Lacustrine Deposition In The Bridger Formation: lake Cosiute Extended'

LEONARD R. BRA ND2

1. M anuscri pt received Septemb er 30. 2004; Accepted M arc h' , 2007 2. Department of Earth and Biological Sci ences, Lom a Lind a U niversity, Loma Linda, CA 92350; lbrand wllu.ed u

ABSTRACT

The Green River Formation was de posited in Lake Gosiute , until the lacustrine system shifted to the more fluvial environment of the Bridger Formation. In the so uthern part of Bridger A exposures, sedi­ ment s become increasingly lacustrine and interfinger with the main bod y of the Laney Shale member of the Green River Formation. High er in Bridger A several widespread limestone marker beds are separated by mud stones. Bridger B also co nsists of mu dstones alternating with limestones. A nu mber of these lime­ stones have no w been mapped . They extend across the entire existing Bridger B exposures, and rep re­ sent basin-wide shallow lakes. The se lakes were filled by volcaniclastic input from episodes of volcanism to the nort h . The lacustrine de posits of the Green River Formation consist largely of laminated ke rogen­ rich micrites (oil sha les) that grad e laterally into massive limes tones or siliciclastic mudstones, whereas the lacustrine deposits of the Bridger Formation are wides pread, massive limestones depos ited in shal­ low, but very large lakes. Thus the large-scale lake that formed the Green River Formation did not really disappear. It became a shallow lake that periodically was filled by an episode of volcan iclastic depositio n in a fluvial-lacustrine system, only to reappear whe n bas in subsidence exceeded volcaniclastic input.

INT RO D UCTION 69 M ODEL 74 LIM ESTO N ES IN TH E BRIDGER FORMATION 70 SUM M ARy 76 BRID GER LAKES AND BASIN TYPE 73 REFE RENCES 76 TU RTLE TAPHONOMY 73

INTRODUCTION replaced the Wasatch , finally covering the GRF and filling the basin (Fig. 2). The lacustrine Green River Formation (G RF) and its The sedime nt in the Bridger Formation has long been relationship with the asso ciated fluvia l de posits of the recognized as prima rily volcaniclastic (Sinclair, 1906; Wasatch an d Bridger Forma tions has been studie d since Koenig, 1960; Bradley, 1964; Gustav, 1974; West, 1976). the late 1800's and ea rly 1900's (Roehler, 1973, 1992a). Most of the volcan ic material apparently is fro m the Eocene Lake Gosiute filled a large part of the Green River Absaroka volca nic field in NWWyoming (Bradley, 1964), basin in SW Wyoming (Fig. 1) , and in this lake the GRF but the tuffs differ in co mposition from the rest of the was dep osited , with its largely laminated kerogen-rich Bridger volcanics and seem to have come from the Challis micrites (o il shales) that grade laterally into massive lime­ volcanic field in Idaho (Evanoff and Rossetti, 1992). stones or siliciclastic mudstones. The primarily fluvial Lake Gos iute fluctuated conside rably in size during its Wasatch Formation was deposited below an d along the history, as reflected in vertical changes in the lateral extent flanks of the GRF, and then the fluvial Bridger Formation of the GRF (Roehler, 1992c)(Fig. 2). During the time of the

'tbe Mou nta in Geologist, Vol. 44, No. 2 (April 2( 07), p 69-78 69 The Rocky Mountain Association of Geologists Leonard R. Brand

Figure 1. Map of the Green River Basin, 0 107 showing the limits of the depositional basin, and the extent of exposures of 25 50 ! ! I J Green River Formation, Bridger units A, Miles B, and CE.

