete skeletons named Matthew’s Bridger E the Cedar Mountain Member, add- from the entire formation was found in the lower part of Horizon ing a third member to the Bridger Formation. Paleontological and B (B2). “Horizon C” was 300 ft thick, “defi ned inferiorly” by the geological studies of Tabernacle Butte, an isolated remnant of Sage Creek white layer, with the Burntfork white layer occurring the Bridger Formation of late Bridgerian age with an important at about its middle, and with abundant and varied fossils. He also fossil fauna, were published by McGrew (1959), McKenna et al. noted that the Sage Creek white layer was the “heavy and persis- (1962), and West and Atkins (1970). tent calcareous stratum” at Sage Creek Spring, thus designating Evanoff et al. (1998), Murphey (2001), and Murphey and a type locality where this unit had been previously described and Evanoff (2007) signifi cantly refi ned Matthew’s (1909) Bridger illustrated, but not named, by Sinclair (1906). “Horizon D” was Formation stratigraphic scheme. Their work included the addition 350 ft thick, composed of harder gray and greenish-gray sandy of newly described marker units; the establishment of new strati- and clayey tuffs, “defi ned inferiorly” by the Lonetree white layer, graphic subdivisions and correlation of marker units across the with the upper white layer ~75 ft from the top, and with abundant southern part of the basin where the most complete stratigraphic and varied fossils. “Horizon E” was 500 ft thick, composed of sequence is exposed; descriptions of detailed stratigraphic sec- soft banded tuffs with heavy volcanic ash layers, with a high gyp- tions measured through the Bridger B, C, D, and E; renaming sum content and nearly barren of fossils. The total thickness of of the Cedar Mountain Member to the Turtle Bluff Member in the Bridger reported by Matthew was 1800 ft. Despite the litho- order to conform with the rules of stratigraphic nomenclature; logic descriptions of the fi ve horizons made by Matthew (1909), stratigraphic positioning of more than 500 fossil localities; iso- subsequent workers have not been able to subdivide the Bridger Middle Eocene rock units in the Bridger and Uinta Basins 131

Formation on the basis of lithologic differences (Bradley, 1964; additional markers were used to redefi ne the base and defi ne the Evanoff et al., 1998; Murphey, 2001, Murphey and Evanoff, top of the Bridger E (Turtle Bluff Member). Four of Matthew’s 2007; Roehler, 1992a). Furthermore, with the exception of the original “white layers” were included in the stratigraphy of the Bridger B-C and D-E boundaries, Matthew’s subdivisions do not Bridger C and D, and these were mapped and redescribed in correspond to major faunal changes (Murphey, 2001; Murphey detail. In conjunction with the latest stratigraphic revision, geo- and Evanoff, 2007; Simpson, 1933; Wood, 1934). logic mapping of ten 7.5 min quadrangles which cover the area The Bridger Formation has been subdivided into three mem- encompassed by the upper Bridger Formation was completed, bers. The Blacks Fork Member, or lower Bridger, is equivalent to and these maps are available from the Wyoming State Geological Matthew’s Bridger A and B; the Twin Buttes Member, or upper Survey. Because many marker units are not continuously exposed Bridger, is equivalent to Matthew’s C and D; and the Turtle Bluff or traceable across the entire basin (from Hickey Mountain, Sage Member, also considered part of the upper Bridger, is equiva- Creek Mountain, and Cedar Mountain east to Twin Buttes and lent to Matthew’s Bridger E. A detailed history of geologic and Black Mountain), a distance of ~40 miles, accurate correlation paleontologic investigations focusing on the Bridger Formation, was made possible by using the mineralogically diagnostic Hen- and the history of stratigraphic nomenclature for this unit, are rys Fork tuff as a datum. provided by Murphey and Evanoff (2007). Rock accumulation rates, isotopic ages of ash-fall tuffs Evanoff et al. (1998), Murphey (2001), and Murphey and (Murphey et al., 1999), and fossils indicate that the 842 m (2763 Evanoff (2007) published the fi rst major stratigraphic revision of ft) thick Bridger Formation was deposited over an ~3.5-million- the Bridger Formation since Matthew’s (1909) stratigraphy. The year interval from ca. 49.09 to 45.57 Ma, and that the faunal most recent stratigraphic subdivisions are based on widespread transition from the Bridgerian to the Uintan Land Mammal Age limestone beds, tuffs, and tuffaceous sheet sandstones which was under way by ca. 46 Ma as indicated by fossils collected are used as marker units. Fifteen such units were described, and from the Turtle Bluff Member (Evanoff et al., 1994; Gunnell et seven of these were considered major markers. These were used al., 2009; Murphey, 2001; Murphey and Evanoff, 2007; Robin- to subdivide the Bridger C and D (Twin Buttes Member) into son et al., 2004). Recognized depositional environments of the lower, middle, and upper informal subdivisions (Fig. 3). Two Bridger Formation include fl uvial, lacustrine, playa lacustrine,

Figure 3. History of Bridger Formation stratigraphic nomenclature Bridger from 1869 until present. The correlation between Mat- thew’s (1909) subdivisions (1–5) and the lower, middle, and upper subdivisions of Murphey and Evanoff (2007) are uncertain. 132 Murphey et al. paludal, marginal mudfl at, basin margin, and volcanic. Murphey Formation (Murphey, 2001; Murphey and Evanoff, 2007). The and Evanoff (2007) concluded that an infl ux of fl uvially trans- shift from dominantly tropical forest environments to more-open, ported volcaniclastic sediment to the Green River basin during savanna-like conditions in the Eocene intermontane basins dur- middle Eocene time led to the fi lling of Lake Gosiute and the ing late Bridgerian (early-middle Eocene) and Uintan (middle development of muddy fl oodplains of low topographic relief, Eocene) times has also been studied by using ecological diversity which persisted for up to 85% of the time during which the upper analysis applied to mammalian faunas (Murphey and Townsend, Bridger was deposited. Occasional lapses in the fl ow of sedi- 2005; Townsend, 2004). ment to the basin permitted the development of shallow, mostly As indicated by fossil distribution and diversity, the Green groundwater-fed lakes and ponds, which accumulated up to four River lakes and their forested margins provided highly favorable times as slowly as fl oodplain deposits. These lapses decreased habitats and preservational environments for both aquatic and in frequency throughout deposition of the upper Bridger Forma- terrestrial organisms. Lake margin habitats, riparian corridors tion. As indicated by fossil distribution and diversity, lakes and and adjacent fl oodplains were apparently vegetated during much their margins provided favorable habitats for both aquatic and of the time of Green River Formation deposition, as indicated by terrestrial organisms during deposition of the Bridger Formation. a paleofl ora that includes a variety of trees and bushes such as palm, cinnamon, oak, maple, lilac, and hazel, as well as cattails Middle Eocene Paleoenvironments of the Green River Basin and rushes. Insects of many varieties lived in the lakes and forests and are locally well preserved in lake sediments. A variety of ter- Numerous studies based on paleontological and geologi- restrial and aquatic mollusks (clams and snails) are also known cal evidence have concluded that the Eocene-age rock units in to have inhabited the Green River lakes (Hanley, 1974). Crayfi sh, the greater Green River basin were deposited in warm temper- prawn, and ostracods inhabited the warm lake waters, as did a ate, subtropical, and tropical climatic conditions (Roehler, 1993). diversity of fi sh species, including relatives of the herring, perch, Perhaps the most reliable information concerning paleoclimates paddlefi sh, bowfi n, gar, catfi sh, and stingray (Grande, 1984; comes from analysis of plant mega- and micro-fossils. According Grande and Buchheim, 1994; McGrew and Casilliano, 1975). to Leopold and MacGinitie (1972), early Eocene fl oras (based Frogs, crocodiles, and turtles were common residents of shal- on palynology of samples collected from the Niland Tongue of lower proximal shoreline waters. A diversity of reptile species, the Wasatch Formation and the Luman and Tipton tongues of the including tortoise, lizards, and snakes, inhabited the forests sur- Green River Formation) suggest a humid subtropical to warm rounding Eocene lakes and ponds. Flamingos, hawks, rails, stone temperate climate with summer rainfall and only mild frost and curlews, and other bird species frequented the forests, wetlands, with a mean annual temperature of 55 °F. Nichols (1987) con- and lakes (Murphey et al., 2001). The forests teemed with the cluded that the climate of the basin fl oor during deposition of the primitive ancestors of many modern mammalian groups, includ- Niland Tongue was subtropical, without freezing temperatures. ing rodents, insectivores, bats, primates, perissodactyls (horse, The earliest middle Eocene climates pertaining to the Cathe- rhinoceros, and tapir), and carnivores, as well as more bizarre dral Bluffs Tongue of the Wasatch Formation and the Wilkins archaic forms such as creodonts, brontotheres, and massive six- Peak Member of the Green River Formation were interpreted as horned uintatheres (Gazin, 1976; Grande and Buchheim, 1994; generally hot and dry (Leopold and MacGinitie, 1972). Climatic Gunnell and Bartels, 1994; McGrew and Casilliano, 1975; Mur- conditions in the early-middle Eocene during deposition of the phey et al., 2001). lower part of the Laney Member of the Green River Formation were characterized as warm and humid with tropical affi nities. Fossils and Biochronology of the Bridger Formation Floras of the upper part of the Laney Member indicate a change to cooler, subhumid conditions (Leopold and MacGinitie, 1972). One of the world’s most abundant and diverse middle Both pollen and leaf data from the Washakie Formation indicate Eocene vertebrate faunas is preserved in the Bridger Forma- a dry but temperate climate (Leopold and MacGinitie, 1972). tion. More than 86 species representing 67 genera, 30 families, Roehler (1993) reported in a written communication that Mac- and 13 orders of fossil mammals are recognized (Gazin, 1976). Ginitie reinterpreted temperature and precipitation ranges on Joseph Leidy’s 1869 description of Omomys carteri was the fi rst the basis of palynology of samples collected from the Washakie scientifi c description of a fossil from the Bridger Formation. basin by Roehler (1992a). His reinterpretation estimated mean Subsequently, Bridger fossils have been the subject of numer- annual temperatures of 65 °F during the early Eocene, 63 °F ous publications, including many classic papers by pioneers of during the earliest middle Eocene, and 62 °F during the middle American vertebrate paleontology (Cope 1872, 1873; Granger, Eocene. Average annual precipitation was estimated at more 1908; Leidy, 1869, 1871, 1872a; Marsh, 1871, 1886; Matthew, than 40 inches during the early Eocene, 25–35 inches during the 1909; Osborn, 1929). Like many other highly fossiliferous for- earliest middle Eocene, and 15–20 inches in the middle Eocene. mations, the Bridger contains an abundance and diversity of fos- Sedimentological evidence of a more arid climate during the sils that make it well suited for paleontological research, most of middle Eocene (transitional Uintan NALMA) includes massive which has focused on the phylogenetics, systematic paleontology, beds of gypsum capping the Turtle Bluff Member of the Bridger and biostratigraphy of the vertebrate fauna (Covert et al., 1998; Middle Eocene rock units in the Bridger and Uinta Basins 133

