Lacustrine Deposition In The Bridger Formation:lake Cosiute Extended'

LEONARD R. BRA ND2

1. Manuscri pt received Septemb er 30. 2004; Accepted M arch' , 20072. Department of Earth and Bio logical Sciences, Lom a Lind a University, Loma Linda, CA 92350; lbra nd wllu.ed u

ABSTRACT

The Green River Formation was de posited in Lake Gosiute , until the lacustrine system shifted to themore fluvial environment of the Bridger Formation . In the so uthern part of Bridger A exposures, sedi­ment s become increasingly lacustrine and interfinger wit h the main bod y of the Laney Shale member ofthe Green River Formation. High er in Bridger A several widespread limestone marker beds are separatedby mud stones. Bridger B also co nsists of mu dstones alternating with limestones. A nu mber of these lime­stones have no w been mapped . They extend across the ent ire existing Bridger B exposures, and rep re­sent basin-wide shallow lakes. The se lakes w ere filled by volcaniclastic input from episodes of volcanismto the nort h . The lacustrine de pos its of the Green River Formation co nsist large ly of laminated ke rog en­rich micrites (o il sha les) that grad e laterally into massive limes tones or siliciclastic mudstones , whereasthe lacustrine deposits of the Bridger Formation are wides prea d, massive limestones depos ited in shal­low, but very large lakes. Thus the large-sca le lake that formed the Green River For mat ion did not reallydisappear. It became a shallow lake that periodically was filled by an episode of volcan iclastic depositio nin a fluvial-lacustrine system, only to reappear whe n bas in subsidence exceeded volcaniclastic input.

INT RO D UCTIO N 69

LIM ESTO N ES IN TH E BRIDGER FORMATION 70BRID GER LAKESAND BASIN TYPE 73

TU RTLE TAPHONOMY 73

INTRODUCTION

The lacustrine Green River Formation (G RF) and itsrelationship with the asso ciated fluvia l de posits of theWasatch an d Bridger Forma tions has been studie d sincethe late 1800's and ea rly 1900's (Roehler, 1973, 1992a) .Eocene Lake Gosiute filled a large part of the Green Riverbasin in SW Wyoming (Fig . 1) , and in this lake the GRFwas dep osited , with its largely laminated kerogen-richmicr ites (o il sha les) tha t grade laterally into mas sive lime­stones or siliciclastic mudstones. The primarily fluvialWasatch Formation was deposited below an d along theflanks of the GRF, and then the fluvial Bridger Format ion

'tbe Mou nta in Geologist, Vol. 44, No. 2 (Apri l 2( 07), p 69-78 69

M ODE L 74

SUM M ARy 76

REFE RENCES 76

replaced the Wasatch , finally covering the GRF and fillingthe basin (Fig. 2).

The sedime nt in the Bridger Formation has long beenrecognized as pr ima rily volcaniclastic (Sinclair, 1906;Koenig, 1960; Bradley, 1964; Gustav, 1974; West, 1976) .Most of the volcan ic material apparently is fro m theAbsaroka volca nic field in NW Wyoming (Bradley, 1964) ,but the tuffs differ in co mposition from the rest of theBridger volcanics and seem to have come from the Challisvo lcanic field in Idaho (Evanoff and Rossetti, 1992).

Lake Gos iute fluctu ated conside rably in size during itshistory, as reflected in vertical changes in the lateral extentof the GRF (Roehler, 1992c) (Fig. 2) . During the time of the

The Rocky Mountain Association of Geo logists

Leonard R. Brand

420

25! ! I

Miles

1070

50J

Figure 1. Map of the Green River Basin,showing the limits of the depositionalbasin, and the extent of exposures ofGreen River Formation, Bridger units A,B, and CE.

Laney shale member of the GRF, the lake was at its largestextent (Surdam and Stanley, 1979), but during that timevolcaniclastic deposition increased, and the formation ofoil shales came to an end. It has been claimed that thebasin was filled and Lake Gosiute disappeared by the endof the Laney interval, replaced by fluvial deposition of theBridger Formation (Surdam and WoUl)auer, 1975; Surdamand Stanley, 1979). Several lines of evidence to be pre­sented here indicate that Lake Gosiute did not end, but justchanged character.

