Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming

LAWRENCE1 A^^HARDIE I department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218

ABSTRACT INTRODUCTION

The Wilkins Peak Member of the Green River Formation of The Eocene Green River Formation is perhaps the best-known Wyoming has been examined in outcrop with the object of recon- and most studied sequence of lacustrine deposits in the world. In- structing its depositional environment. Based on their assemblages vestigators with a great diversity of backgrounds and aims have of sedimentary structures, seven rock units are described, six of concerned themselves with these complex rocks, which present which define depositional subenvironments. These units are (1) problems for many specialties such as paleontology, stratigraphy, flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil sedimentation, mineralogy, and inorganic and organic geochemis- shale, (5) trona-halite, (6) siliciclastic sandstone, and (7) volcanic try. The geologic and paleolimnologic aspects of the formation tuff. Their respective subenvironments are (1) rapid transgression have occupied Bradley (1929, 1964) for a lifetime. Recent impor- of a shallow lake, (2) lake shore oscillating over a mud flat (slow tant contributions to field relations and stratigraphy have been transgression), (3) playa mud flats, (4) shallow lake with occasional made by Pipiringos (1962), Roehler (1965), and Culbertson desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) (1971). Gazin (1965), MacGinitie (1969), and McGrew (1971) not specific. have discussed the paleontologic aspects and climate. The trona These subenvironment deposits are arranged in depositional cy- beds found in the Green River Formation constitute not only the cles. We have observed four types of cycles involving flat-pebble world's largest deposit of sodium carbonate (Deardorff and Man- conglomerate (A), oil shale (B), mudstone (C), lime sandstone (D), nion, 1971), but they also present fascinating mineralogical and and also trona. These cycles are I: A-B-C, II: D-B-C, III: D-C, and geochemical problems (Bradley and Eugster, 1969). The assem- IV: B-trona-C. Individual cycles have been correlated over dis- blages of unusual minerals associated with the saline facies were tances of up to 24 km. described by Milton and Eugster (1959), Fahey (1962), and Milton The Wilkins Peak Member is thought to have been deposited in a (1971); the authigenic minerals of the tuffs by Surdam and Parker playa-lake complex, which consisted of a shallow, central playa (1972); and the clay minerals by Tank (1972). Special attention has lake that was surrounded by vast, normally exposed mud flats been paid to the oil shales so characteristic of the Green River For- fringed by alluvial fans. Evaporative concentration of mation (Bradley, 1931, 1970, 1973). A large number of organic bicarbonate-rich inflow waters led to saturation with respect to compounds have been identified (for example, see Robinson, 1969) calcite, most of which must have been deposited as cement within in these rocks. Economic significance is based on the fact that the alluvial fans. Evaporation continued in the capillary zone of the oil shales of Wyoming, Utah, and Colorado represent the largest mud flats, precipitating calcite first, then magnesian calcite, and potential reserve of hydrocarbons in the world (Duncan and Swan- eventually protodolomite. The carbonates accumulated as a soft son, 1965). micritic mud at the fringes of the playa mud flats. In spite of these extensive multidisciplinary efforts, many aspects During periods of desiccation, the muds were subject to crack- of the Green River rocks have remained mysterious, foremost ing, and the mud-crack polygons contributed sand- and silt-size among them the origin of the oil shales and the dolomitic mud- dolomitic micrite intraclasts that were transported to the central stones that are associated with both the oil shales and the evapo- lake by the next storm. When the central lake was large, oil shale rites. Further insight into these problems may be gained by a con- accumulated in it, with the organic matter derived from a flocculent sideration of the sedimentary environments in which these rocks ooze consisting of bottom-dwelling blue-green algae and fungi. were deposited. The general aspects of these environments have During dry periods the lake shrank, and trona and halite precipi- been discussed by Bradley (1964) and Roehler (1965). Most rocks tated in the central portions. are interpreted as lacustrine; those of the Bridger basin, for in- An understanding of the Wilkins Peak sediments can be achieved stance, are believed to have been deposited in Lake Gosiute, a lake only by considering as inseparable the hydrologic, sedimentary, thought to have existed throughout most of Green River time, ap- geochemical, and biologic processes responsible for their forma- proximately 4 m.y. tion. Key words: sedimentary petrology, Eocene, oil shale, deposi- Bradley and Eugster (1969) focused particularly on trona ac- tional environments, sedimentary cycles, hydrochemistry, alkaline cumulation and suggested the existence of a stratified lake, with the brines, carbonate deposition, trona. deeper parts of the basin occupied by a strong brine of sodium

Geological Society of America Bulletin, v. 86, p. 319-334, 19 figs., March 1975, Doc. no. 50307.