Evanoff et al., 1994; Gazin, 1957, 1958, 1965, 1968, 1976; Gun- tan and Duchesnean mammalian taxa based on the most up to nell et al., 2009; Krishtalka et al., 1987; McGrew and Sullivan, date information (modifi ed from Gunnell et al., 2009). 1970; Robinson et al., 2004; West and Hutchison, 1981). Pre- The fossil assemblages of the Bridger Formation and other served in a variety of sedimentary environments, preservational Eocene rock units in the greater Green River basin provide an states, associations, and in locally varying abundances, these fos- unprecedented opportunity to study ancient communities and sils include primarily vertebrates and mollusks, with less com- environments. Studies of these fossils and the rocks in which they mon plants and ichnofossils. Plant fossils include leaves, seeds, are preserved are the source of much of our knowledge of the and wood, which is sometimes algal covered (see Murphey et al., Eocene Epoch of North America. The vertebrate faunas are of 2001). Ichnofossils include solitary bee cases, earthworm pellets, particular scientifi c importance because they represent an excep- caddis fl y larvae, and fi sh pellets. Vertebrate fossils include fi sh, tional record of early Tertiary mammalian evolution and diversi- amphibians, reptiles (lizards, snakes, turtles, and crocodilians), fi cation spanning the Wasatchian, Bridgerian, and earliest Uintan a diversity of birds (see Murphey et al., 2001), and mammals. NALMAs. Mammalian fossils include apatotheres, artiodactyls, chiropter- ans, carnivores, condylarths, dermopterans, dinoceratans (uin- Bridger Formation Field Trip Stops tatheres), edentates, insectivores, leptictids, marsupials, pantoles- tids, perissodactyls, primates, rodents, taeniodonts, and tillodonts The fi eld trip route travels through the Bridger basin in an (Gazin, 1976; Woodburne et al., 2009a, 2009b; unpublished approximately stratigraphic manner. After leaving historic Fort paleontological data, University of Colorado Museum, compiled Bridger, the staging area for many of the early fossil collecting in 2002). Despite the relative ease with which diverse and statis- expeditions to the Bridger Formation, the route travels east along tically signifi cant fossil samples can be collected, and the large Interstate 80 crossing through badland outcrops of the Bridger historical collections of Bridger vertebrates available in many B that are stratigraphically close to the Bridger A-B boundary museums, taphonomic and paleoecologic studies of Bridger ver- (Blacks Fork Member). We examine the base of the Bridger B, tebrate faunas are relatively few (Alexander and Burger, 2001; defi ned by the Lyman limestone, near Little America. Travel- Brand et al., 2000; Gunnell, 1997; Gunnell and Bartels, 1994; ing back west along I-80, we then visit exposures of the Bridger Murphey et al., 1999; Murphey and Townsend, 2005; Townsend, B near historic Church Butte. We then continue west to Lyman 2004; Townsend et al., 2010). and then head south along Wyoming State Highway 414 to the Over the past twenty years, stratigraphically documented historic Grizzly Buttes badlands in the Bridger B. We continue fossil collections made by workers from the University of Colo- south along Wyoming 414, climbing stratigraphically through rado Museum, Denver Museum of Nature and Science, Univer- the Bridger C and D (Twin Buttes Member), and examine expo- sity of Michigan Museum of Paleontology, and more recently by sures of this interval in the vicinity of Sage Creek and Hickey the San Diego Natural History Museum, have added signifi cantly mountains, and at the “Lonetree Divide” (base of Bridger D). to existing biostratigraphic knowledge of the Bridger Formation. Weather permitting, we will then make our way to the southwest These collections, together with precise provenance data, have rim of Cedar Mountain and visit exposures of the Bridger E (Tur- made it possible to defi ne formal biochronologic units for the tle Bluff Member). Finally, we will head east along Highway 414 Bridgerian NALMA, most of which are based upon stratotype along the south side of Cedar Mountain with excellent vistas of sections that are located in the Bridger Formation. Gunnell et al. the Bridger C, D, and E that are overlain by the Oligocene Bishop (2009) have divided the Bridgerian into four “biochrons.” For- Conglomerate. The fi eld trip concludes after visiting exposures merly referred to as Gardnerbuttean land mammal sub-age, or of the Bridger C at the base of Black Mountain. Br0, biochron Br1a is the only Bridgerian biochron not found Note that all fi eld trip distances are provided in statute in the Bridger Formation. Its stratotype section is the Eotitan- (miles), whereas stratigraphic thicknesses are provided in both ops borealis interval zone of the Davis Ranch section of the statute and metric units. All distances were measured using a Wind River Formation. Biochron Br1b is equivalent to the lower handheld GPS device calibrated to the NAD27 datum. (Note that Blacksforkian, and its stratotype spans the Bridger A (lower part throughout this fi eld trip guide, the mileages given refer to the of the Blacks Fork Member). Biochron Br2 is equivalent to the prior stop unless specifi ed otherwise.) upper Blacksforkian, and its stratotype section spans the Bridger B (upper part of the Blacks Fork Member). Biochron Br3 is Stop 1. Fort Bridger State Historic Site Parking Lot (0.0 mi; equivalent to the Twinbuttean, and its stratotype section spans the cumulative 0.0 mi) entire Bridger C and D (Twin Buttes Member). The uppermost Fort Bridger was originally established as a trading post in member of the Bridger Formation the Turtle Bluff Member, or 1843 by trapper Jim Bridger and his guide Louis Vasquez. The Bridger E, is the stratotype section for the earliest Uintan bio- U.S. Army acquired the trading post in 1857 during the Mormon chron, Ui1a (Gunnell et al., 2009; Walsh and Murphey, 2007). War. It was located along the emigrant trail to Oregon, Califor- In summary, the mammalian fauna of the Bridger Formation has nia, and Utah, and more than twenty years after the establish- been used to formally defi ne biochrons Br1b, Br2, Br3, and Ui1a. ment of the trading post, the route of the newly constructed Appendix A is a biochronologic range chart for Bridgerian, Uin- Union Pacifi c Railroad passed not far to the north. As discussed 134 Murphey et al. in greater detail above, many of the early scientifi c expeditions Optional Stop to the Bridger Formation were based out of Fort Bridger. Yale Approximately 18 m (59 ft) stratigraphically above the University paleontologist O.C. Marsh and his fi eld classes stayed Lyman limestone 1.7 mi to the southeast along Old Highway at Fort Bridger before heading out to the Bridger badlands in 30 is an unusual type of deposit for the Bridger Formation. Park 1870, 1871, and 1873. Rival paleontologist E.D. Cope stayed at approximately one-third of the way up the hill and look for abun- the fort in 1872 during his only fossil collecting expedition to dant dark-brown rock fragments littering the slopes underlain the Bridger. Joseph Leidy, often regarded as the father of North by a thick green mudstone interval. The thin, dark-brown bed American Vertebrate Paleontology and the fi rst paleontologist contains abundant fossil caddis-fl y larval cases and other more to formally describe a Bridger Formation fossil, made his only enigmatic fossils preserved in what appear to be algal covered fossil collecting trip to the west in 1872, and also stayed at Fort logs (SDSNH Loc. 5783). The taphonomy and paleoecology Bridger. Today, Fort Bridger is a state historical site and has been of this unit has yet to be adequately studied. The fossil-bearing partially reconstructed. bed is overlain by a 2.8 m (9 ft) thick sequence of green to tan, The low butte just to the west of Fort Bridger is Bridger well-indurated, platy, fi ne-grained, silty sandstone. It is underlain Butte, which is capped with Quaternary gravels and is composed by a 1.5 m (5 ft) thick, platy, grayish-brown, non-fossiliferous, of Bridger B strata. mudstone with a distinct top contact. Insect and plant fossils are Turn west out of the fort parking lot along the Interstate sparse in the Bridger Formation, and this bed contains the most 80 business loop and then turn east onto I-80 (toward Green abundant insect fossils known from the formation. River). Drive for ~32 mi and take the Granger Junction exit (Exit Return to I-80 and head west. Heading west along I-80, 66) heading north along U.S. Highway 30. Follow U.S. 30 for the fi rst prominent butte you come to south of the interstate is 1.8 mi after exiting the interstate. Then turn east and cross the Jagged Butte, which is capped by the Jagged Butte limestone. cattle guard onto a dirt road for 0.2 mi at which point you will The second prominent butte you come to (approximate highway arrive at the route of old Highway 30 (unmarked gravel road milepost 56.5) is Wildcat Butte, which is capped by the Sage that is still paved in places). Park immediately after turning right Creek limestone (Sage Creek white layer of Matthew, 1909), and (southeast) onto old Highway 30. Outcrops of the Lyman lime- which forms the base of the Twin Buttes Member. Exit I-80 at stone are located just to the east. the Church Butte exit (Exit 53) and turn north onto Church Butte Road (no sign). At 19.2 mi from Stop 2, with Church Butte just Stop 2. The Lyman Limestone at Granger Junction (36.3 mi to the east of your location, turn left (southwest) onto Granger from Stop 1; cumulative 36.3 mi) Road, Uinta County Road (CR) 233. At mile 21.0, turn southeast This stop provides a close-up look at the Lyman limestone, off of CR 233 onto a two-track road heading toward the west- which marks the boundary between the Bridger A and the lower ernmost point of Jackson Ridge. Park where the two-track road Bridger B within the Blacks Fork Member (Figs. 3 and 4). Here, crosses the pipeline right-of-way at 21.2 mi from Stop 2. the Lyman limestone is a gray limestone with locally abundant shells of the gastropod Goniobasis. The presence of this high- Stop 3. Church Butte and Jackson Ridge (21.2 mi from spired snail is a useful diagnostic indicator for this marker unit Stop 2; cumulative 57.5 mi) at many localities in the Bridger basin. The Lyman limestone is Church Butte is a large-linear badland knob formed by the widespread in its distribution. It is exposed to the west where it erosion of rocks of the middle Bridger Formation (lower Bridger forms the bench that is visible to the south of I-80 upon which B beds; Figs. 4, 5 and 7A). The butte was a landmark along the its namesake, the town of Lyman, is situated, almost as far east old Oregon-California-Mormon Trail, now Uinta County Road as the Rock Springs uplift, at least 15 mi north of Granger, and 233. Just to the west of the butte is a north-south–trending rim almost as far south as the town of Manila, Utah. separating Porter Hollow on the east with the valley of the Blacks Stratigraphically below the Lyman limestone are strata of Fork River on the west. The trail dropped off the rim just to the the Bridger A. This interval has been problematic for paleon- southwest of Church Butte, and the outcrops of Bridger Forma- tologists because it is sparsely fossiliferous. P.O. McGrew and tion below the rim were easily accessed by early geologists and R. Sullivan worked on the stratigraphy and paleontology of the paleontologists who traveled along the trail. Bridger A in the late 1960s and published the results of their Church Butte and the rim exposures are all within the mid- work in 1970. More recently, Gregg Gunnell and colleagues dle part of the Blacks Fork Member of the Bridger Formation from the University of Michigan Museum of Paleontology have of Wood (1934), or the lower Bridger B of Matthew (1909). The greatly expanded the known diversity of the Bridger A (Gun- rocks in the area are primarily interbedded brown to green mud- nell, 1998; Gunnell and Bartels, 2001). This has made possible stone sheets and brown to gray sandstone ribbons and sheets. the recent formalization of new biochronologic units (Gunnell The sequence includes two sandstone-dominated intervals and et al., 2009). As discussed above in “Fossils and Biochronon- three mudstone-dominated intervals (Fig. 6). Four thin but logy,” the Bridger A contains a mammalian fauna (biochron regionally widespread marker units occur in the sequence that Br1b) that is biostratigraphically distinct from the fauna of the is 120 m (394 ft) thick. The following descriptions are of the Bridger B (Br2). Bridger exposures along the west side of the rim, over an area Middle Eocene rock units in the Bridger and Uinta Basins 135 approximately three square miles south of where the county 84% sandstone within a minor amount of mudstone. The thick road crosses the rim. sandbodies within these sandstone-dominated sequences are The two sandstone-dominated intervals are characterized highly connected both laterally and vertically. The thick sand- by a series of thick ribbons to broadly lenticular sheet sandstone bodies have a reticulate pattern, with some sandbodies oriented bodies within a sequence of stacked, thin, muddy sandstone and toward the south-southeast (vector mean of 173°) and others ori- mudstone sheets. Sandstones can comprise 100% of the total ented toward the east-southeast (vector mean 118°). The sinu- thickness within the sandstone-dominated interval, but the lower osities of the individual sandbodies are low (mean 1.02). These interval averages 58% sandstone and the upper interval averages sandstone-dominated intervals represent a river system with

Figure 4. Generalized stratigraphic sec- tion of the Bridger Formation in the southern Green River basin, southwest- ern Wyoming. Isotopic ages reported by Murphey et al. (1999) have been re- calculated using the current 28.201 Ma sanidine standard for the Fish Canyon Tuff (Renne et al., 1998). 136 Murphey et al.

Figure 5. Index map of the western Bridger basin, Uinta and Sweetwater counties, Wyoming. Middle Eocene rock units in the Bridger and Uinta Basins 137

Figure 6. Geologic map of the Church Butte–Jackson Ridge area showing major marker beds in the lower Bridger B interval, later- ally extensive sandstone sheet intervals, and trends of major sandstone channel-belt deposits. Also shown are important sites of the Hayden 1870 expedition, including known sites where W.H. Jackson took photos. See the text for details. 138 Murphey et al. numerous splays and local avulsions. The two intervals outcrop 10.1 and 11.7 m (33 and 39 ft) below the upper limestone and as cliffs in the badland exposures. ranges in thickness from 0.5 to 0.7 m (1.6 to 2.3 ft) thick. It is a The mudstone-dominated intervals are characterized by tan to olive clayey mudstone bed that weathers gray and contains thick ribbon sandstone bodies that are typically separated from abundant euhedral crystals of biotite and hornblende. Sanidine adjacent sandstones by extensive mudstone beds. Mudstone- in this tuff has produced a 40Ar/39Ar age of 48.27 Ma (Murphey dominated intervals have sandstone contents that range from et al., 1999, given as 47.96 ± 0.13 Ma) recalculated using the 10% to 35% of total interval thickness. The sandstone ribbons current 28.201 Ma sanidine standard for the Fish Canyon Tuff represent large channels carrying mostly medium sand within (Renne et al., 1998). This upper tuff is called the Church Butte a mud-dominated system. The paleocurrent indicators in these tuff, with the type locality located at the north side of the point on ribbons (mostly medium- to thick-trough crossbed sets) and the east end of the long ridge called Jackson Ridge (UTM coor- sandbody orientations indicate an original fl ow toward the east- dinates of Zone 12T, 572123mE, 4592105mN, WGS 84). The southeast (vector mean of 120°). The sinuosities of the sand- Church Butte tuff occurs throughout the Bridger basin wherever stone bodies are low (mean 1.03) and their geometry is in a lower Bridger B rocks are exposed. “broken stick” pattern with long straight reaches and short sharp Many of the fi rst fossils collected from the Bridger Forma- bends. Sandbody widths and thicknesses are relatively small tion came from the Church Butte area. The fi rst geologist known in straight reaches, but at bends the sandbodies are thicker and to have collected fossils from the area was Ferdinand V. Hayden. wider and contain well-developed lateral accretion sets. Fossil In 1868 he collected fragments of a fossil turtle that were later bones typically accumulate near the bases of these bends. Thin described as Trionyx guttatus by Leidy (1869). Hayden returned sandstone sheets representing overbank splay deposits are rare to the area as part of the Geological and Geographical Survey of and are limited to near their source channels. The mudstone- the Territories of 1870. The survey camped just to the west of dominated intervals outcrop as benches and slopes in the bad- the area along the Blacks Fork River and collected fossils in the land exposures. Church Butte area on September 10 and 11 of 1870 (Hayden, The Eocene streams which deposited the lower Bridger B 1872, p. 41). The 1870 survey was the fi rst time the pioneer pho- channel sandstones in this area were perennial and fl ooded every tographer W.H. Jackson accompanied Hayden. Years later, Jack- year. This is indicated by the abundance of freshwater turtles, son recalled these two days: gar-pike scales (and other fi sh bones), and a large freshwater snail fauna in the overbank deposits. The channel-belt deposits Twelve miles farther on we came to Church Buttes, a remarkable for- also contain the shells of numerous freshwater mussels (unio- mation in the Bad Lands and a famous landmark along the old trail. While Gifford [an artist of the 1870 expedition who assisted Jackson] nid clams), which indicate perennial, well-oxygenated waters and I were making pictures of the interesting scenes, the geologists in streams and rivers. Fossil plants of this time (MacGinitie and under the lead of Dr. Hayden were digging for fossils. They collected Leopold, 1972) indicate subtropical temperatures and mesic a wagon load of ancient turtles, shell fi sh, and other creatures…. For moisture with seasonal precipitation. my part, I made seventeen negatives during the day, something of a There are four regionally widespread marker beds in the record for wet plate work, considering the many changes of location I had to make in getting the different views. (Jackson and Driggs, 1929, Bridger exposures in this area. Two widespread thin limestone p. 89, 91) sheets occur at the base and top of the section in the Church Butte area. The lower limestone is the Lyman limestone at the base of the Bridger B (along the Blacks Fork), and in this area it is a The best known of Jackson’s photos from the area (Fig. 7B) brown to gray ostracodal limestone with scattered catfi sh bones. was taken near the end of a long badlands ridge that is herein The upper limestone occurs on the fl at-surface on top of the rim, named Jackson Ridge in honor of the photographer. The upper just south of the county road. This upper limestone is a brown limestone bed marker in the area is named the Jackson Ridge micrite with brown to black banded chert masses and scattered limestone. large planorbid snail shells (Biomphalaria sp.). Both limestone Jackson’s photos of the area document the type area of such beds can be mapped over much of the Bridger basin in lower fossil mammals as Notharctus tenebrosus, Palaeosyops paludo- Bridger B exposures. The predominantly fl uvial sequence pre- sus, Hyrachyus agrestis, and Microsus cuspidatus, all described served in the Church Butte area was bracketed by these wide- by Leidy (1870, 1872b) and illustrated in 1873. The mollusk spread lacustrine deposits. type specimens collected at Church Butte by the Hayden sur- Two lithifi ed volcanic ashes (tuffs) occur in the section. The vey include Physa bridgerensis, “Viviparus” wyomingensis (a lower tuff is represented by a red clayey mudstone that ranges land snail that is similar in form to the aquatic Viviparus), and from 0.2 to 0.6 m (0.6 to 2 ft) thick, 33 m (108 ft) above the “Unio” leanus described by Meek (1870, 1871, 1872). Seven Lyman limestone. The bed does not contain euhedral crystals in other species of fossil mammals have their type area in or near this area, but in other parts of the Bridger basin this bed thickens the Church Butte area, and these were collected by such pale- and is white with euhedral biotite crystals. This bed has not been ontologists as E.D. Cope, O.C. Marsh, and J. Wortman. Type radiometrically dated. This red tuff has been mapped over much species of fossil mammals collected from the Church Butte area of the western Bridger basin. A second tuff bed occurs between are listed in Table 1. Middle Eocene rock units in the Bridger and Uinta Basins 139

Drive back onto CR 233, and turn left (southwest) toward the Blacks Fork Member. Continue southeast on Highway 414 Lyman. Turn left at mile 5.5 onto CR 237 which then crosses and the highway rises onto the Cottonwood Bench. Immediately the Blacks Fork River, winding south to I-80. Turn westbound after reaching the top of this bench, at 29.7 mi from Stop 3, turn (toward Evanston) onto I-80 at mile 7.4. Pass the Lyman exit east and then immediately north. At 0.2 mi from the turn off, do and drive to the Mountain View-Fort Bridger (Exit 39, 15.5 mi not turn east on Burnt Fork Road (Bureau of Land Management from Stop 3). Turn south onto Wyoming State Highway (SH) Road [BLM] 4315) and instead continue traveling north. At 30.4 414, crossing the Blacks Fork River and climbing up onto the mi from Stop 3, turn west onto the two-track road and follow it Lyman limestone at the top of the hill. Continue through Urie and for 0.6 mi to the Grizzly Buttes overlook. Mountain View, where the highway will bend to the east near the center of town. Refer to Figure 8 for a map that shows the major Stop 4. Grizzly Buttes (31.0 mi from Stop 3; cumulative 88.5 mi) geographic features of the remainder of the fi eld trip route. As Heading southeast from Mountain View, Wyoming State you drive east from the center of Mountain View along Highway Highway 414 rises through a panel of badland exposures and 414, the badlands to the south that are visible beginning at SH climbs onto a high fl at, called the Cottonwood Bench. The bench 414 milepost 105 were known to the early residents and explor- is capped by gravels derived from the Bishop Conglomerate and ers as “Grizzly Buttes” (lower and middle Bridger B). The north transported to the area by Cottonwood and Sage Creeks. Below end of the badlands to the northeast constitute the type area of the gravel-fl at is a series of badlands cut by Leavitt Creek, Little

Figure 7. (A) William H. Jackson photo of Church Butte taken on 10 or 11 Sep- tember 1870. View is to the east north- east. UTM location of the photo site is Zone 12T, 571955mE, 4595101mN, WGS84 datum (U.S. Geological Survey photo jwh00462). (B) William H. Jack- son photo of the west end of Jackson Ridge, taken mid-day either on 10 or 11September 1870. The view is toward the northwest, and includes the Hayden Survey campsite along the Blacks Fork River. Notice the crack that was in the original glass-plate negative. The UTM location of the photo site is Zone 12T, 571318mE, 4592360mN, WGS84 da- tum (U.S. Geological Survey photo jwh00309). 140 Murphey et al.