The Bridger Formation consists of largely tuffaceousfloodplain deposits, with associated channel sandstones,deltaic and lacustrine sandstone and siltstone, and lime­stone units (Koenig, 1960; Bradley, 1964; Gustav, 1974;Buchheim et al., 2000). The Sage Creek Limestone, separat­ing Bridger Band C, was mapped across the basin byBradley (964), but the other limestones were believed tobe local in extent (West, 1976; Roehler, 1992b). Morerecently a number of sedimentary units, primarily lime­stones, have been mapped, and it has become clear thatmost limestones and some other units are continuous andbasin-wide (Brand, 1997; Evanoff et al., 1998; Murphey,2001; Murphey et al. in press a & b; Brand et al. in press a

The Rocky Mountain Association of Geologists 70

& b) (Fig. 3). From study of the sediments and the taphon­omy of the abundant fossil turtles a depositional model forBridger B has been proposed (Buchheim et al., 2000;Brand et al., 2000). This paper will enlarge on this modeland show why it indicates that Lake Gosiute extended intoBridger time.

LIMESTONES IN THE BRIDGER FORMATION

Bridger A contains several prominent, widespread lime­stones (Fig. 4), separated by mudstones (McGrew and Sul­livan, 1971). In the southern part of Bridger A (northwestcorner of Fig. 3) the sediments become increasingly lacus­trine and interfinger with the Laney Member of the GRF(McGrew and Sullivan, 1971). Some limestone beds weretraced from Bridger A into the GEF in this region (Wolf­bauer and Surdam, 1974). Bridger B also contains a num­ber of limestone units separated by mudstones (Fig. 4).Most of these limestones are continuous across all of theexisting Bridger B exposures (as determined by walkingout the limestone exposures during mapping) (Fig. 3),rather than interfingering with other sediments as would

LACUST1IINE DEPOSfT70N IN THE BRlDGh1? FORMA710N: LAKE G051rJ1E EX77XNDED

E

D

A

400

~

1i$ 200~

o

~rir1(rpr Formation

Wasatch Formation

Rock Springs

? ?

?

? ?

?

I?

Figure 2. Cross section throughthe Eocene Green River Forma­tion and associated sediments inthe Green River Basin (afterRoehlerI992a). The upper halfof the diagram shows the rela­tionship between Green River,Wasatch, Washakie, and BridgerFormations as proposed in thispaper. Relationships betweenBridger and Washakie Forma­tions are uncertain, but Roehler(1992b) suggested that somelimestones can be correlatedbetween these two formations.Stratigraphy for Bridger A fromMcGrew and SuiIivan (1971),and for Bridger C - E from Mur­phey (2001).

be expected for limestones generated in local lakes. In afew cases, however, the unit is a resistant limestone overpart of the basin, and continues across the rest of the basinas a continuous limey mudstone that can be dearly tracedinto the limestone unit. The Church Butte tuff in the south­ern part of the basin is underlain by a thick, resistant lime­stone, which thins toward the north, and in the northernhalf of Bridger 13 exposures it is a limey mudstone directlybelow the Church Butte tuff. The same phenomenonapplies to the Black Mountain turtle layer. The Goldenbench limestone changes in the opposite direction. It is aprominent limestone through most of the basin, but in theSE part of Bridger 13 exposures it thins and becomes lessdistinct. In the SW part of the basin, in T15N and south, itdisappears. These facies changes are exceptions to thegeneral character of Bridger 13, in which facies changestratigraphically, as described below, but each facies is lat­erally uniform and continuous across the basin.

Some limestones (BMtl, Gbl, SCL) occur in sets of twoor three limestone units, separated by a few meters ofmudstone, Total thickness of these limestone/mudstonesets varies from 1.5 to 9 m. This seems to indicate thatsome volcanic episodes began with one or two preliminary

71

bursts, separated by sufficient time for a thin limestone toform in the lake. The upper limestone in each set tends tobe the most prominent one. However, where the Goldenbench limestone thins to the SE, the second limestonebelow it becomes more prominent and is the primarybench-forming unit in that area.