420

Laney shale member of the GRF, the lake was at its largest & b) (Fig. 3). From study of the sediments and the taphon­ extent (Surdam and Stanley, 1979), but during that time omy of the abundant fossil turtles a depositional model for volcaniclastic deposition increased, and the formation of Bridger B has been proposed (Buchheim et al., 2000; oil shales came to an end. It has been claimed that the Brand et al., 2000). This paper will enlarge on this model basin was filled and Lake Gosiute disappeared by the end and show why it indicates that Lake Gosiute extended into of the Laney interval, replaced by fluvial deposition of the Bridger time. Bridger Formation (Surdam and WoUl)auer, 1975; Surdam and Stanley, 1979). Several lines of evidence to be pre­ sented here indicate that Lake Gosiute did not end, but just LIMESTONES IN THE BRIDGER FORMATION changed character. The Bridger Formation consists of largely tuffaceous Bridger A contains several prominent, widespread lime­ floodplain deposits, with associated channel sandstones, stones (Fig. 4), separated by mudstones (McGrew and Sul­ deltaic and lacustrine sandstone and siltstone, and lime­ livan, 1971). In the southern part of Bridger A (northwest stone units (Koenig, 1960; Bradley, 1964; Gustav, 1974; corner of Fig. 3) the sediments become increasingly lacus­ Buchheim et al., 2000). The Sage Creek Limestone, separat­ trine and interfinger with the Laney Member of the GRF ing Bridger Band C, was mapped across the basin by (McGrew and Sullivan, 1971). Some limestone beds were Bradley (964), but the other limestones were believed to traced from Bridger A into the GEF in this region (Wolf­ be local in extent (West, 1976; Roehler, 1992b). More bauer and Surdam, 1974). Bridger B also contains a num­ recently a number of sedimentary units, primarily lime­ ber of limestone units separated by mudstones (Fig. 4). stones, have been mapped, and it has become clear that Most of these limestones are continuous across all of the most limestones and some other units are continuous and existing Bridger B exposures (as determined by walking basin-wide (Brand, 1997; Evanoff et al., 1998; Murphey, out the limestone exposures during mapping) (Fig. 3), 2001; Murphey et al. in press a & b; Brand et al. in press a rather than interfingering with other sediments as would

The Rocky Mountain Association of Geologists 70 LACUST1IINE DEPOSfT70N IN THE BRlDGh1? FORMA710N: LAKE G051rJ1E EX77XNDED

Figure 2. Cross section through Rock Springs the Eocene Green River Forma­ tion and associated sediments in ~rir1(rpr Formation the Green River Basin (after E RoehlerI992a). The upper half D of the diagram shows the rela­ ?? tionship between Green River, ? Wasatch, Washakie, and Bridger Formations as proposed in this ? I? paper. Relationships between Bridger and Washakie Forma­ ?? tions are uncertain, but Roehler (1992b) suggested that some limestones can be correlated between these two formations. A Stratigraphy for Bridger A from McGrew and SuiIivan (1971), and for Bridger C - E from Mur­ phey (2001).

400

~ 1i$ 200 ~

o Wasatch Formation

be expected for limestones generated in local lakes. In a bursts, separated by sufficient time for a thin limestone to few cases, however, the unit is a resistant limestone over form in the lake. The upper limestone in each set tends to part of the basin, and continues across the rest of the basin be the most prominent one. However, where the Golden as a continuous limey mudstone that can be dearly traced bench limestone thins to the SE, the second limestone into the limestone unit. The Church Butte tuff in the south­ below it becomes more prominent and is the primary ern part of the basin is underlain by a thick, resistant lime­ bench-forming unit in that area. stone, which thins toward the north, and in the northern There are several widespread limestones in the lower half of Bridger 13 exposures it is a limey mudstone directly and middle Bridger C (Fig. 4), but limestones are much below the Church Butte tuff. The same phenomenon less common above middle Bridger C. It also appears that applies to the Black Mountain turtle layer. The Golden there are more localized limestones in Bridger C and D bench limestone changes in the opposite direction. It is a than in unit 13 (Murphey, 2001). With the exception of the prominent limestone through most of the basin, but in the Sage Creek Limestone, exposures of limestones in Bridger SE part of Bridger 13 exposures it thins and becomes less C-E are largely limited to the southern part of the Bridger distinct. In the SW part of the basin, in T15N and south, it Basin, so it cannot be determined whether some of these disappears. These facies changes are exceptions to the limestones once extended as widely over the basin as the general character of Bridger 13, in which facies change Bridger 13 limestones. stratigraphically, as described below, but each facies is lat­ The Bridger Formation limestones almost never contain erally uniform and continuous across the basin. oil shale (laminated micrite), but are massive limestones Some limestones (BMtl, Gbl, SCL) occur in sets of two (as determined by analysis of polished sections), usually a or three limestone units, separated by a few meters of few em to a few m thick. Also some Bridger limestones mudstone, Total thickness of these limestone/mudstone contain abundant ostracods and/or gastropods of the sets varies from 1.5 to 9 m. This seems to indicate that genus Goniobasis throughout their lateral extent. These are some volcanic episodes began with one or two preliminary both indicators of shallow water. Gastropods of the genus