TABLE 1. TYPE SPECIES OF FOSSILS The Church Butte tuff is a prominent gray band about half way FROM THE CHURCH BUTTE–BLACKS FORK AREA down the escarpment. Notice that channel sandstones are not as MAMMALS abundant in the lower Bridger B rocks below you as they are in Marsupialia Peratherium innominatus (Simpson) 1928 the Church Butte area. Pantolesta To the east is a prominent escarpment rising far above the Pantolestes longicaudus Cope 1872 Cottonwood Bench. This escarpment is capped by the Sage Primates Creek White Layer, the boundary between the Blacks Fork and Notharctus tenebrosus Leidy 1870 Tillodontia Twin Buttes members of the Bridger Formation (the boundary Tillodon fodiens (Marsh) 1875 between Matthew’s Bridger B and C). Almost all the upper half Rodentia of the Bridger B is exposed in the west face of the escarpment. Microparamys minutus (R.W. Wilson) 1937 Sciuravus bridgeri R.W. Wilson 1937 Return to Wyoming State Highway 414 and travel north for Carnivora 5.7 mi. Then turn east and drive for 0.2 mi and park on the north Miacis parvivorus Cope 1872 side of the road. A short walk to the northeast will lead you to Sage Hyaenodontida Creek limestone and the type locality of the Sage Creek white layer. Sinopa major Wortman, 1902 Condylarthra Hyopsodus paulus Leidy 1872 Stop 5. Sage Creek White Layer Type Locality (5.9 mi from Perissodactyla Stop 4; cumulative 94.4 mi) Orohippus major Marsh 1874 Palaeosyops paludosus Leidy 1870 This outcrop of the Sage Creek white layer is located Cetartiodactyla next to site of the old Sage Creek stage station and Sage Creek Microsus cuspidatus Leidy 1870 Spring along the old Lonetree stage road. It was fi rst described TURTLE and photographed by Sinclair in 1906 (Fig. 9), and then named Trionychidae “Aspideretes” guttatus Leidy 1869 and mapped by Matthew (1909). The Sage Creek white layer is MOLLUSKS the base of Matthew’s Bridger C, the base of the Twin Buttes Bivalvia, Unionidae Member, and the base of the upper Bridger Formation as pres- “Unio” leanus Meek 1870 ently defi ned. Since Matthew’s (1909) work, this unit has been Gastropoda, Pulmonata renamed the Sage Creek limestone, and is the base of the lower Physa bridgerensis Meek 1872 “Viviparus” wyominensis Meek 1871 Bridger C of Evanoff et al. (1998), Murphey (2001), and Mur- Note: Data compiled from Leidy (1872a); Meek (1872); phey and Evanoff (2007). The general stratigraphy of the upper Henderson (1935); and Gazin (1976). Bridger Formation in the Sage Creek Mountain area is illustrated in Figure 10. At its type locality, the Sage Creek limestone is 4.1 m (13.5 ft) thick. It consists of a lower massive tan micritic limestone, a mid- Dry Creek and their tributaries. The badland hills directly west dle shaly limestone with dark-gray to black chert bands, and an of the overlook comprise the traditional “Grizzly Buttes” of upper platy to shaly limestone. Elsewhere, it includes massive to the early explorers, but the name is not known to the modern blocky marly and micritic limestone, ledgy marlstone, and platy population of the Smith’s Fork valley (see history of paleonto- calcareous shale, and is locally interbedded with green to brown logical investigations). Matthew (1909, p. 297) stated about the mudstone and claystone and thin carbonaceous shale. Fossils of buttes: “This is the richest collecting ground in the basin; thou- this unit consist of scattered gastropods, bone fragments (mostly sands of specimens have been taken from it, and many skulls and fi sh), and turtle shell fragments, and the limestone within it is skeletons more or less complete.” Type species of fossil mam- locally stromatolitic. The Sage Creek limestone supports a very mals collected from the Grizzly Buttes area are listed in Table 2. widespread bench, and it is the thickest and most widespread The lower half of the Bridger B is exposed in the Grizzly lacustrine deposit in the upper Bridger Formation. Buttes and along the Cottonwood Bench escarpment. Not far Stratigraphically overlying the Sage Creek limestone within below the Quaternary gravels at this overlook is a widespread the lower Bridger C are two other limestone beds that are much limestone that was named by Matthew (1909) the Cottonwood thinner but are also widespread: the Whisky Reservoir limestone white layer (now known as the Cottonwood limestone). It is a and the Butcher Knife limestone (see Fig. 10). The lower Bridger white micritic limestone that is very widespread but is locally C is the least fossiliferous subunit of the upper Bridger Formation absent in the Church Butte area. The Cottonwood limestone (Twin Buttes and Turtle Bluff members), despite the fact that it is is typically 5 m (16 ft) above the Church Butte tuff, but in this by far the most geographically widespread. area it is 10.4 m (34 ft) above the tuff. The thickness of intervals Continue south along Highway 414 for 3.2 mi. Traveling between the widespread marker beds increases from the Church south, the highway route travels up section through the lower Butte area toward the southwest. The Jackson Ridge limestone Bridger C and into the middle Bridger C. Sage Creek Mountain has been eroded by Cottonwood Creek on the bench, but in this is the highest point on the west side of the highway and Hickey area it is typically 6 m (20 ft) above the Cottonwood White Layer. Mountain is the highest point on the east side of the highway. Middle Eocene rock units in the Bridger and Uinta Basins 141

Both of these mountains are capped by the Oligocene Bishop Formation reference section, Murphey and Evanoff, 2007), Conglomerate. At 3.2 mi from Stop 5, pull into the Henry #1 gas the Hickey Mountain limestone is a well studied and very well pad on the east side of the road. important unit paleontologically. It has a relatively limited areal distribution, occurring over a distance of ~5.6 mi north Stop 6. Soap Holes and Hickey Mountain and Limestones of Hickey Mountain and west of Sage Creek Mountain, and (3.2 mi from Stop 5; cumulative 97.6 mi) is the upper limestone bed exposed on the cliff at this stop. The Soap Holes limestone, the lower of the two thin rusty This unit provides an excellent example of one of the most brown limestone beds visible at this cliffy exposure, is a wide- paleontologically prolifi c depositional settings in the upper spread marker unit that forms the base of the middle Bridger C Bridger Formation. (Evanoff et al., 1998; Murphey, 2001; Murphey and Evanoff, The early fossil collectors were the fi rst to notice the close 2007). It is believed that Matthew (1909) considered this bed to association between vertebrate fossils and the “white layers,” be equivalent to his Burnt Fork limestone, which is a lithologi- which are typically limestone and marlstone beds that were cally similar unit that is exposed to the southeast in the Henrys deposited in shallow lakes and ponds. More recently, paleon- Fork Valley but is actually not present in the section in this part tologists observed that it is not the marlstone beds that contain of the basin. In the Henrys Fork Valley, however, it is in fact is the majority of vertebrate fossils, but the immediately overly- 33 m (108 ft) higher than the Soap Holes limestone. The Soap ing and underlying mudstone beds. These mudstones, which are Holes limestone contains few fossils, but it is noteworthy that it occasionally carbonaceous, are inferred to have been deposited is stratigraphically closely associated with fossil logs at several along lake margins during lake transgressions and regressions localities. Fossils of the Soap Holes limestone include isolated, (Murphey, 1995; Murphey et al., 2001). Typically, the lime- disarticulated and poorly preserved bones of fi sh, reptiles (espe- stone and marlstone beds contain the remains of mostly aquatic cially turtles), and mammals within and on top of the unit. In the organisms such as snails, clams, fi sh, amphibians, pond turtles, Black Mountain area it is locally underlain by thin carbonaceous and crocodilians. The lake margin mudstones contain a mixed shale beds which preserve plant fragments. The Sage Creek and aquatic and terrestrial assemblage, and the terrestrial elements Soap Holes limestones have in fact yielded the fewest vertebrate include locally abundant reptiles such as lizards, as well as bird fossils of any upper Bridger lacustrine deposits. bones and mammal bones and teeth. One particularly prolifi c fos- Situated within the middle Bridger C 10.5 m (34 ft) above sil locality, the Omomys Quarry, is located approximately ½ mi the base of the Soap Holes limestone (in the upper Bridger west of this stop in the Hickey Mountain limestone and overlying

Figure 8. Map of a part of the southern Green River basin encompassing most of the area of outcrop of the upper Bridger Formation. Map shows both modern and historic geographic terminology (from Murphey and Evanoff, 2007). 142 Murphey et al. mudstone. This unusual fossil accumulation has produced over (4) a predator accumulation dominated by Omomys but prob- 2,300 specimens of vertebrates, gastropods, and plants from an ably including other vertebrates formed by owls in close prox- 8–10-cm-thick deposit in a 4 m2 area (Murphey et al., 2001). imity to a nest, day roost or night feeding station. The fauna and What makes the assemblage so unusual is that it contains a high fl ora of the Omomys Quarry is listed in Table 3. concentration of dental and post-cranial remains of the primate The same pattern of fossil distribution observed in the Omomys, avian skeletal remains, and eggshell fragments. The Hickey Mountain limestone occurs throughout the upper Bridger unusual components of the fauna are superimposed on a more Formation as illustrated in Figure 11. Most fossils are found typical Bridger fauna that occurs at the quarry and lateral to in association with lacustrine deposits, although stream chan- it in the same stratigraphic interval. Four taphonomic agents nels are also productive. Least productive are the volcaniclastic have been postulated for the formation of the Omomys Quarry mudstone and claystone beds that were deposited on low relief fossil accumulation: (1) an attritional accumulation of aquatic fl oodplains, and, together with stream channel deposits, comprise taxa in lacustrine sediments; (2) an attritional accumulation 95% of the total thickness of the upper Bridger. Examples of the of both aquatic and terrestrial taxa in shoreline sediments; fl oodplain deposits, here consisting of green and gray mudstone (3) an attritional accumulation consisting primarily of bird bones and claystone beds, are well exposed at this stop above and below and eggshell formed in close proximity to a nesting area; and the Soap Holes and Hickey Mountain limestones. Both the Soap Holes and Hickey Mountain limestones are better exposed, with some minor faulting, on the east side of the highway just to the TABLE 2. TYPE SPECIES OF north of this location. FOSSIL MAMMALS FROM GRIZZLY BUTTES Continue south on Highway 414 for 1.1 mi and turn east

Lipotyphla onto the gas well road. Follow this road to the east and it will Entomolestes grangeri Matthew 1909 bend to the north for a total distance of 1.6 mi from Stop 6. Park Nyctitherium serotinum (Marsh) 1872 Nyctitherium dasypelix (Matthew) 1909 on the north side of the Henry #10 well pad. The Henrys Fork Plesiadapiformes limestone (type locality of this unit) and the underlying Henrys Mycrosyops elegans (Marsh) 1871 Fork tuff are exposed above the well pad on the badland hill just Primates to your north. Look for the gray weathered bed near the top of Smilodectes gracilis (Marsh) 1871 the badland slope and an overlying thin light-gray marlstone, and Tillodontia bring a shovel to examine the tuff. Trogosus castoridens Leidy 1871 Pholidota Metacheiromys marshi Wortman 1903 Stop 7. Type Locality of the Henrys Fork Limestone (1.6 mi Metacheiromys tatusia Osborn 1904 from Stop 6; cumulative 99.2 mi) Metacheiromys dasypus Osborn 1904 The Henrys Fork limestone (and associated shore margin Rodentia deposits) is another highly fossiliferous unit, and has produced Thisbemys plicatus A.E. Wood 1962 hundreds of fossil mollusks and vertebrates across its distri- Leptotomus parvus A.E. Wood 1959 2 Reithroparamys delicatissimus (Leidy) bution. It is quite widespread, covering an area of ~402 km 1871 (250 mi2), and was deposited in an elongate east-west–trending Pseudotomus robustus (Marsh) 1872 basin which formed in the downwarp along the Uinta Mountain Ischyrotomus horribilis A.E. Wood 1962 Mysops minimus Leidy 1871 front. At this location, which is near the western edge of ancient Mysops parvus (Marsh) 1872 Henrys Fork Lake, the deposit is only 3 cm (1.2 in) thick, but Sciuravus nitidus Marsh 1871 it attains a maximum thickness of 1.65 m (5.4 ft) on the south Tillomys? parvidens (Marsh) 1872 side of Cedar Mountain near the center of its depositional basin. Hyaenodontida It is of taphonomic interest that the upper Bridger Formation Sinopa rapax Leidy 1871 Sinopa minor Wortman 1902 with its abundant vertebrate fossils preserved in lacustrine and Tritemnodon agilis (Marsh) 1872 associated shore margin deposits contains few articulated skel- Limnocyon verus Marsh 1872 etons or even partially articulated vertebrate remains, most of Carnivora which have been collected in the Bridger B (see Alexander and Thinocyon velox Marsh 1872 Viverravus gracilis Marsh 1872 Burger, 2001). Oödectes proximus Mattehw 1909 Immediately underlying the Henrys Fork limestone is the Vulpavus profectus Matthew 1909 Henrys Fork tuff, a unit that was fi rst discovered by Emmett Perissodactyla Evanoff while conducting fi eld work in the Sage Creek Moun- Palaeosyops major Leidy 1871 tain area in 1991. This ash-fall tuff is the most analyzed tuff Limnohyops priscus Osborn 1908 Helaletes nanus (Marsh) 1871 in the Bridger Formation, and it is beyond the scope of this Cetartiodactyla paper to report the various ages that have been published. How- Helohyus plicodon Marsh 1872 ever, Murphey et al. (1999) and Murphey and Evanoff (2007) Note: Compiled from Gazin (1976). reported a 40Ar/39Ar age of 46.92 ± 0.17 Ma based on single Middle Eocene rock units in the Bridger and Uinta Basins 143

Figure 9. Photograph of the Sage Creek white layer taken by W.H. Sinclair in 1906 (Sinclair, 1906, plate 38). Note the unit numbers penned in near the left edge of the photo.

Figure 10. Generalized stratigraphic section of the upper Bridger Formation in the Sage Creek Mountain area, Uinta County, Wyoming. The diagram shows widespread and more localized markers, as well as informal submembers of Mat- thew (1909). Thicknesses taken from the reference section of the Twin Buttes Member and the type section of the Tur- tle Bluff Member (from Murphey and Evanoff, 2007). 144 Murphey et al.