There are several widespread limestones in the lowerand middle Bridger C (Fig. 4), but limestones are muchless common above middle Bridger C. It also appears thatthere are more localized limestones in Bridger C and Dthan in unit 13 (Murphey, 2001). With the exception of theSage Creek Limestone, exposures of limestones in BridgerC-E are largely limited to the southern part of the BridgerBasin, so it cannot be determined whether some of theselimestones once extended as widely over the basin as theBridger 13 limestones.

The Bridger Formation limestones almost never containoil shale (laminated micrite), but are massive limestones(as determined by analysis of polished sections), usually afew em to a few m thick. Also some Bridger limestonescontain abundant ostracods and/or gastropods of thegenus Goniobasis throughout their lateral extent. These areboth indicators of shallow water. Gastropods of the genus

The Rocky Mountain Association of Geologists

Leonard R. Brand

HIGHWAYS AND DIRT ROADS

RIVERS OR CREEKS

-- SCI SAGE CREEK LIMESTONETbBu UPPER BRIDGER B

-- BMtI BLACK MOUNTAIN TIJRTLE LAYERTbBm MIDDLE BRIDGER B

CB CHURCH BUTTE TUFFTbBI LOWER BRIDGER B

-- LI LYMAN LIMESTONE

WYOMING

Figure 3. Map of several of thelimestones in Bridger B. The ChurchButte tuff and the Black Mountainturtle layer are each underlain by alimestone.

R 108 WRI09WR 1I0W

NOT MAPPED

RIIIWR uz WRll3WRlI4W

~ TbBl

f-

Biornpbalaria are also abundant in most Bridger lime­stones (Murphey, 2001). These snails are most abundant inwater less than 1.8 m deep, and usually are not foundbelow 4.5 m depth (Hanley, 1974; Murphey, 2001). Itappears that the Bridger lakes were shallow over the entirebasin. The oil shale in the GRF indicates a deeper lake

than the Bridger massive limestones. There were minorexceptions, however. A thin oil shale unit was found lowin Bridger C, indicating an interval with a deeper lake, andthe F marker bed in Bridger A (Fig. 4) consists partly ofrich oil shales (McGrew and Sullivan, 1971).

The Rocky Mountain Association of Geologists 72

LACUSY7UNE DZ,POSf11ON IN Trw Btaocm: FORMA7JON: LAKE GOSlUYE EX7J,NDED

760Top of Bridger E- ~~-"-

E 740- Basal E limestoneno--

]00

680- Upper limestone-

D 660-

640"--

620--

600-

580-~~-~ Lonetree limestone

560-

540-

520

C -500-

-480 ---- Soap Holes limestone-460

-440""

420Sage Creek limestone

... 1312vc,

400 1311 - Upper turtle layerc,::J ~ -

tJ 380-B9 - Black Mountain tuttle layerI-- ::>: -

360

340 138 - Golden bench limestonev;;S -"0 320

B ;;§ -300 B7 - Lower turtle layer

B6280 B5 Cottonwood white layer

-

I-- 260- B4 - Church Butte tuff- B3... 240v

"0 220-"-i-

200-- 131 - Lyman limestone (= G marker bed)

180

... 160 Fvp,

140c,E::J -

120-

A 100-'--- """ ----- [)GRF 80 - Lacustrine tongueof GreenRiverFmc-- -

60 C... Bv 40

'"0 A-i 20 -

o -

Figure 4. Stratigraphy of the Bridger Formation, units A-E, showingthe stratigraph ic relationsh ips of laterally persistent limestones.The Cottonwood white layer is a limestone. "White layer" is anolder general term for various prominent marker beds. The ChurchButte tuff, Black Mountain turtle layer and Upper turtle layer areunderlain by limestones. B3, B6 and B12 are limestones withunpublished names (Emmett Evanoff, personal communication).Bridger Unit A stratigraphy from McGrew and Sullivan (1971)(showing mean thicknesses of intervals between limestones), andUnits C-E from Murphey (2001).

73

BRIDGER LAKES AND BASIN TYPE

The presence of the basin-wide limestones indicate thebasin was not filled with sediment by the end of Laneytime, and Lake Gosiute did not disappear, but onlychanged character. There was continuing subsidence, andthe subsidence was fairly even across the basin. This isindicated by massive limestones and gastropods across thebasin the lakes were shallow throughout, with no facieschanges across the basin. In contrast, the GRF has at leastfour facies within the lake in the Green River Basin (Eug­ster and Hardie, 1975; Surdam and Stanley, 1979; Smoot,1983). Buchheim (1994) found that in a single lithofaciesassociation in the GRF of Fossil Lake, there are up to fouror five carbonate facies that interfinger with each otherfrom center to margin of the lake.