71 The Rocky Mountain Association of Geologists Leonard R. Brand

Figure 3. Map of several of the

HIGHWAYS AND DIRT ROADS limestones in Bridger B. The Church RIVERS OR CREEKS Butte tuff and the Black Mountain

-- SCI SAGE CREEK LIMESTONE turtle layer are each underlain by a TbBu UPPER BRIDGER B limestone. -- BMtI BLACK MOUNTAIN TIJRTLE LAYER TbBm MIDDLE BRIDGER B CB CHURCH BUTTE TUFF TbBI LOWER BRIDGER B -- LI LYMAN LIMESTONE

WYOMING

~ TbBl

f-

NOT MAPPED

RlI4W Rll3W R uz W RIIIW R 1I0W RI09W R 108 W

Biornpbalaria are also abundant in most Bridger lime­ than the Bridger massive limestones. There were minor stones (Murphey, 2001). These snails are most abundant in exceptions, however. A thin oil shale unit was found low water less than 1.8 m deep, and usually are not found in Bridger C, indicating an interval with a deeper lake, and below 4.5 m depth (Hanley, 1974; Murphey, 2001). It the F marker bed in Bridger A (Fig. 4) consists partly of appears that the Bridger lakes were shallow over the entire rich oil shales (McGrew and Sullivan, 1971). basin. The oil shale in the GRF indicates a deeper lake

The Rocky Mountain Association of Geologists 72 LACUSY7UNE DZ,POSf11ON IN Trw Btaocm: FORMA7JON: LAKE GOSlUYE EX7J,NDED

760 BRIDGER LAKES AND BASIN TYPE - ~~-"- Top of Bridger E E 740 - Basal E limestone The presence of the basin-wide limestones indicate the no- - basin was not filled with sediment by the end of Laney ]00 time, and Lake Gosiute did not disappear, but only 680- - Upper limestone changed character. There was continuing subsidence, and D 660- the subsidence was fairly even across the basin. This is 640"- indicated by massive limestones and gastropods across the - 620- basin the lakes were shallow throughout, with no facies - 600 changes across the basin. In contrast, the GRF has at least - 580- four facies within the lake in the Green River Basin (Eug­ ~~-~ Lonetree limestone ster and Hardie, 1975; Surdam and Stanley, 1979; Smoot, 560 - 1983). Buchheim (1994) found that in a single lithofacies 540- association in the GRF of Fossil Lake, there are up to four 520 - or five carbonate facies that interfinger with each other C 500- - from center to margin of the lake. 480 ---- Early in Laney time the basin was a hydrographically - Soap Holes limestone 460 closed, underfilled basin (Carroll and Bohacs, 1999), then - 440"" in the later, freshwater, phase of the Laney, the basin Sage Creek limestone drained south into the Piceance Creek basin (Surdam and 420 1312 ...v c, Stanley, 1979), and was an overfilled basin. GRF subsi­ c, 400 1311 - Upper turtle layer ::J ~ - dence was greater close to the Uinta uplift, producing the tJ 380- I-- ::>: - B9 - Black Mountain tuttle layer thicker accumulation of alluvial and lacustrine rocks in the 360 southern part of the basin (Bradley, 1964). By the end of v 340 138 - Golden bench limestone the Laney and beginning of Bridger deposition, it was a ;;S - "0 320 shallow, overfilled basin; the lake overflowed at each cycle B ;;§ - 300 B7 - Lower turtle layer and then there was enough subsidence to develop another B6 lake and then fill it with sediment. This sediment was vol­ 280 B5 Cottonwood white layer - I-- 260- B4 - Church Butte tuff caniclastic, and thus the amount of sediment input to the - B3 basin was dependent on the volume of volcanic input ...v 240 0 from the Absaroka volcanic field. The nature of the lake -i 220-" " - and basin type depended on the balance between subsi­ 200- - 131 - Lyman limestone (= G marker bed) dence and volcanic activity. The rate of subsidence in the 180 basin during Bridger time could have been the same as in 160 F Laney time, but what changed was the episodic increase in v... p, c, 140 volcaniclastic sediment input that periodically filled the ::J - E 120- lake. The Bridger lake was quite alkaline (bicarbonate-rich), A 100- '--- """ ----- [) as indicated by common tufa deposits around logs, turtles, GRF 80 - Lacustrine tongueof GreenRiverFm c-- - etc., tufa mounds, and tufa coating on algae. The Laney 60 C member also contains tufa-coated objects, tufa mounds, ...v B 40 and tufa spring deposits (Surdam and Stanley, 1979, p. 0 A -i'" 20 - 107). o -