TABLE 3. FAUNA AND FLORA OF THE OMOMYS QUARRY

TAXA NISP* PLANTAE Division Chlorophyta Chara sp. Division Tracheophyta Dennstaedtiopsis aerenchymata (fern) 2 types of dicotyledenous wood ANIMALIA Phylum Mollusca Class Gastropoda TABLE 3. (continued) Order Lymnophila TAXA NISP* (freshwater pulmonates) Class Mammalia Biomphalaria sp. Mammalia undet. 150 Omalodiscus sp. Order Marsupialia Stagnicola? spp. Peratherium sp. 13 Physa spp. Order Geophilia (land pulmonates) Peradectes sp. 1 Gastrocopta spp. Order Rodentia Oreoconus? sp. Rodentia undet. 13 Phylum Chordata Paramys sp. 1 Coprolites 4 Thisbemys sp. 1 Class Microparamys sp. 1 Osteichthyes Ischyromyidae undet. 1 Osteichthyes undet. 200 Sciuravus sp. 9 Order Amiiformes Pauromys sp. 2 Amia sp. 8 Sciuravidae undet. 6 Order Lepisosteiformes Order Apatemyida Lepisosteus sp. 98 Apatemys sp. 1 Order Siluriformes 14 Order Lipotyphla Class Amphibia Lipotyphlan undet. 28 Amphibia undet. 4 Scenopagus sp. 2 Order Anura Entomolestes sp. 7 Anura undet. 4 Centetodon sp. 6 Class Reptilia Apternodontidae undet. 1 Order Chelonia Nyctitherium sp. 4 Chelonia undet. 5 Order Plesiadapiformes Trionychidae undet. 1 Uintasorex sp. 1 Echmatemys sp. 1 Order Primates Palaeotheca sp. 2 Notharctus sp. 2 Order Squamata Omomys sp. nov. 214 Lacertilia undet. 117 Order Condylarthra Iguanidae undet. 3 Hyopsodus sp. 22 Tinosaurus sp. 1 Order Cetartiodactyla Saniwa sp. 1 Cetartiodactyla undet. 1 Serpentes undet. 2 Homacodon sp. 1 Order Crocodilia Order Perissodactyla Allognathosuchus sp. 34 Hyrachyus sp. 1 Crocodilia undet. 4 TOTAL 1,183 Class Aves Note: Eggshell is not included. From Murphey et al. (2001). Order Ciconiiformes *Number of identifiable specimens for vertebrates. Juncitarsus gracillimus 4 Order Falconiformes Accipitridae (2 species) 7 Order Charadriiformes Burhinidae 5 Order Gruiformes Rallidae 5 Geranoididae 8 Total Aves (includes undet. (191) specimens not listed above) (continued) Middle Eocene rock units in the Bridger and Uinta Basins 145 crystal laser fusion analysis of sanidine, plagioclase, and biotite. The base of the Henrys Fork tuff forms the base of the upper Recalculated using the current 28.201 Ma sanidine standard for Bridger C, and is 121 m (397 ft) above the base of the Sage Creek the Fish Canyon Tuff (Renne et al., 1998), the age of the Henrys limestone in upper Bridger Formation reference section (Mur- Fork tuff is 47.22 Ma. Ash-fall tuff deposits comprise less than phey and Evanoff, 2007). Weathered badland exposures of the 1% of the total thickness of the upper Bridger Formation, and Henrys Fork tuff form a distinctive dark-gray weathering bed that based on their mineralogy, are believed not to have originated in is readily discernable from other Bridger lithologies, especially the Absaroka volcanic fi eld to the north like other volcaniclastic when wet. Bridger Formation sediments, but rather in the Challis volcanic Return to Highway 414 and drive south for 3.0 mi (note fi eld located in central Idaho. At its type locality on the south the exposures of Henrys Fork tuff which is visible as a subtle side of Cedar Mountain, the tuff is 0.95 m (3.1 ft) thick (Mur- gray bed on both sides of the highway just above road level after phey and Evanoff, 2007). The tuff is a blocky, non-calcareous, turning back onto the highway). Then turn west onto the access gray to white biotitic claystone with a distinct bottom contact road for Conoco Fed #20-2 gas well pad. Proceed to the well and a diffuse top contact. It contains biotite, zircon, allanite, and pad and park (3.2 mi from Stop 7). The route you just drove apatite crystals. Plagioclase is the most abundant feldspar. It continued up section through the upper Bridger C to the level of typically consists of a structureless lower unweathered portion the Lonetree limestone (base of lower Bridger D) which is at the with coarse euhedral biotite (up to 1.3 mm in diameter) which approximate level of the highway at the Lonetree Divide. This grades upward into a reworked portion with less coarse and less is the stratigraphically highest point that Highway 414 attains in abundant biotite. the Bridger Formation.

Stop 8. The Lonetree Divide (3.2 mi from Stop 7; cumulative 102.4 mi) This area provides some excellent vistas of the upper Bridger Formation and its marker units, especially from the top of the ridge just to your north. The base of the lower Bridger D, the Lonetree limestone (Lonetree white layer of Matthew, 1909), is well exposed at the base of the badland slopes at road level. The base of the middle Bridger D, the Basal blue sheet sandstone, is exposed on the slopes of the prominent conical butte to your west as well as on parts of the ridges to your north and south. The prominent butte, called “Old Hat Mountain” by the locals, is an erosional remnant of Hickey Mountain (Fig. 12A) to which it is still attached. The “rim of the hat” is the Upper White lime- stone (Upper white layer of Matthew, 1909). The butte is capped by a thin interval of red mudstone of the Turtle Bluff Member (Bridger E of Matthew, 1909). To your northeast is Sage Creek Mountain, with a thick sequence of Bridger E (red beds overly- ing gray beds of Bridger D) visible near its summit. The Basal Bridger E tuff (40Ar/39Ar age of 46.16 ± 0.44 Ma; Murphey and Evanoff, 2007) occurs just below the base of the Bridger E on Sage Creek Mountain. O.C. Marsh called Sage Creek Mountain “Big Bone Butte” because of the abundance of uintathere bones found in the area. Visible to your east is Cedar Mountain, with the thickest and best exposed sequence of Turtle Bluff Member. All three of the mountains in this area (Hickey, Sage Creek, and Cedar) are capped by Oligocene Bishop Conglomerate. Numerous fossil localities have been documented in the Lonetree Divide area. These include the classic Lonetree locali- ties of Matthew (AMNH expeditions of 1903–1906) and Gazin Figure 11. Stratigraphic distribution of catalogued mammalian speci- (United States National Museum [now the National Museum of mens from the upper Bridger Formation (University of Colorado Mu- Natural History] expeditions between 1941 and 1969. This area seum collections). BELS—Basal Bridger E limestone; HFLS—Hen- was also worked by Robert M. West of the Milwaukee Public rys Fork limestone; HMLS—Hickey Mountain limestone; HRLS— Hickey Reservoir limestone; LTLS—Lonetree limestone; SCLS— Museum during the 1970s, and by crews from the University of Sage Creek limestone; SHLS—Soap Holes limestone; ULS—Upper Colorado Museum during the 1990s. Channel sandstones in this White limestone. area indicate paleocurrent directions to the southeast. 146 Murphey et al.

Turn south on Highway 414 for 1.9 mi and turn east onto that if the ground is wet, it is not advisable to leave the paved Cedar Mountain Rim road (BLM Road 4314). At 2.8 mi from highway (SH 414). Stop 8, the road bends to the north and travels stratigraphically through the upper Bridger C, crossing the Lonetree limestone, Stop 9. The Turtle Bluff Member on Cedar Mountain (6.4 mi and continuing up through the lower and middle Bridger D. from Stop 8; cumulative 108.8 mi) At the junction of Cedar Mountain Rim Road and Sage Creek Looking east from this location affords an excellent view Mountain Road (5.2 mi from Stop 8), turn south onto a two- of the upper Bridger D, the highest sub-unit in the Twin Buttes track road. Drive south on the two track, keeping straight at Member, overlain by the Turtle Bluff Member of the Bridger miles 6.2 and 6.3 where other tracks diverge, until you reach Formation (Matthew’s Bridger E). The contact between the two the Turtle Bluff Member overlook (6.4 mi from Stop 8). Note members is shown on Figure 12B and is defi ned on the basis of

Figure 12. (A) View of the Bridger D and E on “Old Hat Mountain,” a prominent butte on the southeast fl ank of Hickey Mountain, Uinta County, Wyoming. Photo taken looking southwest. (B) View of the Bridger D and Bridger E on the southwest fl ank of Cedar Mountain, Uinta County, Wyoming. Photo taken looking east. BBS—Basal blue sheet sandstone; BELS—Basal Bridger E limestone; BRGB— Behunin Reservoir Gypsum bed; Tbe—Bridger E; Tbi— Bishop Conglomerate; Tbdm—middle Bridger D; Tbdu—upper Bridger D; ULS— Upper White limestone. Middle Eocene rock units in the Bridger and Uinta Basins 147 a limestone that occurs at the approximate level of the lowest on the age of the member, saying that its few mammal fossils red bed (note that some of the strata you see are slumped). The prove suffi ciently that it “belongs to the Bridger Age” (Matthew, limestone that supports the bench that you are standing on is the 1909, p. 296). Osborn (1929) correlated the Bridger E with the Upper White limestone. Washakie B and Uinta B, although he cited no evidence to sup- Consisting primarily of banded red, gray, and tan beds of port this correlation. Simpson (1933), Wood et al. (1941), and gypsiferous claystone and mudstone, rocks of the Turtle Bluff Gazin (1976) also regarded the Bridger E as Uintan, although Member are the least volcaniclastic in the Bridger Formation. this was apparently not based on fossil evidence. Based on eight Lthologically, the Turtle Bluff Member is somewhat distinct isolated rodent teeth identifi ed as Paramys cf. P. delicatior, and from the rest of the formation, being similar in appearance to the “several” bone fragments identifi ed as brontothere, West and red and brown sandstone, mudstone, and claystone beds of parts Hutchison (1981) concluded that the Bridger E (their Cedar of the Washakie and Uinta Formations of similar age. The Tur- Mountain Member) was Bridgerian. Subsequent work during tle Bluff member occurs only on Hickey Mountain, Sage Creek the 1990s by crews from the University of Colorado Museum Mountain, the south end of Black Mountain, and Twin Buttes, (Evanoff et al., 1994; Murphey and Evanoff, 2007) and in the but by far the most extensive and thickest exposures occur here 2000s by crews from the San Diego Natural History Museum on the southwest side of Cedar Mountain. The type section for (Walsh and Murphey, 2007) have now documented a much the Turtle Bluff Member on Cedar Mountain is a 131.5 m (431 ft) more diverse faunal assemblage from multiple stratigraphic lev- thick sequence of reddish-brown and gray claystone beds with a els within the Turtle Bluff Member (Table 4). Donna’s locality high gypsum content. This gypsum is both primary and second- (UCM Loc. 92189) is located near the base of the member and ary. Secondary gypsum consists of selenite and satin spar crys- is the only locality thus far to produce specimens of the newly tals which are abundant on the upper slopes of Cedar Mountain. described species of omomyid primate Hemiacodon engardae Primary gypsum occurs in thin beds, but the Turtle Bluff Member (Murphey and Dunn, 2009). Located 105 m (344 ft) above the on Cedar Mountain is capped by a thick and laterally extensive base of the member, Roll the Bones (SDSNH Loc. 5844) and gypsum bed. The mostly fi ne-grained reddish Turtle Bluff sedi- Red Lenses (SDSNH Loc. 5844) are the stratigraphically high- ments were probably derived from the adjacent Uinta Mountains est localities to yield identifi able fossils in the member. Hun- based on their color, unlike those of the Bridger A–D, which dreds of fossils have now been collected from these and other were largely derived from more distal volcanic sources. localities mostly via screenwashing of sedimentary matrix. The Turtle Bluff Member contains two markers: The Basal Non-Bridgerian taxa include Epihippus, Metanoiamys, Pareu- Bridger E limestone, which marks the base of the member (base mys, Triplopus, Sespedectinae indet., and Oromerycidae gen. of Matthew’s Bridger E), and the Behunin Reservoir Gypsum and sp. nov. The faunal assemblage of the Turtle Bluff Member Bed, which is the youngest and stratigraphically highest well is now considered to be earliest Uintan in age (biochron Ui1a exposed rock unit in the Bridger Formation (note that Behunin of Gunnell et al., 2009), although efforts to obtain additional is pronounced “Buhannan” by locals). Here on southwest Cedar fossils from this biochronologically important interval member Mountain, the Turtle Bluff Member contains four additional on Cedar Mountain and other locations within the Bridger basin unnamed limestone beds, and on Twin Buttes there are three. A are ongoing. 2.3 m (7.5 ft) thick laterally extensive, quartz arenite bed that lies The Bridger Formation is unconformably overlain by the 75 m (246 ft) above the base of the member on Cedar Mountain is Bishop Conglomerate, which is visible from this stop capping the only sandstone bed. Similar sandstone beds in the Turtle Bluff Cedar Mountain. To the east of this location, it forms massive Member also occur on the northwest fl ank of Hickey Mountain cliffs and spectacular columns. This unit is a very coarse con- and the south fl ank of Sage Creek Mountain, and may be roughly glomerate composed primarily of arkosic cobbles and boulders stratigraphically equivalent. derived from the Proterozoic Uinta Mountain Group, with locally The Behunin Reservoir Gypsum Bed is lithologically unique common cobbles and boulders of Paleozoic limestone (Bradley, for the Bridger Formation. Although other gypsum beds occur in 1964). It is as much as 40 m (131 ft) thick. The Bishop Conglom- the Turtle Bluff Member, they are much thinner. Restricted to just erate is unfossiliferous, but currently believed to be Oligocene below the southwest rim of Cedar Mountain (below the Bishop in age (K/Ar 29.50 ± 1.08 Ma, biotite) based on isotopic ages Conglomerate), this unit consists of a 7 m (23 ft) thick sequence obtained from a tuff that occurs within it on the south side of the of gray and tan unfossiliferous bedded gypsum beds interbedded Uinta Mountains (Hansen, 1986). with gypsiferous mudstones and marlstones. It is visible from a Return to Wyoming State Highway 414, and continue great distance as a prominent white bed high on Cedar Mountain. south. The highway crosses the Henrys Fork of the Green This bed is interpreted as an evaporitic playa lacustrine deposit, River, passes by the hamlet of Lonetree, and bends to the east. and may indicate changing climatic conditions near the end of There is an excellent view of the Henrys Fork tuff and Henrys Bridger Formation deposition. Fork limestone on the north side of the highway at SH 414 Because of its sparse fossils and steep, limited exposures, milepost 128. The Henrys Fork tuff is the prominent gray bed the biochronologic affi nity of the Turtle Bluff Member has been exposed low on the slopes of Cedar Mountain not far above diffi cult to determine. Matthew was the fi rst worker to comment road level, and the Henrys Fork limestone is a prominent white 148 Murphey et al. bed immediately overlying the tuff. Continuing east, the high- lying lacustrine strata of unknown thickness represents a fi nal way passes through the hamlet of Burntfork. At highway mile- transgressive phase of Lake Gosiute (Laney Shale Member of post 130.6 there is a point of historic interest on the south side Green River Formation) (Brand, 2007), although there is little of the highway. This location is near the site of the fi rst (1825) supporting evidence. Whatever the case, this sequence, combined mountain man fur trading “rendezvous” led by General Wil- with evidence provided by other upper Bridger lacustrine depos- liam Ashley and attended by a then “green” Jim Bridger and its (thicknesses and areal distribution), suggests that lacustrine Jedediah Smith. At the McKinnon Junction (16.6 mi from Stop deposition during upper Bridger deposition was most prevalent 9), turn north onto Sweetwater County Highway 1. Proceed to just to the north of the Uinta Mountain front. 18.8 mi from Stop 9 and pull over on the east shoulder next to Continue north on Sweetwater County Highway 1. You will the road cut. be driving through rocks of the lower Bridger C and will descend into upper Bridger B strata at approximate Sweetwater County Stop 10. The McKinnon Roadcut (18.8 mi from Stop 9; Highway 1 milepost 16.7 before climbing stratigraphically again cumulative 127.6 mi) lower Bridger C strata at highway milepost 15.8. At mile 11.5 This roadcut features the thickest lacustrine sequence in from Stop 10, turn east onto a two-track road toward the north the upper Bridger Formation. The Sage Creek limestone is the end of Black Mountain. Twin Buttes is the conical peak to the thick blocky limestone near the top of the cut. Underlying it are south of Black Mountain. Bear right at mile 12.1. Take the left at least 30 m (98 ft) of lacustrine shale, mudstone, marlstone, fork at mile 13.0 (look for the BLM Wilderness Study Area sign). and limestone, and the total thickness of the lacustrine sequence Park at the base of Black Mountain at mile 13.8. Note that if the is unknown. It has been postulated that this sequence and under- ground is wet, it is advisable to stay on the paved highway.

TABLE 4. TAXONOMIC SUMMARY OF THE TURTLE BLUFF MEMBER, BRIDGER FORMATION (BIOCHRON UI1A)

Designation Number Taxa Index Species 2 Hemiacodon engardae Oromerycidae gen. and sp. nov.