Early in Laney time the basin was a hydrographicallyclosed, underfilled basin (Carroll and Bohacs, 1999), thenin the later, freshwater, phase of the Laney, the basindrained south into the Piceance Creek basin (Surdam andStanley, 1979), and was an overfilled basin. GRF subsi­dence was greater close to the Uinta uplift, producing thethicker accumulation of alluvial and lacustrine rocks in thesouthern part of the basin (Bradley, 1964). By the end ofthe Laney and beginning of Bridger deposition, it was ashallow, overfilled basin; the lake overflowed at each cycleand then there was enough subsidence to develop anotherlake and then fill it with sediment. This sediment was vol­caniclastic, and thus the amount of sediment input to thebasin was dependent on the volume of volcanic inputfrom the Absaroka volcanic field. The nature of the lakeand basin type depended on the balance between subsi­dence and volcanic activity. The rate of subsidence in thebasin during Bridger time could have been the same as inLaney time, but what changed was the episodic increase involcaniclastic sediment input that periodically filled thelake.

The Bridger lake was quite alkaline (bicarbonate-rich),as indicated by common tufa deposits around logs, turtles,etc., tufa mounds, and tufa coating on algae. The Laneymember also contains tufa-coated objects, tufa mounds,and tufa spring deposits (Surdam and Stanley, 1979, p.107).

TURTLE TAPHONOMY

Fossil turtles are velY abundant in several horizons ofthe Bridger Formation, and their distribution and taphon­omy provide insights into the Bridger depositional regime(Brand et al., 2000). There seems to be a background ofattritional turtle bones, and overprinted on this back­ground are several stratigraphic levels with turtle massmortalities that were buried rapidly, but after sufficient

The Rocky Mountain Association of Geologists

Leonard R. Brand

time for some appendages to separate from the bodies.Turtles in the mass mortalities have relatively completeshells, but rarely have skulls, and not many limb bones.Also, the turtle bones have very little abrasion or weather­ing, and almost no predator or scavenger tooth marks.These lines of evidence indicate the turtles were all buriedat about the same point in the sequence of decay and dis­articulation events (see Brand et al., 2003). They representmass deaths and burials, with burial occurring within afew months of death.

Another relevant feature of the turtles is their verynon-uniform stratigraphic distribution. They are concen­trated in a few horizons, in very continuous, basin-widemudstone units just above limestones. This indicates arepeating paleoenvironmental sequence, which is abasin-wide process. The Black Mountain turtle layer wasstudied over the entire basin, and turtle abundance inthis unit showed a geographic cline extending over theentire basin, with turtles more abundant in the south(Fig. 5).

MODEL

A depositional model for Bridger B has been proposed(Buchheim et al., 2000), that explains the sedimentary evi­dence and the turtle taphonomy. In this model a series ofshallowing upward sequences were formed by therepeated filling of a large lake by volcaniclastic sediments,followed by reforming of the lake (Fig. 6). Each sequence(Fig. 7) began with a limestone formed in a widespread,shallow, carbonate-precipitating floodplain lake. The lakewas then rapidly filled by episodic volcaniclastic deposi­tion, delivered via air-fall and/or prograding fluvial-deltaicsystems. Each sequence consists of a lithofacies associa­tion, which includes from base to top: limestone, clay­stone, thin bedded sandstone and siltstone, and across-bedded sandstone facies consisting of laterallyextensive channel sandstones representing anastomosingriver deposits (Fig. 7). Buchheim et a1. (2000) present evi­dence indicating that at least the initial part of the deposi­tion in a sequence represents deltaic shoestring sands that

Figure 5. Map of fossil tur­tle bone abundance acrossthe Black Mountain turtlelayer (reprinted from Brandet al., 2000, with permis­sion from Elsevier).