Figure 4. Stratigraphy of the Bridger Formation, units A-E, showing TURTLE TAPHONOMY the stratigraph ic relationsh ips of laterally persistent limestones. The Cottonwood white layer is a limestone. "White layer" is an Fossil turtles are velY abundant in several horizons of older general term for various prominent marker beds. The Church the Bridger Formation, and their distribution and taphon­ Butte tuff, Black Mountain turtle layer and Upper turtle layer are underlain by limestones. B3, B6 and B12 are limestones with omy provide insights into the Bridger depositional regime unpublished names (Emmett Evanoff, personal communication). (Brand et al., 2000). There seems to be a background of Bridger Unit A stratigraphy from McGrew and Sullivan (1971) attritional turtle bones, and overprinted on this back­ (showing mean thicknesses of intervals between limestones), and ground are several stratigraphic levels with turtle mass Units C-E from Murphey (2001). mortalities that were buried rapidly, but after sufficient

73 The Rocky Mountain Association of Geologists Leonard R. Brand time for some appendages to separate from the bodies. MODEL Turtles in the mass mortalities have relatively complete shells, but rarely have skulls, and not many limb bones. A depositional model for Bridger B has been proposed Also, the turtle bones have very little abrasion or weather­ (Buchheim et al., 2000), that explains the sedimentary evi­ ing, and almost no predator or scavenger tooth marks. dence and the turtle taphonomy. In this model a series of These lines of evidence indicate the turtles were all buried shallowing upward sequences were formed by the at about the same point in the sequence of decay and dis­ repeated filling of a large lake by volcaniclastic sediments, articulation events (see Brand et al., 2003). They represent followed by reforming of the lake (Fig. 6). Each sequence mass deaths and burials, with burial occurring within a (Fig. 7) began with a limestone formed in a widespread, few months of death. shallow, carbonate-precipitating floodplain lake. The lake Another relevant feature of the turtles is their very was then rapidly filled by episodic volcaniclastic deposi­ non-uniform stratigraphic distribution. They are concen­ tion, delivered via air-fall and/or prograding fluvial-deltaic trated in a few horizons, in very continuous, basin-wide systems. Each sequence consists of a lithofacies associa­ mudstone units just above limestones. This indicates a tion, which includes from base to top: limestone, clay­ repeating paleoenvironmental sequence, which is a stone, thin bedded sandstone and siltstone, and a basin-wide process. The Black Mountain turtle layer was cross-bedded sandstone facies consisting of laterally studied over the entire basin, and turtle abundance in extensive channel sandstones representing anastomosing this unit showed a geographic cline extending over the river deposits (Fig. 7). Buchheim et a1. (2000) present evi­ entire basin, with turtles more abundant in the south dence indicating that at least the initial part of the deposi­ (Fig. 5). tion in a sequence represents deltaic shoestring sands that

Figure 5. Map of fossil tur­ g HIGHWAYS ANDDIRTROADS tle bone abundance across 0"- BLACK MOUNTAIN TURTLE LAYER o the Black Mountain turtle -41'30' T18N\-, Dp·6 LOCALITY NUMBERS Wyoming T17N TURTLE DENSITY layer (reprinted from Brand ISOPACH LINES et al., 2000, with permis­ Green River 4,000 NUMBER OFTURTLE BONES PER HECTARE II1II:"-' Study area sion from Elsevier).