Genus LRD 6 Epihippus Pareumys Oromerycidae gen. and sp. nov. Sespedectinae indet. Metanoiamys Triplopus

Genus HRD 7 Entomolestes Paramys Hemiacodon Taxymys Oromerycidae gen. and sp.nov. Uintasorex Mysops

Species LRD 3 Epihippus gracilis Triplopus cubitalis Metanoiamys sp.

Species HRD 8 Entomolestes grangeri Sciuravus nitidus Paramys delicatior Taxymys lucaris Pauromys perditus Thisbemys corrugatus Pontifactor bestiola Uintasorex parvulus

Range-Through 36 Apatemys sp. Nyctitherium sp. Antiacodon sp. Omomys carteri Brontotheriidae spp. Orohippus sylvaticus Centetodon bembicophagus Pantolestes longicaudus Chiroptera indet. Pantolestes natans Copedelphys innominatum Pantolestes sp. Dilophodon minusculus Pauromys sp. Harpagolestes sp. Peradectes chesteri Helohyus sp. Scenopagus priscus Herpetotherium knighti Thisbemys corrugatus Herpetotherium marsupium Tillomys senex Hyopsodus sp. Trogolemur sp. Hyrachyus eximius Uintacyon vorax Isectolophus sp. Uintaparamys bridgerensis Mesonyx obtusidens Uintaparamys caryophilus Microparamys minutus Uintatherium anceps Microsyops annectens Viverravus minutus Notharctus robustior Washakius sp. Note: From Gunnell et al. (2009). LRD—lowest range datum; HRD—highest range datum. Middle Eocene rock units in the Bridger and Uinta Basins 149

Stop 11. Twin Buttes and Black Mountain (13.8 mi from layers” did not align as expected when using the Sage Creek Stop 10; cumulative 141.4 mi) limestone as a datum. This problem was fi nally solved by locat- Although the classic Bridger badlands and collecting areas ing the mineralogically diagnostic Henrys Fork tuff not far from we have already visited are located far to the west, the Twin here on Black Mountain, and using it as a stratigraphic datum. Buttes Member and the Twinbuttean land mammal subage was The reason that earlier workers had diffi culties establishing a named for Twin Buttes. Because of this, Murphey and Evanoff correlation between the Twin Buttes and Black Mountain area (2007) designated their type section of the Twin Buttes Member with exposures to the west using the “white layers” is that the for the upper Bridger sequence on the south side of Twin Buttes, lower Bridger C thins dramatically from the west to the east as and established their Twin Buttes Member reference section of evidenced on Twin Buttes, where the thickness between the Sage the upper Bridger for the sequence in the Sage Creek and Hickey Creek limestone and Soap Holes limestone is 21 m (69 ft) less Mountain area. However, the reference section is thicker and than in the nearest correlative sequence to the west. contains more marker units. The major stratigraphic features of You are stratigraphically located within the middle the upper Bridger in the Twin Buttes and Black Mountain area Bridger C, and the Henrys Fork tuff is located 42 m (138 ft are shown in Figure 13. above this level). In fact, all of the major marker units present You are standing in front of another upper Bridger marker in the Twin Buttes reference section are present in the Twin bed. The Horse Ranch red bed occurs only in the eastern part of Buttes type section except for the Basal blue sheet sandstone the basin (east side of Twin Buttes [Mass Mountain], Black Moun- (base of middle Bridger D). The Lonetree limestone is very tain and Twin Buttes). It is an ~4 m (13 ft) thick sequence of non- well exposed in the saddle between Black Mountain and Twin calcareous brick red, greenish-gray, and light-brown claystone, Buttes, and the Upper white limestone is exposed near the top blocky mudstone, and blocky fi ne-grained muddy sandstone of Twin Buttes. Only 21 m (69 ft) of Turtle Bluffs Member (Murphey and Evanoff, 2007). It is locally fossiliferous. occurs at Twin Buttes, which is capped by a thin remnant of For many years, paleontologists were vexed by the diffi culty Bishop Conglomerate. The hike from this stop to the saddle of correlating between Twin Buttes and Cedar Mountain to the between Twin Buttes and Black Mountain is well worth the west, especially considering the classic “layer cake geology” of effort if you have the time. the Bridger with very low dips and laterally persistent marker This is the end of the Bridger basin portion of the fi eld trip. units. This was because the stratigraphic positions of the “white From here we will head south to Manila, Utah, via Sweetwater

Figure 13. Generalized stratigraphic section of the upper Bridger Formation in the Twin Buttes area, Sweetwater County, Wyoming. The diagram shows widespread and more localized markers, as well as informal submembers of Mat- thew (1909). Thicknesses taken from the type section of the Twin Buttes Member, which includes the Turtle Bluff Member on Twin Buttes (from Murphey and Evanoff, 2007). 150 Murphey et al.

County Highway 1 and Wyoming State Highway 414, and then con- (Johnson, 1985). Throughout most of these fi ve stages, the tinue south over the Uinta Mountains to Vernal and the Uinta basin. salinity of Lake Uinta increased steadily. Ultimately, salinity increased to the point at which nahcolite and halite were pre- PART II. UINTA BASIN FIELD TRIP cipitated. The gradual fi lling of Lake Uinta during the middle Eocene was due to an infl ux of volcaniclastics from the Absa- This portion of the fi eld trip will examine rocks of the Uinta roka Volcanic Field in Wyoming to the north and from volcanic and Duchesne River formations in the Uinta basin. The following centers farther to the west, and then later by an increase in sedi- sections of the fi eld trip guide provide a summary of the early ment resulting from rejuvenated local Laramide uplifts. There Cenozoic geologic history of the Uinta basin, as well as the his- is no direct geologic evidence that Lakes Uinta and Gosiute tory of investigations, stratigraphy, depositional and paleoenvi- were ever physically connected. However, fl uvially deposited ronmental history, and fossils of the Uinta and Duchesne River volcaniclastics in Lake Uinta, demonstrably derived from the formations. This is followed by a detailed fi eld trip road log. Absaroka Volcanic Field in northwestern Wyoming, would be a persuasive line of evidence. This, however, would not rule Early Cenozoic Geologic History out the possibility of transport of volcaniclastic material from Lake Gosiute south to Lake Uinta via streams over the lowest By the late Paleocene (Clarkforkian NALMA), large shal- drainage divides after the greater Green River basin had been low lakes and ponds occupied a large portion of both the Uinta fi lled (Surdam and Stanley, 1980). and Piceance Creek basins. During the latest Paleocene and As lacustrine deposition in Lake Uinta diminished because early Eocene (late Clarkforkian and early Wasatchian NALMA), of basin fi lling, fl uvial sedimentation resulted in deposition of there was a return to fl uvial sedimentation in both basins, which the Uinta Formation in the Piceance Creek and Uinta basins is represented by rocks of the Wasatch Formation (also called (Stokes, 1986). At this time, fl uvial deposition began to domi- DeBeque Formation by some paleontologists). This was fol- nate in the basin as streams fl owed into it from the north and east lowed by a return to dominantly lacustrine conditions in the early (Stokes, 1986). The east-west fl ow resulted in the formation of Eocene with the establishment of two large freshwater lakes (one limey siltstone beds and fi ne-grained Uinta Formation shales in each basin). Bradley (1931) named the tongue of the freshwa- in the eastern part of the Uinta basin, contrasting with the red ter lacustrine unit which was deposited during this brief time of sandstone, shale, siltstone, conglomerate, and limestone beds maximum transgression the “basal tongue” of the Green River in the west (Stokes, 1986). Today, fl uvial rocks of the middle Formation where it crops out in Indian Canyon in the western Eocene Uinta Formation (Uintan NALMA) are preserved above part of the Uinta basin. The two early freshwater lakes in the Green River Formation strata only in the northern part of the Uinta and Piceance Creek basins appear to have been similar Uinta basin. The late-middle Eocene (Duchesnean NALMA) to the approximately contemporaneous Lake Luman, the early Duchesne River Formation overlies rocks of the Uinta Forma- freshwater stage of Lake Gosiute, which existed in the greater tion in the north-central and western parts of the Uinta basin, Green River basin in Wyoming. and the Cretaceous-age Mesaverde Group on Asphalt Ridge. The sediments deposited during lacustrine maximums Both the Uinta and Duchesne River formations were deposited in both the Uinta and Piceance Creek basins almost connect in river channels and on adjacent fl oodplains (Stokes, 1986) and over the top of the Douglas Creek arch, suggesting that during in river deltas (Townsend, 2004). Riparian environments were at least in part of the freshwater period the two basins were forested, and the rivers and streams fl owed through a savanna- hydrologically connected (Johnson, 1985). During this period, type environment with some swampy and marshy areas and for- the two basins are believed to have drained externally since the ested highlands (Hamblin, 1987). lake waters in both basins remained fresh enough to support abundant freshwater mollusks (Johnson, 1985). At the end of Uinta Formation the early Eocene (Wasatchian-Bridgerian boundary), a major transgression associated with expansion and deepening of lake The Uinta Formation is exposed at the southern base of the water marked the beginning of Lake Uinta as an unbroken body Uinta Mountains in an ~80 mi (~130 km) long, and rather nar- of water across the Douglas Creek Arch in both the Piceance row band which can be traced as far westward as Duchesne near Creek and Uinta basins (Johnson, 1985; Moncure and Surdam, the Wasatch Mountain Range and as far eastward as the Utah- 1980). Johnson (1984) named this the Long Point transgres- Colorado state line (Bryant et al., 1989; Hamblin et al., 1999; sion. After the maximum transgression, Lake Uinta extended Peterson and Kay, 1931). It overlies the lacustrine Green River close to the margins of the combined Uinta and Piceance Creek Formation, with an interfi ngering contact between the two forma- structural and sedimentary basins; it covered a much larger tions that refl ects their closely related depositional history (Hin- area than that of the maximum transgression during the earlier tze, 2005; Ryder et al., 1976). The Uinta Formation is ~1298 m freshwater stage of the lake. Five stages in the developmen- (4257 ft) thick (including subsurface and exposed rock) and con- tal history of the larger saline Lake Uinta have been identifi ed tains bitumen in the form of gilsonite veins (Cashion, 1986; Hin- based on changes in water chemistry and depositional events tze, 2005; Sprinkel, 2007). The Uinta Formation was deposited Middle Eocene rock units in the Bridger and Uinta Basins 151 under fl uvial conditions as the lake depositing the Green River (740 ft) in thickness (Cashion, 1974). Within in Uinta A, there is Formation was receding (Ryder et al., 1976). a 1.8 m (6 ft) thick tuffaceous bed (“a” tuff), and the base of this bed occurs 56 m (185 ft) below the top of the Uinta A (Cashion, History of Paleontological Investigations 1974). Uinta B is composed of light-gray, light greenish-gray, light-brown, light-purple mudstones and claystones forming O.C. Marsh led the fi rst paleontological expedition to the non-resistant slopes (Sprinkel, 2007). These are interbedded Uinta basin in 1870 as part of a Yale College Scientifi c Expedi- with green-gray, yellow and brown fi ne-grained sandstones tion (Betts, 1871; Marsh, 1875a, 1875b). In an unexplored area that form thin resistant ledges (Sprinkel, 2007; Townsend et al., between the Green and White rivers, he and his party collected 2006). There are prominent gilsonite veins in this unit (Cashion, numerous fossil mammals, the beginnings of an assemblage that 1986). In the eastern part of the basin, the Uinta B is capped by would later defi ne the Uintan North American Land Mammal Age a massive fi ne- to coarse-grained gray and brown sandstone bed (NALMA), a time period that spans ~40–46.5 Ma (Murphey and that weathers to dark brown, and was named the “Amynodon” Evanoff, 2007; Prothero, 1996; Wood et al., 1941). In the 1880s, sandstone (Riggs, 1912). This massive sandstone unit is over Francis Speir of Princeton University collected in this same area one mile in length, and was originally defi ned as the boundary and the mammalian fossils he collected were studied by W.B. between the Uinta B and Uinta C horizons (Riggs, 1912), but is Scott and H.F. Osborn, whose efforts produced the fi rst com- now known to be local in extent. Uinta C is distinctive in that prehensive publications on mammals from the Uinta Formation the lower intervals and upper intervals are strikingly different in (Rasmussen et al., 1999a; Scott and Osborn, 1887, 1890). At the terms of rock type and coloration (Townsend et al., 2006). The turn of the century, the Carnegie Museum sent expeditions to the lower part of Uinta C is comprised of light-gray, light green- Uinta basin at different times led by O.A. Peterson and then Earl ish gray, and light-brown mudstone and claystone that interbed Douglass (Douglass, 1914; Peterson, 1919). Peterson (1919) ini- with fi ne-grain sandstone beds, which are also greenish-gray or tially described many of the medium and smaller-sized mammals commonly yellow or brown (Sprinkel, 2007, Townsend et al., from the Uinta Formation, many now recognized as index taxa 2006). The upper part is composed of mainly deep red orange, for the Uintan NALMA and therefore crucial for understanding red, dark-brown, grayish-purple and yellow mudstone and clay- Uintan biochronology on both the regional and continental scales stone with thin gray and green fi ne grain sandstone beds that (Walsh, 1996). After these initial expeditions, museums across form thin ledges (Sprinkel, 2007; Townsend et al., 2006). North America sent parties to collect small samples of Uintan The stratigraphy of the Uinta Formation has been less well mammals, the most notable of these being the Smithsonian Insti- studied than that of the Green River Formation, and most of tution. In 1993, an expedition from Washington University in St. the stratigraphic work has been undertaken by paleontologists. Louis began what would be a 15+ year project (still ongoing) in The early work that produced the initial description of the for- the Uinta and Duchesne River Formations, with the goal of col- mation has been confused by changes in terminology describ- lecting small mammals (Rasmussen et al., 1999a; Townsend et ing the lithologic horizons and by shifting boundaries of the al., 2006). These latest expeditions have amassed one of the larg- units that divide these horizons (Cashion, 1986; Osborn, 1929; est assemblages of Uintan mammals from the Rocky Mountain Prothero, 1996; Riggs, 1912; Stagner, 1941; Townsend, et al., region (Townsend, 2004). 2006). Osborn (1895) cited Peterson as fi rst dividing the forma- tion into informal units Uinta A, B, and C, and all units were Stratigraphy reported to be fossiliferous. All early fossil collections have these horizon designations assigned as stratigraphic data (Pro- While the Uinta Formation has been formally divided into thero, 1996). Osborn (1929) removed the top 400 ft of Peter- a lower Wagonhound Member and an upper Myton Member, son’s Uinta A, and called it Uinta B1, leaving the remaining many geologists and paleontologists continue to utilize the sandstones of Uinta A without any known record of fossil verte- informal tripartite scheme in which the formation was initially brates. Additionally, he designated the “Amynodon” sandstone subdivided: Uinta A, B and C, from lowest to highest (Pro- of Riggs (1912) as the boundary unit between Uinta B (now thero, 1996; Sprinkel, 2007; Wood et al., 1941). The tripartite B2) and Uinta C, further altering Peterson’s initial scheme. division of the formation is a modifi cation of Osborn’s (1929) Osborn’s (1929) Uinta B1 had an upper limit bound by another stratigraphic nomenclature: Uinta A, Uinta B1, Uinta B2, and massive unit, the “Metarhinus” sandstone. The massive sand- Uinta C (Peterson and Kay, 1931; Prothero, 1996; Sprinkel, stone units of Uinta B1 are not laterally extensive, and a red 2007; Townsend et al., 2006; Walsh, 1996). The lowermost shale and siltstone unit described by Cashion (1974) might be unit, Uinta A, intertongues with the underlying Parachute a more appropriate boundary unit. The various changes in ter- Creek Member of the Green River Formation and is com- minology and marker units have caused signifi cant confusion in prised of resistant fi ne-grained sandstones that are yellowish- museum locality data, and have adversely affected the accuracy brown and yellowish-gray (Sprinkel, 2007). These beds are of the stratigraphic ranges of numerous Uintan taxa. medium to massive sandstones that weather to cliffs, ledges, Prothero (1996) constructed a magnetostratigraphic section and softer gray slopes and have been measured to be ~226 m correlating the various historical Uinta Formation localities. With 152 Murphey et al.