Wyoming

Kilometers

3 4 5 6

GreenRiverII1II:"-' Study area

TlSN

T 16N

HIGHWAYS ANDDIRTROADS

BLACK MOUNTAIN TURTLE LAYER

LOCALITY NUMBERS

TURTLE DENSITYISOPACH LINES

NUMBER OFTURTLEBONES PER HECTARE

Dp·6

4,000

8,000

9,000

10,000

12~0 _

14,000

g0"-o

7,000

T1SN

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

_..'T 14N

---5= ----

--

.~.....,

T18N\-,

T17N

, .\ RR·4 --........... ~\ J4,700~ ~

~3:~ __ $

~ § )~ .~

-41'30'

The Rocky Mountain Association of Geologists 74

TIME!

LACUS7J?lNE DliPOS1770N IN TIm BNlIX;hNFoNMA770N: LAKli GOSIU'lli h).:nWDJilJ

TIME 3

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

20 ill

15

10

5

BMtl-~e~i

MSwl = Meadow Springswhite layer

BMtl Black Mountainturtle layer

Ltl = Lower turtle layer

~ Abundant turtles

Tuffaceous sandstone

--- Organic-rich laminae

Sandstone

Alternating sandstone andclaystone/siltstone

Siltstone

Claystone

Limestone

Figure 7. Two sedimentarycycles representing the Iitho­facies association in BridgerB. Modified from Buchheim etal., 2000.

75 The Rocky Mountain Association of Geologists

Leonard R. Brand

prograded large distances out into a very shallow lake, asseen in modern rapid deposition in the Salton Sea, Califor­nia, and in shallower parts of Lake Turkana in the AfricanRift Valley. This produced fluvial type deposits in a lacus­trine environment. In each sequence the early deposits arevery laterally continuous and contain few significant chan­nels, but further sedimentation became more typical ofanastomosing river deposits. A lithofacies sequence wascompleted when volcaniclastic deposition stopped orslowed relative to the rate of basin subsidence, andanother basin-wide lake developed. The periodic natureof the volcanism is indicated by the repeated alternationbetween unbroken, basin-wide limestones Oakes) andsequences of sedimentary facies, of volcaniclastic origin.

When some volcanic episodes began, large numbers ofturtles were apparently killed by volcanic processes, per­haps from suffocation by ash or by poisonous gasses,forming the mass mortalities in the initial mudstonedeposits that began filling the lake. The scarcity of chan­nels in this facies may indicate an ash fall origin for muchof the sediment. In at least the Black Mountain turtle layer,this mass mortality was a basin-wide process, as indicatedby the cline of turtle abundance across the basin (Fig. 5).The turtle mass deaths resulting from the beginning of vol­canic episodes accounts for the repeating phenomena ofthe turtle concentrations just above limestones, and is con­sistent with the sedimentary sequence described above.

It has been previously suggested that "Lake Gosiutebegan in the medial Wasatchian and persisted late into theBridgerian" (Lillegraven and Ostresh, 1988, 317). The map­ping of limestones and other research summarized hereprovides a stronger basis for this concept (Buchheim et al.,2000).

During Laney time, sedimentary processes in LakeGosiute changed from largely laminated micrite accumula­tion to predominantly volcaniclastic deposition, whichtemporarily filled the lake. Alternation of lacustrine lime­stone and volcaniclastic deposition continued as theBridger Formation was deposited. Perhaps the primarychange near the end of the Laney was not the disappear­ance of Lake Gosiute, but the beginning of large scale,episodic volcanism in the region, that began to fill thelake periodically (Fig. 2). This input of volcaniclasticseliminated the stable lake conditions that favored lami­nated micrite formation, and resulted in Bridger type ofsediments throughout the basin - massive limestones in ashallow lake, alternating with fluvial-lacustrine sequencesand anastomosing river deposits.

The volcaniclastic deposition in the lake, apparentlycombined with fairly even subsidence across the basinproduced lakes that were shallow across the basin, shal­lower than Lake Gosiute during GRF time. This accountsfor the more massive limestones, instead of laminatedmicrite. This depositional regime continued throughout

The Rocky Mountain Association of Geologists 76

Bridgerian time, although the limestones are less abundantin Bridger C-E.