Kilometers 3 4 5 6

1,500 ,fj---t+--:-::-c;:---

T 16N

TlSN

7,000

_..' T1SN .~ ....., T 14N 8,000

9,000

, .\ RR·4 --...... ~ 10,000 J4,700~ ~ --- \ 12~0 _ ~3:~ __ $ 5= ---- ~ § )~ .~ -- 14,000

The Rocky Mountain Association of Geologists 74 LACUS7J?lNE DliPOS1770N IN TIm BNlIX;hNFoNMA770N: LAKli GOSIU'lli h).:nWDJilJ

TIME! TIME 3

Figure 6. Depositional model for Bridger unit B (reprinted from Buchheim et al., 2000, with permission from Elsevier).

Figure 7. Two sedimentary MSwl = Meadow Springs cycles representing the Iitho­ white layer facies association in Bridger BMtl Black Mountain B. Modified from Buchheim et al., 2000. turtle layer Ltl = Lower turtle layer BMtl-~e~i ~ Abundant turtles Tuffaceous sandstone --- Organic-rich laminae

20 ill Sandstone Alternating sandstone and 15 claystone/siltstone 10 Siltstone

5 Claystone Limestone

75 The Rocky Mountain Association of Geologists Leonard R. Brand prograded large distances out into a very shallow lake, as Bridgerian time, although the limestones are less abundant seen in modern rapid deposition in the Salton Sea, Califor­ in Bridger C-E. nia, and in shallower parts of Lake Turkana in the African Rift Valley. This produced fluvial type deposits in a lacus­ trine environment. In each sequence the early deposits are SUMMARY very laterally continuous and contain few significant chan­ nels, but further sedimentation became more typical of The Bridger Formation consists of widespread massive anastomosing river deposits. A lithofacies sequence was limestones alternating with thick sequences of volcaniclas­ completed when volcaniclastic deposition stopped or tic sediments deposited in a fluvial-lacustrine system. slowed relative to the rate of basin subsidence, and When each episode of volcanism ceased, a shallow, car­ I another basin-wide lake developed. The periodic nature bonate-precipitating lake again formed across the basin. of the volcanism is indicated by the repeated alternation The beginning of some volcanic. episodes apparently between unbroken, basin-wide limestones Oakes) and killed large numbers of turtles. They were preserved as sequences of sedimentary facies, of volcaniclastic origin. turtle mass mortalities in mudstones just above limestones. When some volcanic episodes began, large numbers of We conclude that Lake Gosiute did not disappear at the turtles were apparently killed by volcanic processes, per­ end of Laney time. The principle change was the sharp haps from suffocation by ash or by poisonous gasses, increase in volcaniclastic deposition, occurring as episodes forming the mass mortalities in the initial mudstone of volcanism that periodically filled the shallow, basin­ wide lake. deposits that began filling the lake. The scarcity of chan­ nels in this facies may indicate an ash fall origin for much of the sediment. In at least the Black Mountain turtle layer, REFERENCES this mass mortality was a basin-wide process, as indicated by the cline of turtle abundance across the basin (Fig. 5). Bradley, W. H., 1964, Geology of the Green River Formation and The turtle mass deaths resulting from the beginning of vol­ associated Eocene rocks in southwestern Wyoming and adja­ canic episodes accounts for the repeating phenomena of cent parts