the local ranges of various mammalian taxa in disorder due to ear- The Uinta B2 fauna is incredibly diverse (see Appendix A) lier stratigraphic nomenclatural changes, Prothero’s (1996) work and constitutes the bulk of the assemblage that is considered early was an essential fi rst step in clarifying and correlating localities Uintan, and corresponds to biochron Ui2 of Gunnell et al. (2009). across the exposed formation in the Uinta basin. Townsend et Sediments that have yielded faunas from the Uinta B2 unit al. (2006) described a series of stratigraphic sections through include White River Pocket and areas around Kennedy’s Hole. the upper intervals of Osborn’s (1929) Uinta B2 and Uinta C, Recent fossil collecting efforts have made it possible to formally up to the contact with the overlying Duchesne River Formation defi ne biochronologic units and stratotype sections for the Uinta (Fig. 14A). These authors suggested a new boundary between the Formation (Gunnell et al., 2009). The beginning of Ui2 is defi ned Uinta B–Uinta C units based on a signifi cant lithologic and color by 26 index taxa (outlined in Gunnell et al., 2009). Numerous change. The suggested boundary is ~73 m higher than the “Amyn- taxa range through Ui2, but only a few are unique to Ui2 and odon” sandstone of Osborn (1929). Sprinkel (2007) mapped the these are from Uinta B2 beds: Amynodon reedi, Eobasileus cor- boundary between the Uinta C and the Duchesne River Forma- nutus; Eomoropus amororum, Hyrachyus eximius, Limnocyon tion higher in the section, and because it is associated with such potens, Mesonyx obtusidens, Metarhinus fl uviatalis, Metarhinus a gradual lithology change in the eastern part of the basin, he sp., Ourayia sp. nov., Pantolestes longicaudus, Paramys sp., Sci- recognized and mapped a transition zone between the forma- uravus popi, Uintatherium anceps (Gunnell et al., 2009). tions. He mapped the Uinta B-C boundary using a distinctive and Fossil mammals found in Uinta C sediments are typically reportedly more laterally persistent greenish mudstone bed that considered to be late Uintan and correlate to biochron Ui3 of occurs just below the “Amynodon” sandstone. Stratigraphic work Gunnell et al. (2009). Strata that have yielded late Uintan faunas continues in conjunction with the current paleontological work. include Myton Pocket, Kennedy’s Hole, Leota Ranch, Devil’s Playground, Leland Bench Draw, and Antelope Draw. The Uinta Paleoenvironmental History, Paleontology C fauna is equally as diverse as the Uinta B2 fauna. Twenty-four and Biochronology taxa defi ne the beginning of Ui3 including Auxontodon, Colo- don, Duchesneodus, Eosictis, Janimus, Mytonius, Mytonomeryx, The Eocene epoch is the time when many modern orders of Pentacemylus, Procyonodictis, Protadjiduamo, Protitanoth- mammals fi rst appear in the fossil record (e.g., primates, bats, erium, and Metatelmatherium. One of the striking differences artiodactyls, perissodactyls, rabbits,). Prior to the Eocene, mam- between Ui2 and Ui3 faunas is that the younger assemblage is malian faunas were composed of archaic forms that had diver- dominated by artiodactyls. Therefore, this biochron was desig- sifi ed just after the demise of the dinosaurs at the Cretaceous- nated the “Pentacemylus progressus Interval Zone” refl ecting the Tertiary boundary. During the early Eocene, global climates were abundance of this particular taxon (Gunnell et al., 2009). While exceedingly warm and produced an almost pole-to-pole tropical the taxonomic composition of Ui3 is quite different than Ui2, greenhouse (Zachos et al., 2001). The Uintan NALMA and the only a few genera make their fi rst appearance during this interval fi nal intervals of the preceding Bridgerian, mark the beginning (Gunnell et al., 2009). Current fi eldwork is focused on collecting of the end of this global greenhouse, and this is refl ected in the the upper intervals of the Uinta C unit via excavation and screen- ecological diversity of the mammals that lived during this time washing in order to increase the sample of small mammals. (Townsend et al., 2010; Woodburne et al., 2009a). Tropical arbo- real forms, typical of the early Eocene greenhouse, began to drop Uinta Formation Field Trip Stops off, and mammals that were more adapted to subtropical, even temperate conditions and habitats began to appear (Townsend et Traveling south across the Uinta basin via State Route al., 2010). The Uintan NALMA also marks a key transition in (SR) 45, we will cross both the Green and White rivers and the evolutionary history of North American mammalian faunas make our way to the base of the Uinta Formation just south of as ~31% of modern mammalian taxonomic families (ancestors Wagon Hound Canyon. Note that the driving distance from the of canids, camelids, felids, and some modern rodents) appeared Utah Field House Museum in Vernal to the fi rst fi eld trip stop in the fossil record (Black and Dawson, 1966). is 42.6 mi. Our examination of the Uinta Formation will com- The Uinta Formation fossils that form the basis of the Uintan mence at the base of the formation where it overlies strata of the NALMA are from Uinta B1, B2, and C units. The fauna from Parachute Creek Member of the Green River Formation along Uinta B1 comes mainly from the Wagonhound-Bonanza area, an the White River. From there we will travel north across Wagon early locality (Well No. 2), and the Willow Creek-Ouray area. Hound Canyon to examine rocks of the Uinta A and interbed- The assemblage from this unit is small and includes Achaenodon ded Green River Formation. Continuing north, we will exam- uintensis, Protoreodon parvus, Oromeryx plicatus, Harp- ine the green siltstones and brown sandstones typical of Uinta agolestes, Eomoropus amarorum, Hyrachyus eximius, a species B1. From here we will be able to see the typical red and gray of Triplopus, and a diverse group of brontotheres and uintatheres beds defi ning Uinta B2 and the orange-red sediments that typify (see Appendix A) (Gunnell et al., 2009; Prothero, 1996). These the Brennan Basin Member of the Duchesne River Formation taxa are considered an early Uintan assemblage corresponding to from a distance. We will spend most of the day exploring typi- biochron Ui2 of Gunnell et al. (2009). cal Uinta B2 localities in the Coyote Wash area and will then Middle Eocene rock units in the Bridger and Uinta Basins 153

Figure 14. (A) Stratigraphic section from Townsend et al. (2006) indicating the level of the Amynodon sandstone of Os- born (1929), the base of which forms the traditional Uinta B2–Uinta C boundary, and the Uinta B2-C boundary at Devil’s Playground suggested by Townsend et al. (2006). (B) View of massive cliff-forming Uinta A sandstones overlying and interfi ngering with Parachute Creek Member of the Green River Formation. (C) Measuring one of the massive sandstone units in Uinta B. (D) Collecting at WU-18, a typical Uinta B2 locality. (E) Sandstone channels associated with the WU- 110 locality complex. 154 Murphey et al. drive to the Red Wash area where the transition to Uinta C rocks bend to the northwest. After another 0.15 mi, there is a fork in is well exposed. From Red Wash, we will make our way up the road. Take the right fork to the top of the hill to your north. At section a high Uinta C fossil locality near the contact with the ~1.8 mi from Stop 2, the spoils pile from Peterson’s 1924 Doli- overlying Duchesne River Formation. Our fi nal Uinta Forma- chorhinus Quarry will be visible 0.2 mi to the east of the road. tion stop is located to the southwest of this location at a classic This historic quarry site was re-located by Vernal paleontologists Uinta Formation fossil locality, White River Pocket. Evan Hall and Sue Ann Bilbey. Return to Stop 2, then get back on SR 45 and head north and northeast past the Bonanza gil- Stop 1. Uinta Formation–Green River Formation Contact sonite mining operation. Park on the east shoulder of the highway along the White River at Wagon Hound Canyon (0.0 mi; (3.4 mi from Stop 2). 0.0 cumulative; SR 45 highway milepost 0.8) Peterson in Osborn (1895) described the contact between Stop 3. Uinta B1 and Metarhinus Sandstone (3.4 mi from the Uinta Formation and the Green River Formation as a series Stop 2; 5.3 cumulative mi; SR 45 highway milepost 5.6) of “hard brown sandstones” that conformably overlie the Green This unit is composed of a massive yellow-gray sandstone River shales. Looking north across the White River, the Para- bed underlain by greenish-gray siltstone (Fig. 14C). The Meta- chute Creek Member of the Green River Formation is exposed as rhinus sandstone is not a widespread unit. It is the boundary sloping yellow-brown, tan, and green-gray mudstone and clay- between Osborn’s (1929) Uinta B1 and B2. Off to the west both stone directly underlying the massive sandstone cliffs of Uinta the Bonanza and Independent gilsonite veins are visible. The A (Fig. 14B). Looking to the east along the White River, the most common vertebrate remains found here are turtle and croco- Green River shales are exposed as dark-brown bands within the dile. Mammals that have been recovered include mostly bronto- member. The “hard brown sandstones” of the Uinta Formation theres and other large Uintan mammals. are markedly different from the underlying Green River Forma- Continue northwest on SR 45. At 3.6 mi from Stop 3 (SR tion in that they form massive resistant cliffs of yellow brown 45 highway milepost 9.3), turn northeast at the intersection with sandstone. There are few reported identifi able vertebrate fossils Uinta County Road 3150. The dark-brown, bench supporting, from the Uinta A with stratigraphically reliable locality data, and sandstone bed to your east and west is the “Amynodon sand- none collected thus far are biostratigraphically diagnostic. The stone,” which has traditionally formed the boundary between the Uinta A may be earliest Uintan (Ui1a) in part (or possibly even Uinta B and Uinta C. Br3 in part), and therefore could be partially equivalent with the Continue down Uinta County Road 3150 for 4 mi. After Turtle Bluff Member of the Bridger Formation (see Murphey and crossing the railroad tracks, there is a dirt road turning south that Daitch, 2007). crosses over the tracks. Head south down this dirt road. At ~5.25 mi Head north on SR 45 and take a left at 1.7 mi (milepost 2.4) from Stop 3, there will be a two-track road on the right, head- and turn west onto a graded dirt road. Bear right at the bottom of ing west. This two-track goes across Quaternary sediments with the hill 0.1 mi ahead (cumulative 1.8 mi from Stop 1), and turn Uinta Formation outcrops to the north. The two-track road will onto a less well maintained road. Pull off after 0.1 mi (cumulative veer to the right, toward the outcrops. This is the region of the 1.9 mi from Stop 1). WU-18 locality. Stop 4 is located 6.4 mi from the junction of SR 45 and Uinta County Road 3150, and 10.49 mi from Stop 3. Stop 2. Uinta A and Cashion’s “a” Tuff (1.9 mi from Stop 1; 1.9 cumulative mi) Stop 4. Uinta B2 and WU-18 (10.49 mi; 15.79 cumulative mi) Driving north on Highway 45 through Wagon Hound Driving into Coyote Wash, sediments characteristic of Canyon we turn west onto a graded dirt road. Here the sloping Uinta B2 lithologies can be observed: greenish-gray, yellow, and yellow-gray siltstones and cliff forming sandstones of Uinta A are light-pink to red mudstone and claystone. The WU-18 locality well exposed. Cashion’s (1974) “a” tuff is exposed on the slope or “Gnat-Out-Of-Hell,” is a typical Uinta B2 locality in terms and is most visible just below the cliffs to the north. This tuff of the lithology (Fig. 14D). WU-18 is the single most produc- has not yet been dated because it is so crystal poor. The bound- tive Uinta B2 locality currently known. The main productive ary between the Uinta A and Uinta B is mapped as occurring level is a dark-brown sandy mudstone that has produced numer- half way up the cliff above the tuff. Beds of lacustrine shale are ous mammals, including the early primate Chipetaia and large interbedded with the lower part of the Uinta A sequence in this bodied amynodont rhinocerotoids and brontotheres (Rasmussen, general area, refl ecting the complex transition from lacustrine 1996; Rasmussen et al., 1999a). Coprolites with small mammal to fl uvially dominated depositional environments. Fragments of bones from WU-18 indicate that this locality may have been par- fossilized bone and wood have been found in some of the side tially accumulated by a diurnal raptor (Thornton and Rasmussen, canyons in this area. 2001). Stratigraphically above and to the northeast of WU-18 are the “Amynodon sandstones,” the classic boundary between Optional Stop Uinta B2 and Uinta C (Osborn, 1929). This massive sandstone Peterson’s 1924 Dolichorhinus Quarry. Continue southwest unit sometimes yields large-bodied ungulates, such as its rhinoc- from Stop 2. After passing a small gilsonite mine the road will erotoid namesake, Amynodon, and brontotheres. Screen washing Middle Eocene rock units in the Bridger and Uinta Basins 155

at WU-18 has resulted in the recovery of multiple bird bones, sediments and are highly fragmented. Small-scale excavations increased samples of primates and small rodents. in this locality complex have recovered a number of articulated Return to SR 45 and head north. Pull over carefully as close skeletons, including the skull and postcrania of the tiny deer- to the guardrails as possible at SR 45 highway milepost 11.6, like Protoreodon and a skull, forelimb, and vertebral column of which is 1.8 mi from Stop 4. medium sized rodent. From Stop 7, turn left, heading northeast for ~2 mi until Stop 5. Devil’s Playground Red Beds (1.8 mi from Stop 4; reaching an intersecting road, and turn left again. After ¼ mi, 17.59 cumulative mi; SR 45 highway milepost 11.6) you will see an intersection of four roads. Take the road to your To the east there is a prominent red bed that Townsend et al. immediate left, which will lead west along a pipeline road. (2006) consider to be the boundary between the Uinta C and the Stay on this road for the next 5–6 mi. This is a main transport Brennan Basin Member of the Duchesne River Formation. road in the oil fi elds. After mile 7, this road will become Glen Heading south on SR 45, about ¾ of a mile, make a sharp Bench Road. At about mile 7¾, the road will jog to the right right and travel a mile and veer to the right at ~1 mi from the turn and you will see an intersecting road. Turn left and begin the off from SR 45. Travel for another 1¼ mi to a well pad and park. steep climb up to the top of Glen Bench. There is a sharp turn at about mile 9¼. After this turn, continue along the road until you Stop 6. Uinta C WU-110 Locality Complex (2.25 mi; come across an intersecting road just past mile 10. Cross this 19.84 cumulative mi) intersecting road and follow it down the northwest face of Glen The WU-110 locality complex spans the Uinta B2–Uinta Bench into Antelope Draw. There will be a three-way intersec- C boundary as defi ned by Townsend et al. (2006). The pri- tion at about the 11¾ mi mark. Veer left, heading northwest. At mary lithology in the area consists of gray-green, tan, and mile 13, take a sharp left, driving for more than a ½ mi. Make yellow-brown mudstones. A distinctive feature of this complex a hard right heading due north, this is another major transport includes large dark-brown fi ne- to coarse-grained channel sand- road. At ~14½ mi, you will come to two roads that veer off to stone beds (Fig. 14E). Numerous fossils have been discovered the right, one heading northeast, the other southeast. Take the in a dark-brown siltstone layer that weathers pink at WU-110. northeast road. After mile 15, the road will loop around a large These fossils include the primate Ourayia, the artiodactyls Pro- canyon. Continue along a series of tall escarpments, and the toreodon and Leptotragulus, as well as numerous rodents. The road will make another loop after mile 17. At this point, fi nd a remaining localities in the area WU-115, 116, 117, have also place off of the main road to park. Looking due east, you will been quite productive. In addition to more small mammal den- see the WU-210 locality complex. taries, large-bodied perissodactyls, carnivore postcrania, and a specimen of Simidectes have been recovered. Stop 8. Uinta C, WU-210, Contact with Duchesne River Return to SR 45. Turn right and travel ¾ of a mile to the Formation (18.71 mi; 55.24 cumulative mi) fi rst dirt road exit on the left, this is the Sand Ridge Road. Take Nearing the top of the Uinta Formation, the sediments are a right. Follow Sand Ridge Road west, and at mile 7 from Stop brightly pigmented as they weather deep brown, bright orange, 6, you will come to a road that veers left. Do not turn here; con- and various shades of red. In the WU-210 area, sloping red, tinue west. After mile 9, the road will turn south, and at mile 10 gray, and tan mudstone beds are interbedded with large, ledge- you will encounter a fork in the road. Make a right here, and in forming blocky yellow-brown sandstone beds. At this level in another ½ mi there will be another fork where you will take a the formation, the Duchesne River and Uinta formations are right again. Continue on this road for 3.5 mi. Close to mile 14, locally interbedded. Conglomeratic resistant sandstone chan- you will see a road veering left, do not turn here, continue straight nels typical of the Brennan Basin member of the Duchesene ahead. At approximately mile 15, there is a road that veers right River Formation can be viewed at the top of WU-210. This (north). Turn here. At the 16.5 mi mark, pull off the road and locality and others surrounding it are highly productive and park. Looking due west, you will see a large gray and orange have yielded numerous perissodactyls including early tapiroids, outcrop in the distance, this is the WU-22 locality complex. such as Isectolophus, and almost complete skeletons of large rodents such as Pseudotomus. Stop 7. Uinta C and WU-22 Locality Complex (16.69 mi; From WU-210, head north toward the loop in the road and 36.53 cumulative mi) make your way back to the main road from which we approached Note the Deseret Power Plant to the northeast and a large this complex of localities. Take a right heading north going bench directly north of the locality complex with strata consist- toward the top of Deadman Bench. This road will intersect with ing of highly variegated red, purple, gray, and orange sediments. the main Deadman Bench road that trends east-west. Take a right These claystone and mudstone beds surround the main drainage on this main road and head back to SR 45 (6.6 mi). of this region of the basin, Red Wash. Fossils at WU-22 are gen- Head southeast on SR 45 for 5.9 mi and turn northeast onto erally found in gray, tan, yellow, and green-gray mudstone beds. a short graded road at SR 45 highway milepost 14.0. There is an Typically, as with other localities, fossils here are found directly old information kiosk and small wind turbine located ~300 ft east on the surface, where they have eroded out of the surrounding of the highway at this location. 156 Murphey et al.