SUMMARY

The Bridger Formation consists of widespread massivelimestones alternating with thick sequences of volcaniclas­tic sediments deposited in a fluvial-lacustrine system.When each episode of volcanism ceased, a shallow, car­bonate-precipitating lake again formed across the basin.The beginning of some volcanic. episodes apparentlykilled large numbers of turtles. They were preserved asturtle mass mortalities in mudstones just above limestones.We conclude that Lake Gosiute did not disappear at theend of Laney time. The principle change was the sharpincrease in volcaniclastic deposition, occurring as episodesof volcanism that periodically filled the shallow, basin­wide lake.

REFERENCES

Bradley, W. H., 1964, Geology of the Green River Formation andassociated Eocene rocks in southwestern Wyoming and adja­cent parts of Colorado and Utah, USGS Professional Paper496-A, 86 p.

Brand, L. E, 1997, Mapping of widespread marker beds in unit 13of the Middle Eocene Bridger Formation, southwesternWyoming, Journal of Vertebrate Paleontology, v. 17, supple­ment 3, Abstracts, p. 33A.

Brand, L. R, M. Hussey, and J. Taylor. 2003, Experimentaltaphonomy of turtles, Journal of Taphonomy, v. 1 (4) 2003(2004), p. 233-245

Brand, L. R, H. T. Goodwin, P. G. Ambrose and H. P. Buchheim,2000, Taphonomy of turtles in the Middle Eocene Bridger For­mation, SW Wyoming, Palaeogeography, Palaeoclimatology,Palaeoecology, v. 162, p. 171-189.

Brand, L., P. C. Murphey, J. E. Hessig, and A. A. Smith, In press a,Bedrock geologic map of the Linwood Canyon 7.5' Quadran­gle, Sweetwater County, Wyoming, Wyoming State GeologicalSurvey Open File Map, scale 1:24,000, 1 sheet.

Brand, L., P. C. Murphey, and J. E. Haessig, In press b, Bedrockgeologic map of the Antelope Wash 7.5' Quadrangle, Sweet­water County, Wyoming, Wyoming State Geological SurveyOpen File Map, scale 1:24,000, 1 sheet.

Buchheim, H. P., 1994, Eocene Fossil Lake, Green River Forma­tion, Wyoming: a history of fluctuating salinity, in Renaut, R.,

and Last, W., eds., Sedimentology and Geochemistry of Mod­ern and Ancient Saline Lakes, Society of Sedimentary Geology,Special Publication 50, p. 239-247.

Buchheim, H. P., L. R. Brand, and H. T. Goodwin, 2000, Lacus­trine to fluvial flood-plain deposition in the Eocene BridgerFormation: Palaeogeography, Palaeoclimatology, Palaeoecol­ogy, v. 162, p. 191-209.

Carroll, A. R., and K. M. Bohacs, 1999, Stratigraphic classificationor ancient lakes: balancing tectonic and climatic controls:Geology, v. 27, p. 99-102.

I

I

LACUI'711JNE DHPOSI770N IN TIlE BRIDGER FORMA71ON: LAKE GOSlUTE hXT1WDED

Eugster, H. P, and L. A. Hardie, 1975, Sedimentation in an ancientplaya-lake complex: The Wilkins Peak Member of the GreenRiver Formation of Wyoming, GSA Bulletin v. 86, p. 319 334.

Evanoff,E., and D. F. Rossetti, 1992, A tale of two distal volcani­clastic sequences I: the fluvial-lacustrine Bridger Formation ofsouthwest Wyom.ing. SEpM Theme Meeting, Mesozoic of theWestern Interior, Abstracts, p. 25.

Evanoff, E., L. R. Brand, and r. C. Murphey, 1998, The BridgerFormation (Middle Eocene) of southwest Wyoming: wide­spread marker units and subdivisions of Bridger B through D,Dakoterra, v. 5, p. 115-122.

Gustav, S. H., 1974, The sedimentology and paleogeography ofthe Bridger Formation, (Eocene) of southwestern Wyoming:Masters thesis, Amherst, University of Massachusetts, 82 p.

Hanley, ]. H., 1974, Systematics, paleoecology, and biostratigra­phy of nonmarine mollusca from the Green River and Wasatchformations (Eocene), southwestern Wyoming and northwest­ern Colorado: Ph.D. dissertation, Laramie, University ofWyoming, 285 p.