of Colorado and Utah, USGS Professional Paper the turtle concentrations just above limestones, and is con­ 496-A, 86 p. sistent with the sedimentary sequence described above. Brand, L. E, 1997, Mapping of widespread marker beds in unit 13 It has been previously suggested that "Lake Gosiute of the Middle Eocene Bridger Formation, southwestern began in the medial Wasatchian and persisted late into the Wyoming, Journal of Vertebrate Paleontology, v. 17, supple­ Bridgerian" (Lillegraven and Ostresh, 1988, 317). The map­ ment 3, Abstracts, p. 33A. Brand, L. R, M. Hussey, and J. Taylor. 2003, Experimental ping of limestones and other research summarized here taphonomy of turtles, Journal of Taphonomy, v. 1 (4) 2003 provides a stronger basis for this concept (Buchheim et al., (2004), p. 233-245 2000). Brand, L. R, H. T. Goodwin, P. G. Ambrose and H. P. Buchheim, During Laney time, sedimentary processes in Lake 2000, Taphonomy of turtles in the Middle Eocene Bridger For­ Gosiute changed from largely laminated micrite accumula­ mation, SW Wyoming, Palaeogeography, Palaeoclimatology, tion to predominantly volcaniclastic deposition, which Palaeoecology, v. 162, p. 171-189. temporarily filled the lake. Alternation of lacustrine lime­ Brand, L., P. C. Murphey, J. E. Hessig, and A. A. Smith, In press a, stone and volcaniclastic deposition continued as the Bedrock geologic map of the Linwood Canyon 7.5' Quadran­ Bridger Formation was deposited. Perhaps the primary gle, Sweetwater County, Wyoming, Wyoming State Geological change near the end of the Laney was not the disappear­ Survey Open File Map, scale 1:24,000, 1 sheet. Brand, L., P. C. Murphey, and J. E. Haessig, In press b, Bedrock ance of Lake Gosiute, but the beginning of large scale, geologic map of the Antelope Wash 7.5' Quadrangle, Sweet­ episodic volcanism in the region, that began to fill the water County, Wyoming, Wyoming State Geological Survey lake periodically (Fig. 2). This input of volcaniclastics Open File Map, scale 1:24,000, 1 sheet. eliminated the stable lake conditions that favored lami­ Buchheim, H. P., 1994, Eocene Fossil Lake, Green River Forma­ nated micrite formation, and resulted in Bridger type of tion, Wyoming: a history of fluctuating salinity, in Renaut, R., sediments throughout the basin - massive limestones in a and Last, W., eds., Sedimentology and Geochemistry of Mod­ shallow lake, alternating with fluvial-lacustrine sequences ern and Ancient Saline Lakes, Society of Sedimentary Geology, and anastomosing river deposits. Special Publication 50, p. 239-247. I The volcaniclastic deposition in the lake, apparently Buchheim, H. P., L. R. Brand, and H. T. Goodwin, 2000, Lacus­ combined with fairly even subsidence across the basin trine to fluvial flood-plain deposition in the Eocene Bridger Formation: Palaeogeography, Palaeoclimatology, Palaeoecol­ produced lakes that were shallow across the basin, shal­ ogy, v. 162, p. 191-209. lower than Lake Gosiute during GRF time. This accounts Carroll, A. R., and K. M. Bohacs, 1999, Stratigraphic classification for the more massive limestones, instead of laminated or ancient lakes: balancing tectonic and climatic controls: micrite. This depositional regime continued throughout Geology, v. 27, p. 99-102.