Stop 9. Uinta Formation—Duchesne River Formation C (Myton Member) and the Brennan Basin Member of the Duch- lower Transition Zone Contact (16.2 mi from Stop 8; esne River Formation, type sequence of the “Randlett Horizon.” 71.44 cumulative mi; SR 45 highway milepost 14.0) For further explanation, see Stop 1 of the Duchesne River Forma- This is the approximate stratigraphic location of the base of tion fi eld trip below. the transition zone between the Uinta C (Myton Member) and the Brennan Basin Member of the Duchesne River Formation as START OF DUCHESNE RIVER FORMATION TRIP mapped by Sprinkel (2007). Drive northwest along SR 45 for 0.7 mi until you come to Duchesne River Formation an intersection with the paved road which leads to the Deseret Power Plant heading southwest and stop. The Duchesne River Formation occurs only in the northern and western parts of the Uinta basin in Utah, where it is wide- Stop 10. Uinta Formation–Duchesne River Formation spread. Its defi nition and stratigraphic history are related to the Upper Transition Zone Contact (0.7 mi from Stop 9; underlying and apparently conformable Uinta Formation. In the 72.14 cumulative mi; SR 45 highway milepost 14.7) northern part of its distribution along the Uinta Mountain front, it Looking east you can view the approximate stratigraphic unconformably overlies rocks of Triassic and Jurassic age. Inter- location of the top of the transition zone between the Uinta C estingly, the Duchesne River Formation contains tar sand in the (Myton Member) and the Brennan Basin Member of the Duch- Asphalt Ridge area (U.S. Geological Survey, 1980). esne River Formation as mapped by Sprinkel (2007). The Duchesne River Formation was deposited under mostly Continue northwest on SR 45 and pull over at SR 45 high- fl uvial conditions but includes some minor lacustrine deposits. way milepost 31.5, 16.7 mi from Stop 10. Andersen and Picard (1972) proposed the presently accepted stratigraphy and described the following members in ascend- Stop 11. View of Asphalt Ridge (16.7 mi from Stop 10; ing stratigraphic sequence: Brennan Basin, Dry Gulch Creek, 88.84 cumulative mi; SR 45 highway milepost 31.5) Lapoint, and Starr Flat. For the last 16.7 mi, you’ve been driving through rocks of The Brennan Basin Member is composed of soft to mod- the Brennan Basin Member of the Duchesne River Formation. erately resistant, light-to-medium red, light-gray, light-brown, This stop affords an excellent view of Asphalt Ridge, a geologi- yellow, and tan ledgy sandstone, mudstone, conglomerate, shale, cally interesting structure composed of uplifted and tilted strata and siltstone, with a maximum thickness of ~600 m (1970 ft) of the Brennan Basin Member of the Duchesne River Forma- south of Vernal. This member thins signifi cantly to the east and tion unconformably overlying uplifted and tilted strata of the late west. The Dry Gulch Creek Member consists of soft to moder- Cretaceous Mesa Verde Group. The structural geology of Asphalt ately resistant, light-to-medium gray, medium red, purplish gray, Ridge documents different episodes of uplift and erosion related and yellow sandstone, mudstone, shale, and conglomerate. It is to Uinta Mountain tectonics. ~149 m (490 ft) thick. The Lapoint Member consists of mostly The fi nal Uinta Formation fi eld trip stop is a visit to the soft, light-red, tan, and yellow sandstone, siltstone, and mudstone classic fossil locality White River Pocket. Detailed directions with minor amounts of conglomerate. It contains diagnostic beds to this fi eld trip stop are not included in this fi eld trip guide of light-gray to medium-gray or bluish-gray bentonite and ranges because there are numerous ways to get there. As a result, the in thickness from ~119–299 m (390–980 ft). The Lapoint ash, mileage to this fi eld trip stop is not connected to the preceding which forms the base of the Lapoint Member (where most of fi eld trip stops. To get to White River Pocket, travel to the point the vertebrate fossils have been discovered), has been dated at at which SR 88 crosses the White River Bridge south of the 39.74 Ma ± 0.07 Ma (Prothero and Swisher, 1992). The Starr hamlet of Ouray. Flat Member consists of moderately resistant, light-to-medium red and tan sandstone, mudstone, with signifi cant red and gray Stop 12. White River Pocket conglomerate, and with a thickness of ~149 m (490 ft) (Rowley Head South on SR 88 for 0.2 mi. Pull over onto the shoulder. et al. 1985). It outcrops spottily along the southern fl ank of the The badland hill and surrounding area to your east is a classic Uinta Mountains. fossil locality called White River Pocket. The fossil assemblage The history of stratigraphic nomenclature for the Duch- from White River Pocket provided the basis for early Uintan fau- esne River Formation is somewhat confusing (Table 5). Clar- nas and are signifi cant because a large number of small mammal ence King (1878) named the Uinta Group for what we now call taxa were found there. the Uinta and Duchesne River formations. Peterson, in Osborn Drive north on SR 88 for 11.7 mi. Just past the Pelican Lake (1895), proposed that the Uinta Group be subdivided into A, B, Café, turn left onto Uinta County Route 2762. Drive west for C horizons, with the C horizon being equivalent in part to what 4.7 mi and park on the north shoulder. To your right are spec- we now know as Duchesne River Formation and equivalent in tacular cliffs that are historically known to paleontologists as part to the upper Uinta Group. Douglass (1914) suggested that “Randlett Point,” with excellent exposures of the sequence that the name Uinta Group be replaced by the name Uinta Tertiary to contains the conformable boundary between the uppermost Uinta avoid confusion with the Precambrian Uinta Mountain Group. Middle Eocene rock units in the Bridger and Uinta Basins 157

Scott in Peterson (1931c) named the Duchesne Formation, and it was renamed Duchesne River Formation by Kay (1934) because the name Duchesne Formation was preoccupied. Kay (1934) defi ned the Duchesne River Formation by removing the red beds Member Member Member Starr Flat

Picard (1972) of Osborn’s (1895) upper Uinta Group, recognizing they have a Brennan Basin Lapoint Member Dry GulchCreek younger mammalian fossil fauna. He also recognized three bio- stratigraphic “horizons:” Randlett, Halfway, and Lapoint. Gazin (1955) removed the Randlett horizon from the Duchesne River Formation, recognizing that it contains a fauna that is almost identical to that of the Uinta C. Warner (1963) recognized two member member members, the Minor Bentonite member being equivalent to the Minor bentonite Major bentonite Brennan Basin and Dry Gulch Creek members, and the Major Bentonite Member being equivalent with the Lapoint Member. Andersen and Picard (1972) named the currently recognized members: Brennan Basin, Dry Gulch Creek, Lapoint, and Starr Flat members. With regard to Andersen and Picard’s (1972) stratigraphic terminology, the Brennan Basin Member is equiva-

Halfway horizon lent to the Randlett Horizon of Kay (1934) and the lower part of Halfway Horizon, the Dry Gulch Creek Member is equiva- lent with the upper part of the Halfway horizon, and the Lapoint Member is equivalent to the Lapoint horizon.

Fossils and Biochronology of the Duchesne (not studied) (not studied) (not studied) Lapoint horizon Lapoint horizon

Halfway horizon

Randlett horizon River Formation Kay (1934) Gazin (1955) Warner (1963) Andersen and

Formation Formation Duchesne River River Duchesne In comparison with the underlying Uinta Formation and cor- relative strata of Duchesnean age in coastal southern California,

FORMATION STRATIGRAPHIC NOMENCLATURE the vertebrate fossil fauna of the type Duchesne River Formation is sparse. However, it is a critically important period in mamma- lian evolution (Lucas 1992; Rasmussen et al., 1999b; Robinson Peterson in (1931) et al., 2004). Although it is the nominal stratotype for the Duch- Duchesne Formation esnean NALMA (Wood et al.,1941), its validity has been the Scott subject of some controversy among paleontologists (see Lucas, 1992; Wilson, 1978). Nevertheless, the Duchesnean NALMA has now been widely accepted by paleontologists (Rasmussen et al., 1999b; Robinson et al., 2004). Lucas (1992. p. 88) made

redbeds the observation that the Duchesne River Formation “has either underlying Kay (1931) Upper Uinta Peterson and Uinta Tertiary separated from been questioned, abandoned, subdivided, or defended.” Scott (1945) regarded the Duchesne River Formation as Oligocene in age. Simpson (1946) and Gazin (1955) considered it Eocene. Faunally, the importance of the Duchesne River Formation and TABLE 5. HISTORY OF DUCHESNE RIVER (1914)

(in part) the Duchesnean NALMA is based on the fact that it records a Douglass

Uinta Tertiary major faunal replacement in North America, as demonstrated by its large number of fi rst and last occurrences, as well as the small number of genera that are restricted to it (Appendix A; Black and Dawson, 1966; Robinson et al., 2004). Robinson et al. (2004) tentatively assigned Duchesnean fi rst appearances to Group include Hyaenodon, Duchesneodus, Duchesnehippus interme- = upper Uinta Osborn (1895)

C horizon (in part) dius, Amynodontopsis, and Eotylopus. The fauna of the Brennan Basin Member, which was previ- ously assigned to the Randlett horizon and lower part of the half- way horizon of Kay (1934), includes a faunal assemblage that

(in part) is generally considered to be intermediate between the Uinta C King (1878) Peterson in Uinta Group (biochron Ui3, Gunnell et al., 2009) and the Lapoint Member 158 Murphey et al.

of the Duchesne River Formation, although most workers regard Roeder and Paul Murphey of the San Diego Natural History it as late Uintan. The Brennan Basin Member outcrops over a Museum discovered a highly fragmented mammal tooth that was fairly large area, and several somewhat productive localities are tentatively identifi ed as a partial upper molar of a large camelid known. It contains a reasonably diverse assemblage of Uintan from near the base of the member (Paul Murphey, 2008, unpub- artiodactyls including Protoreodon, Pentacemylus, Diplobunops, lished fi eld notes). and Leptotragulus. Perissodactyls include a mix of Uintan and Although the Lapoint and Brennan Basin members of the Duchesnean brontotheres, as well as tapirs and rhinos. Speci- Duchesne River Formation are the most fossiliferous, fossils are mens of the Uintan lagomorph Mytonolagus and the rodents generally scarce throughout the formation. Any new discoveries Pareumys and Mytonomys have also been collected (Rasmussen would be highly signifi cant, especially from the Dry Gulch Creek et al., 1999b). and Starr Flat members. It is also noteworthy that reptiles in gen- Until recently, only two published specimens had ever been eral are poorly represented from the Duchesne River Formation. reported from the Dry Gulch Creek Member, and these repre- Much work remains to be done in order to document the faunal sented the “Halfway Fauna.” The specimens include the holotype changes within the Duchesne River and ascertain the position of of Duchesnehippus intermedius, a genus of horse that is pur- the Uintan-Duchesnean boundary. portedly intermediate between the Uintan horse Epihippus and the Chadronian horse Mesohippus, and a femur and astragalus Duchesne River Formation Field Trip Stops tentatively identifi ed as (?) hyracodontid rhinoceros. Walsh and Murphey (2007) discovered a locality that produced a diversity of Our exploration of the Duchesne River Formation begins at small mammals in the Dry Gulch Creek Member (67 identifi able Randlett Point where the contact between the Myton Member specimens). SDSNH Loc. 5939 (“Halfway Hollow One”) is situ- of the Uinta Formation (informal member C of Uinta Forma- ated roughly in the stratigraphic middle of the Tdr-dgc, at about tion) and the overlying Brennan Basin Member of the Duchesne the same level as the type locality of D. intermedius. Taxa from River Formation is well exposed and relatively easy to discern. this locality include Nanodelphys sp., Lipotyphlan indet., Pareu- From there we travel north through strata of the Brennan Basin mys cf. P. guensburgi, cf. Janimus sp., Metanoiamys sp., Protad- Member in Halfway Hollow, and then examine exposures of the jidaumo typus, Protadjidaumo sp. (large), Eomyidae, unidenti- Dry Gulch Creek Member and discuss the paleontology of this fi ed gen. and sp., Eomyidae? new gen. and sp., Geomorpha, new sparsely fossiliferous unit that appears to contain the boundary gen. and sp., cf. Griphomys sp., cf. Passaliscomys sp., Mytono- between the Uintan and Duchesnean NALMA’s although the lagus sp., and Artiodactyla indet. Thus, the number of identifi - precise stratigraphic position of the boundary has not yet been able mammal specimens from the Dry Gulch Creek Member has determined. From a distance, we then view the Lapoint Member been increased dramatically. Fourteen additional mammalian and overlying Starr Flat Member in Halfway Hollow. Heading taxa are now known from this member, nine of which are new east along the Lapoint highway to Twelvemile Wash, our last stop records for the Duchesne River Formation as a whole. Impor- is at the site of the famous Carnegie Museum Titanothere Quarry tantly, Pareumys cf. P. guensbergi, Protadjidaumo typus, and cf. just above the base of the Lapoint Member, as well as the Lapoint Passaliscomys sp. suggest a Duchesnean age for most or all of ash at quarry level. the Dry Gulch Creek Member, and that the Uintan-Duchesnean boundary occurs somewhere within the upper part of the Brennan Stop 1. Randlett Point (0.0 mi; cumulative 0.0 mi) Basin Member or the lower part of the Dry Gulch Creek Member. Peterson and Kay (1931) defi ned the base of the Duch- Additional stratigraphically controlled screen washing efforts esne River Formation at this location. The boundary between are needed to determine the precise stratigraphic location of the the Uinta C (Myton Member) and overlying Brennan Basin Uintan-Duchesnean boundary in the Uinta basin. Member in this part of the Uinta basin is easier to discern The mammalian fauna of the Lapoint Member is considered than further to the east in the type area of the Uinta Forma- the type Duchesnean fauna, although misidentifi cations, data tion in the Coyote basin. Here the contact is marked by the loss, and mistakes made by earlier workers have resulted in con- change from slope-forming pink, purple, light-gray, and red siderable confusion regarding what was actually collected at this variegated beds of mudstone and siltstone to orange, reddish- horizon. Most of these problems seem to have been resolved by brown and gray ledge forming siltstone and sandstone beds of the efforts of Rasmussen et al. (1999b). The Lapoint fauna con- the overlying Brennan Basin Member (Fig. 15A). The Carn- tains diverse assemblages of artiodactyls such as Protereodon, egie Museum’s Randlett Quarry is ~1 mile to the north of this Agriochoerus, Simimeryx, and Brachyops, and perissodactyls location. With the exception of the large Duchesnean bronto- such as Colodon, Hyracodon, Duchesnehippus, and Duchesneo- there Duchesneodus, the fauna of the Brennan Basin Member dus. Also present are carnivores, the mesonychid Hessolestes, the resembles that of the Uinta C, and the fauna of the lower part hyaenodontan Hyaenodon, the lipotyphlan Centetodon, and the of the Brennan Basin Member is regarded by most workers to rodents Pareumys and Protadjidaumo. be Uintan in age. No published accounts of fossils have been reported from Drive east on the Ouray-Randlett Road (Uinta County Road the stratigraphically highest Starr Flat Member. However, Mark 2762) for 4.7 mi to the junction of State Route (SR). Middle Eocene rock units in the Bridger and Uinta Basins 159

Stop 2. Brennan Basin Member–Dry Gulch Creek Member thick dark-red indurated fi ne-grained sandstone, interpreted by Boundary (19.0 mi from Stop 1; cumulative 35.6 mi) Andersen and Picard (1972) as a paleosol, can be seen ~18 m Andersen and Picard (1972) defi ned this boundary as (60 ft) above the top of the sandstone. Fossil snails and bone the base of the lowest bed of fi ne-grained rock overlying the fragments have been obtained from screenwashing sediments highest resistant sandstone of the Brennan Basin Member collected from between the boundary sandstone bed and red (Fig. 15B). This contact was mapped by Rowley (unpub- paleosol ~0.25 mi to the southwest of this location. However, lished fi eld maps), Rowley et al. (1985), and Sprinkel (2007) no identifi able mammal fossils have been found to date in as the sandstone bed visible at this stop. This tan sand- these samples. stone and conglomerate is highly variable in thickness, and Continue driving north on Halfway Hollow Road for 2.4 mi. crosses Halfway Hollow Road at UTM Zone 12, 609901 mE, You will stop to the east of a large evaporation pond with 5 tank 4470184 mN (NAD 27). A widespread but thin 0.5 m (1.6 ft) batteries located along its southern edge.