Koenig, K. ]., 1960, Bridger Formation in the Bridger Basin,Wyoming, in Wyoming Geological Association Guidebook:15th Annual Field Conference, p. 163-168.

Lillegraven, J A., and L. M. Ostresh, 1988, Evolution of WyomingEarly Cenozoic topography and drainage patterns, NationalGeographic Research, 4 (3):303-327.

McGrew, P.O., and R. Sullivan, 1971, The stratigraphy and pale­ontology of Bridger A, Wyoming University Contributions toGeology, v. 9, p. 66-85.

Murphey, P. c., 2001, Stratigraphy, fossil distribution, and deposi­tional environments of the upper Bridger Formation (MiddleEocene) of southwestern Wyoming, and the taphonomy of anunusual Bridger microfossil assemblage: Ph.D. dissertation,University of Colorado, Boulder, Colorado, 345 p.

Murphey, P. c., L. Brand,]. E. Haessig, and A. A. Smith, In pressa, Bedrock geologic map of the Devils Playground 7.5' Quad­rangle, Sweetwater County, Wyoming, Wyoming State Geolog­ical Survey Open File Map, scale 1:24,000, 1 sheet.

Murphey, P. c., L. Brand, J E. Haessig, In press b, Bedrock geo­logic map of the Black Spring Reservoir 7.5' Quadrangle,Sweetwater County, Wyoming, Wyoming State Geological Sur­vey Open File Map, scale 1:24,000, 1 sheet.

Roehler, H. W., 1973, Stratigraphy of the Washakie Formation inthe Washakie Basin, Wyoming, USGS, v. 1369, p. 1-40.

Roehler, H. W., 1992a, Introduction to greater Green River Basingeology, physiography, and history of investigations, USGSProfessional Paper 1506-A, 14 p.

Roehler, II. W., 1992b, Description and correlation of Eocenerocks in stratigraphic reference sections for the Green Riverand Washakie Basins, southwest Wyoming, USGS ProfessionalPaper 1506-D, 83 p.

Roehler, H. W., 1992c, Correlation, composition, areal distribu­tion, and thickness of Eocene stratigraphic units, greater GreenRiver Basin, Wyoming, Utah, and Colorado, USGS ProfessionalPaper, 49 p.

Sinclair, W. J, 1906, Volcanic ash in the Bridger beds ofWyoming, Bulletin of the American Museum of Natural His­tory, v. 22, p. 273-280

Smoot, J p., 1983, Depositional subenvironments in an arid

77

closed basin; the Wilkins Peak Member of the Green RiverFormation (Eocene), Wyoming, U.S.A., Sedimentology, v.30(6), p. 801-827.

Surdam, R c., and C. A. WolflJauer, 1975, Green River Formation,Wyoming: a playa-lake complex, GSA Bulletin, v. 86, p. 335-345.

Surdam, R c., and K. O. Stanley, 1979, Lacustrine sedimentationduring the culminating phase of Eocene Lake Gosiute, Wyoming(Green River Formation), GSA Bulletin, v. 90, p. 93-110.

West, R M., 1976, Paleontology and geology of the Bridger For­mation, southern Green River Basin, southwestern Wyoming.Part 1. History of field work and geological setting, MilwaukeePublic Museum Contributions to Biology and Geology, v. 7, p.1-12

Wollbauer, C. A., and E. C. Surdam, Origin of nonmarinedolomite in Eocene Lake Gosiute, Green River Basin,Wyoming, GSA Bulletin, v. 85, p. 1733-1740.

THE AUTHOR

LEONARD R. BRAND

LEONARD BRAND received hisPh.D. in ecology and evolu­tionary biology at CornellUniversity. Since then hehas taught and conductedresearch at Loma Linda Uni­versity for over 30 years, aswell as serving as depart­ment chair for part of thattime. His teaching areas arein vertebrate biology andpaleontology and philoso­phy of science. His researchfor about the last 20 yearshas focused on taphonomyof vertebrate fossil assem­blages in Wyoming (Eocene

Bridger Formation) and Peru (Miocene/Pliocene Pisco For­mation), paleoenvironmental implications of fossil verte­brate trackways in the Coconino Sandstone of Arizona,and geological mapping of the research areas in Wyomingand Peru.

The Rocky Mountain Association of Geologists