The Rocky Mountain Association of Geologists 76 LACUI'711JNE DHPOSI770N IN TIlE BRIDGER FORMA71ON: LAKE GOSlUTE hXT1WDED

Eugster, H. P, and L. A. Hardie, 1975, Sedimentation in an ancient closed basin; the Wilkins Peak Member of the Green River playa-lake complex: The Wilkins Peak Member of the Green Formation (Eocene), Wyoming, U.S.A., Sedimentology, v. River Formation of Wyoming, GSA Bulletin v. 86, p. 319 334. 30(6), p. 801-827. Evanoff,E., and D. F. Rossetti, 1992, A tale of two distal volcani­ Surdam, R c., and C. A. WolflJauer, 1975, Green River Formation, clastic sequences I: the fluvial-lacustrine Bridger Formation of Wyoming: a playa-lake complex, GSA Bulletin, v. 86, p. 335-345. southwest Wyom.ing. SEpM Theme Meeting, Mesozoic of the Surdam, R c., and K. O. Stanley, 1979, Lacustrine sedimentation Western Interior, Abstracts, p. 25. during the culminating phase of Eocene Lake Gosiute, Wyoming Evanoff, E., L. R. Brand, and r. C. Murphey, 1998, The Bridger (Green River Formation), GSA Bulletin, v. 90, p. 93-110. Formation (Middle Eocene) of southwest Wyoming: wide­ West, R M., 1976, Paleontology and geology of the Bridger For­ spread marker units and subdivisions of Bridger B through D, mation, southern Green River Basin, southwestern Wyoming. Dakoterra, v. 5, p. 115-122. Part 1. History of field work and geological setting, Milwaukee Gustav, S. H., 1974, The sedimentology and paleogeography of Public Museum Contributions to Biology and Geology, v. 7, p. the Bridger Formation, (Eocene) of southwestern Wyoming: 1-12 Masters thesis, Amherst, University of Massachusetts, 82 p. Wollbauer, C. A., and E. C. Surdam, Origin of nonmarine Hanley, ]. H., 1974, Systematics, paleoecology, and biostratigra­ dolomite in Eocene Lake Gosiute, Green River Basin, phy of nonmarine mollusca from the Green River and Wasatch Wyoming, GSA Bulletin, v. 85, p. 1733-1740. formations (Eocene), southwestern Wyoming and northwest­ ern Colorado: Ph.D. dissertation, Laramie, University of Wyoming, 285 p. Koenig, K. ]., 1960, Bridger Formation in the Bridger Basin, Wyoming, in Wyoming Geological Association Guidebook: 15th Annual Field Conference, p. 163-168. THE AUTHOR Lillegraven, J A., and L. M. Ostresh, 1988, Evolution of Wyoming Early Cenozoic topography and drainage patterns, National Geographic Research, 4 (3):303-327. LEONARD R. BRAND McGrew, P.O., and R. Sullivan, 1971, The stratigraphy and pale­ LEONARD BRAND received his ontology of Bridger A, Wyoming University Contributions to Ph.D. in ecology and evolu­ Geology, v. 9, p. 66-85. tionary biology at Cornell Murphey, P. c., 2001, Stratigraphy, fossil distribution, and deposi­ University. Since then he tional environments of the upper Bridger Formation (Middle has taught and conducted Eocene) of southwestern Wyoming, and the taphonomy of an research at Loma Linda Uni­ unusual Bridger microfossil assemblage: Ph.D. dissertation, versity for over 30 years, as University of Colorado, Boulder, Colorado, 345 p. well as serving as depart­ Murphey, P. c., L. Brand,]. E. Haessig, and A. A. Smith, In press ment chair for part of that a, Bedrock geologic map of the Devils Playground 7.5' Quad­ time. His teaching areas are rangle, Sweetwater County, Wyoming, Wyoming State Geolog­ in vertebrate biology and ical Survey Open File Map, scale 1:24,000, 1 sheet. paleontology and philoso­ Murphey, P. c., L. Brand, J E. Haessig, In press b, Bedrock geo­ phy of science. His research logic map of the Black Spring Reservoir 7.5' Quadrangle, for about the last 20 years Sweetwater County, Wyoming, Wyoming State Geological Sur­ has focused on taphonomy vey Open File Map, scale 1:24,000, 1 sheet. of vertebrate fossil assem­ Roehler, H. W., 1973, Stratigraphy of the Washakie Formation in blages in Wyoming (Eocene the Washakie Basin, Wyoming, USGS, v. 1369, p. 1-40. Bridger Formation) and Peru (Miocene/Pliocene Pisco For­ Roehler, H. W., 1992a, Introduction to greater Green River Basin mation), paleoenvironmental implications of fossil verte­ geology, physiography, and history of investigations, USGS brate trackways in the Coconino Sandstone of Arizona, Professional Paper 1506-A, 14 p. and geological mapping of the research areas in Wyoming Roehler, II. W., 1992b, Description and correlation of Eocene and Peru. rocks in stratigraphic reference sections for the Green River and Washakie Basins, southwest Wyoming, USGS Professional Paper 1506-D, 83 p. Roehler, H. W., 1992c, Correlation, composition, areal distribu­ tion, and thickness of Eocene stratigraphic units, greater Green River Basin, Wyoming, Utah, and Colorado, USGS Professional Paper, 49 p. Sinclair, W. J, 1906, Volcanic ash in the Bridger beds of Wyoming, Bulletin of the American Museum of Natural His­ tory, v. 22, p. 273-280 Smoot, J p., 1983, Depositional subenvironments in an arid

77 The Rocky Mountain Association of Geologists