Figure 15. (A) The contact between the Uinta C (Tu-c) and Brennan Basin Member of the Duchesne River Forma- tion (Tdr-bb) at Randlett Point, Uintah County, Utah. (B) View looking north- east at the sandstone bed (Ss) that di- vides the Brennan Basin Member of the Duchesne River Formation (Tdr-bb) from the overlying Dry Gulch Creek Member (Tdr-dgc) in Halfway Hollow, Uintah County, Utah. (C) View looking west at the Carnegie Museum Teleodus Quarry (TQ), the lower bentonite bed (LB) that marks the boundary between the Dry Gulch Creek Member (Tdr-dgc) of the Duchesne River Formation and the overlying Lapoint Member (Tdr- lp), and the Lapoint tuff (LpT), Uintah County, Utah. 160 Murphey et al.

Stop 3. Dry Gulch Creek Member Fauna (2.4 mi from sils have been reported. That said, it has yet to be thoroughly Stop 2; cumulative 38.0 mi) prospected. It is possible that the fauna of the Starr Flatt Mem- This part of Halfway Hollow has produced the only known ber is younger than the Duchesnean NALMA. mammal fossils from the Dry Gulch Creek Member. A rela- Turn east onto the Lapoint highway and drive for 2.5 mi. Just tively fossil-rich micromammal quarry was discovered just to past SR 121 highway milepost 31, turn north onto a two-track the east of the road (Halfway Hollow One, SDSNH Loc. 5939) road and drive for 0.7 mi for a total of 3.2 mi from Stop 4. The in 2006, and is discussed above. These fossils are currently Carnegie Teleodus Quarry pit is located near the summit of the being formally described, and it is likely that additional screen large hill to the northwest of your location. washing of sediments from this locality will yield additional mammal fossils. Peterson (1931c, p. 70–71) described the type Stop 5. Carnegie Titanothere Quarry and Lapoint Ash specimen of Epihippus (Duchesnehippus) intermedius (along (3.2 mi from Stop 4; cumulative 42.1) with specimens he tentatively identifi ed as a partial femur and This is the site of the famous Carnegie Titanothere Quarry astragalus of a hyracodontid rhinoceros) as coming from “the (also known as the Teleodus Quarry and the Duchesneodus west side of Halfway Hollow, from a red clay about 8 feet Quarry). For many years prior to the discovery of this quarry, thick, overlain by a massive brown sand and underlain by a there remained a recognized hiatus between the earlier mam- reddish nodular clay associated with astragalus and horse jaw.” malian faunas of the Rocky Mountain intermontane basins and The specimens were collected by John Clark on 21 July 1931 the younger mammalian faunas of the White River badlands of (Alan Tabrum, Carnegie Museum of Natural History, 2006, South Dakota and Nebraska. In discovering and working this written commun.). quarry, J.L. Kay of the Carnegie Museum made the fi rst impor- Continue driving north/northeast on Halfway Hollow Road tant paleontological discovery to begin to fi ll this knowledge for 0.9 mi until you reach SR 121 (Lapoint Highway). Stop just gap. Peterson (1931a, 1931b, 1931c) and Peterson and Kay beyond the cattle guard. (1931) described the fossils from this locality and from other localities in the then Lapoint horizon. These workers regarded Stop 4. View of Lapoint and Starr Flat Members (0.9 mi the fauna as “perfectly transitional” (Peterson, 1931c, p. 62). from Stop 3; cumulative 38.9 mi) In addition to the brontothere fossils collected from the quarry, Looking north from this spot affords an excellent view of bunodont and selenodont artiodactyls, a rodent, an insecti- the Lapoint Member, the Starr Flatt Member, and the distant vore, hyracodontid rhinos, and a creodont and mesonychid Uinta Mountains with 3668 m (12,031 ft) high Leidy Peak were discovered and described. Note that a recent thorough re- on the skyline. The base of the Lapoint Member is defi ned as inspection of the quarry and surrounding area has failed to yield the lowest widespread bentonite bed. Thick, laterally persis- any additional fossils. tent bentonite beds are characteristic of this member. Several The thin bentonite bed visible on the slope below the quarry additional American Museum of Natural History and Carnegie is considered to be the base of the Lapoint Member by Ander- Museum fossil localities are located in this part of Halfway sen and Picard (1972). However, the Lapoint ash (39.74 Ma ± Hollow to the north of the Lapoint highway, although recent 0.07 Ma, Prothero and Swisher, 1992) is actually the thicker, efforts to relocate them have proven unsuccessful. The con- higher bentonite bed that is cut by the red sandstone channel in tact between the Lapoint and overlying Starr Flat members is which the Carnegie Quarry fossils were preserved (Fig. 15C). located at the approximate level of the tops of the red cliffs you Therefore, the Carnegie Titanothere Quarry is located strati- see in the distance at the north end of Halfway Hollow. How- graphically just above the base of the Lapoint Member and is ever, the top of the Duchesne River Formation is not visible slightly younger than the Lapoint tuff that was cut by the red from this location. As discussed above, the Starr Flatt member sandstone channel. has yielded tooth fragments from one locality, but no other fos- End of Uinta basin fi eld trip. Middle Eocene rock units in the Bridger and Uinta Basins 161

APPENDIX A. BIOCHRONOLOGIC RANGES OF BRIDGERIAN, UINTAN, AND DUCHESNEAN MAMMALIAN GENERA Order Family Genus Br1a Br1b Br2 Br3 Ui1a Ui1b Ui2 Ui3 Du Cetartiodactyla Agriochoeridae Agriochoerus X Diplobunops X X X Protoreodon X X X Camelidae Poebrodon X X Dichobunidae Auxontodon X Diacodexis X Bunomeryx X X Hylomeryx X X Mytonomeryx X Neodiacodexis X Pentacemylus X X Entelodontidae Brachyhyops X Dyscritochoerus X Helohyidae Achaenodon X X Helohyus X X X X Homacodontidae Antiacodon X X X X X Homacodon X Mesomeryx X Microsus X X X Hypertragulidae Simimeryx X Oromerycidae Oromeryx X X X Protylopus X X X Protoceratidae Leptoreodon X X X Leptotragulus X X X Poabromylus X Carnivora Miacidae Miacis X X X X X X X Miocyon X X Oodectes X X X Procyonodictis X Tapocyon X X Uintacyon X X X X X X Vulpavus X X X Viverravidae Didymictis X Viverravus X X X X X Cimolesta Apatamyidae Apatemys X X X X X X X Pantolestidae Pantolestes X X X X X X X Simidectes X X Leptictida Leptictidae Palaeictops X X X Lipotyphla Sespectidae Crypholestes X Scenopagus X X X X X X Geolabididae Centetodon X X X X X X X Marsholestes X X X Erinaceidae Entomolestes X X X Talpavus X X X X X Micropternodontidae Micropternodus X Nyctitheriidae Nyctitherium X X X X X X X X Pontifactor X X Chiroptera Icaronycteridae Icaronycteris X Condylarthra Hyopsodontidae Hyopsodus X X X X X X X X Creodonta Oxyaenidae Patriofelis X X X X Hyaenodontidae Apataleurus X Hyaenodon X Limnocyon X X X X X X Oxyaenodon X X Proviverra X Sinopa X X X X Tritemnodon X X X X Machaeroides X (continued) 162 Murphey et al. APPENDIX A. (continued) Order Family Genus Br1a Br1b Br2 Br3 Ui1a Ui1b Ui2 Ui3 Du Dinocerata Uintatheriidae Bathyopsis X X Eobasileus X X Uintatherium X X X X Lagomorpha Leporidae Mytonolagus X X X Marsupialia Didelphidae Armintodelphys X Copedelphys X X X X X X X Peradectes X X X X X X Herpetotherium X X X X X X X X Nanodelphys X Mesonychia Mesonychidae Mesonyx X X X X X X Harpagolestes X X X X X Hessolestes X Palaenodonta Epoicotheriidae Tetrapassalus X X X X Metacheiromyidae Brachianodon X X X Metacheiromys X X X Perissodactyla Amynodontidae Amynodon X X X Megalamynodon X Brontotheriidae Diplacodon X X Dolicorhinus X Duchesneodus X X Eotitanops X Metarhinus X X Metatelmatherium X Paleosyops X X X X Protitanotherium X X Mesatirhinus X X Sphenocoelus X X Sthenodectes X Eomoropidae Eomoropus X X Equidae Hyracotherium X Orohippus X X X X X Epihippus X X X X Duchesnehippus X Isectolophidae Isectolophus X X X X X X Family indet. Desmatotherium X Helaletidae Colodon X X Helaletes X X X X Heptodon X Hyracodontidae Dilophodon X X X X X X Epitriplopus X X X Hyrachyus X X X X X X Hyracodon X Triplopus X X X X Primates Notharctidae Cantius X Notharctus X X X X X X Smilodectes X X X Omomyidae Ageitodendron X Anaptomorphus X X X Artimonius X Chipetaia X Gazinius X X X Hemiacodon X X X Macrotarsius X X Mytonius X Omomys X X X X X X Ourayia X X Shoshonius X Sphacorhysis X Trogolemur X X X X X X X Uintanius X X X Utahia X X Washakius X X X X Wyomomys X (continued) Middle Eocene rock units in the Bridger and Uinta Basins 163

APPENDIX A. (continued) Order Family Genus Br1a Br1b Br2 Br3 Ui1a Ui1b Ui2 Ui3 Du Plesiadapiformes Microsyopsidae Microsyops X X X X X X Uintasorex X X X Rodentia Allomyidae Spurimus X Eutypomyidae Janimus X X Family indent. Uintamys X Natrona X Ischyromyidae Acritoparamys X X X Microparamys X X X X X X X Mytonomys X X X Paramys X X X X X X Pseudotomus X X X X X Quadratomus X X X Reithroparamys X X X X X Thisbemys X X X X X X X Uintaparamys X X X X X X X Sciuravidae Knightomys X Pauromys X X X X Sciuravus X X X X X X Taxymys X X X Tillomys X X X X Cylindrodontidae Mysops X Pareumys X X X X X Eomyidae Metanoiamys X X X Protadjidaumo X X Protopychidae Protoptyhcus X X X Family indent. Griphomys X Heliscomyidae Passaliscomys X Tillodontia Esthonychidae Tillodon X Trogosus X X X Taeniodonta Stylinodontidae Stylinodon X X X X X Note: Modified from Gunnell et al. (2009).

ACKNOWLEDGMENTS Betts, C.W., 1871, The Yale College expedition of 1870: Harper’s New Monthly Magazine, v. 43, p. 663–671. Black, C.C., and Dawson, M.R., 1966, A review of late Eocene mammalian The authors wish to express their deepest gratitude to the faunas from North America: American Journal of Science, v. 264, p. 321– many colleagues, students, and volunteers who have assisted 349, doi:10.2475/ajs.264.5.321. with our fi eld work in the Bridger and Uinta basins over the Bradley, W.H., 1931, Origin and microfossils of the Green River Formation of Colo- rado and Utah: U.S. Geological Survey Professional Paper 168, p. 58. years. In particular, we are grateful for the participation of Bradley, W.H., 1964, Geology of Green River Formation and associated Eocene faculty, staff, and students from the University of Colorado rocks in southwestern Wyoming and adjacent parts of Colorado and Utah: Museum, Washington University in St. Louis, San Diego Nat- U.S. Geological Survey Professional Paper 496-A, p. 1–86. Brand, L.R., 2007, Lacustrine deposition in the Bridger Formation: Lake Gos- ural History Museum, College of Charleston, and Lamar Uni- iute extended: The Mountain Geologist, v. 44, p. 69–77. versity. We also thank the Wyoming and Utah State Offi ces of Brand, L.R., Goodwin, H.T., Ambrose, P.D., and Buchheim, H.P., 2000, Tapho- the U.S. Department of Interior Bureau of Land Management, nomy of turtles in the Middle Eocene Bridger Formation: Palaeogeogra- phy, Palaeoclimatology, Palaeoecology, v. 162, p. 171–189, doi:10.1016/ and the Kemmerer, Rock Springs and Vernal fi eld offi ces. S0031-0182(00)00111-5. Without the support of the BLM, our fi eld work would not Bryant, B., Naeser, C.W., Marvin, R.F., and Mehnert, H.H., 1989, Upper Cre- have been possible. Finally, we thank Justin Strauss and Mar- taceous and Paleogene sedimentary rocks and isotopic ages of Paleogene tuffs, Uinta Basin, Utah: U.S. Geological Survey Bulletin 1787-J, p. 1–22 p. garet Madsen for their assistance with the compilation of the Cashion, W.B., 1974, Geologic map of the Southam Canyon quadrangle, Uin- fi eld trip mileage log. tah County, Utah: U.S. Geological Survey Miscellaneous Field Studies Map, MF-579. Cashion, W.B., 1986, Geologic map of the Bonanza quadrangle, Uintah County, REFERENCES CITED Utah: Miscellaneous Field Studies Map, MF-1865. Chadey, H.F., 1973, Historical Aspects of the Green River Basin, Wyoming, in 25th Alexander, J.P., and Burger, B.J., 2001, Stratigraphy and taphonomy of Grizzly Field Conference on the geology and mineral resources of the greater Green Buttes, Bridger Formation, and the middle Eocene of Wyoming; Eocene River Basin, Wyoming: Wyoming Geological Association, p. 27–33. biodiversity; unusual occurrences and rarely sampled habitats: Topics in Cope, E.D., 1872, Descriptions of some new Vertebrata from the Bridger Group Geobiology, v. 18, p. 165–196. of the Eocene: Proceedings of the American Philosophical Society, v. 12, Andersen, D.W., and Picard, M.D., 1972, Stratigraphy of the Duchesne River p. 460–465. Formation (Eocene-Oligocene), northern Uinta Basin, Northwestern Cope, E.D., 1873, On the extinct Vertebrata of the Eocene of Wyoming, Utah: Bulletin of the Utah Geological and Mineral Survey, v. 97, p. 1–29. observed by the expedition of 1872, with notes on the geology, in Hayden, 164 Murphey et al.

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