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NOHTHHN MARGIN CENTER OF BASIN alents at the margins of the Green River basin belong to the Wasatch Formation, whose main body underlies the Green River Formation. The stratigraphic relations are shown schematically in Figure 1. The lateral extent of the Bridger basin is shown in Figure 2, which approximately delineates the hydrographic basin and the maximum extents of the Wilkins Peak and Laney stages. The basin is essentially free of structural complications, with all rocks dipping very gently (less than 2°), except for the immediate area of the Rock Springs uplift, at the south margin, and along the northwest mar- gin. Bradley (1964) considered the uplift to have formed an island in Lake Gosiute, although later geochemical and sedimentological evidence indicates that uplifting postdates Tipton and probably Wilkins Peak time as well (Wolfbauer, 1971, p. 6). Only a small percentage of the Wilkins Peak rocks are accessible in outcrop, the principal belt being shown in Figure 3. However, the outcrop belt, together with section A located in the Big Island mine, provides Figure 1. Stratigraphie relations of Green River Formation in Bridger coverage from the alluvial fans to near the center of the basin. basin (after McGrew, 1971). ROCK UNITS AS RECORDS OF carbonate—sodium chloride composition. Based on a preliminary DEPOSITIONAL SUBENVIRONMENTS examination of the sedimentary structures of the Wilkins Peak Member, Eugster and Sufdam (1973) challenged this model and Bradley (1931, 1964) recognized a number of rock types in the suggested that it be replaced by a playa-lake model. In their view, Wilkins Peak Member of the Bridger basin. The following oil shale and trona are the only true lacustrine deposits, whereas lithologies were defined: claystone, siltstone, mudstone, marlstone, the dolomitic mudstones and calcareous siltstones were deposited shale, papery shale, and oil shale. Additional terms used were on the broad playa flats fringing the central lake and thus were sub- limestone, limy sandstone, sandstone, and edgewise conglomerates. jected to frequent drying and wetting. An understanding of the physical conditions under which these rocks accumulated has implications beyond the origin of a particu- lar rock type. They involve questions of hydrology, climate, geochemical balance, brine evolution, mineral formation and transport, as well as biologic processes. What is most needed at this point is to clarify some of these questions by a detailed investiga- tion of the sedimentary and geochemical features that are diagnos- tic of a particular depositional process and environment. This paper presents such an analysis for the Wilkins Peak Member of the outcrop belt west of the Rock Springs uplift.

GEOLOGIC SETTING

The Green River Formation is middle-early to early-middle Eocene in age (Gazin, 1965) and consists of three principal mem- bers: Tipton Shale, Wilkins Peak, and Laney, with the following re- spective maximum thicknesses: 60, 370, and 520 m. Lateral equiv-

N

Figure 2. Inferred outline of hydrographic basin of Gosiute Lake. Approximate maximum extent of Lake Gosiute during Laney and Wilkins Peak stages. [Modified after Bradley and Eugster (1969).] For map showing Figure 3. Outcrop belt of Wilkins Peak Member and locations of mea- probable extent of oil shale, see Culbertson (1969). sured sections; see Fig. 15 (after Bradley, 1964).

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Subsequent workers have adopted this terminology (for example, Stuart, 1965; Roehler, 1965; Culbertson, 1971), which is essen- tially a pétrographie field terminology. Stratigraphie sections are measured in terms of this field classification. In addition, in his type sections, Bradley (1964) noted the presence of sedimentary struc- tures such as mud cracks, ripple bedding, and salt casts. In contrast to this approach, our objective was to group together all rocks that are the product of the same set of depositional pro- cesses. Hence we have chosen to group rock types according to their assemblages of sedimentary structures and textures rather than their pétrographie nature. Such groups of rocks interpreted as be- longing to the same depositional subenvironment of the overall Wilkins Peak environmental complex are called rock units (=lithofacies) in this paper, and our measured stratigraphie sec- tions are reported in terms of these rock units. We have recognized the following major lithofacies in the Wil- kins Peak Member: (1) flat-pebble conglomerate, (2) lime sand- stone, (3) mudstone, (4) oil shale, (5) trona-halite, and (6) siliciclas- tic sandstone. The essential diagnostic features of each of these units are presented in Appendix 1. Our interpretations of the dep- ositional subenvironments of the major lithofacies have been de- termined on the basis of these data and are described as follows.

Flat-Pebble Conglomerate Facies

The flat-pebble conglomerate facies consists of a single rock type — a 5- to 20-cm-thick bed of pebbles of dolomite mud. The indi- Figure 4A. Bedding surface view of flat-pebble conglomerate (bed 153, vidual particles are thought to be intraclasts derived from underly- section F). Width of sample is 10 cm (see pencil point for scale). Pebbles are ing mudstone units and therefore represent abraded mud-cracked dolomitic mudstone; matrix is dolomitic peloids with quartz cement stand- polygons and fragments of polygons (Figs. 4A and 4B). This is ob- ing out in relief. vious from the fact that the flat-pebble conglomerates observed are always in erosional contact with mud-cracked mudstones and that the particles have the same internal peloidal 1 structure and com- position as the mudstones. At the time the particles were deposited, they were cohesive but not lithified. This is demonstrated by curled pebbles with drying cracks on their convex side, which are reminis- cent of algal mat-bound supratidal mud chips (Figs. 5A and 5B). In the Bahamas, such chips are produced by storm waves ripping up polygonally cracked, algal-bound laminated sediments and are re- deposited a few meters away. The clasts are made coherent enough for high-energy wave transport by both drying and algal mat bind- ing (Hardie and Eugster, 1971, p. 210). Although the Green River flat-pebble conglomerate units are very thin, some have been traced over a north-south distance of more than 30 km. This areal extent, in combination with the intra- clast nature, suggests that this facies represents a transgressive lag deposit formed when a very shallow lake expanded over an ex- posed mud flat.

Lime Sandstone Facies

The lime sandstone units form prominent ledges 10 cm to 2 m thick, which are traceable along strike for over 35 km. This facies consists of two interbedded rock types: thin-bedded, rippled, dolomite-rich calcarenites, and mud-cracked dolomite mudstones (Figs. 6A and 6B). The alternation of these two rock types indicates short periods of very shallow water interrupted by periods of ex- posure and playa mud accumulation. Thus, this facies must repre- sent a lake shore oscillating over a mud flat. The morphology of the ripples (Fig. 7) points to a wave-dominated regime (Harms, 1969) during onlap, in which fine mud was winnowed out of the bottom sediment. Ripple crests are oriented north-south in the outcrop belt examined, at right angles to the transport direction of the siliciclas- tic sandstones (see below), which indicates that directions were Figure 4B. Thin section of flat-pebble conglomerate (bed 10, section G). controlled by shoreline configuration rather than by sediment sup- Dark peloids are of clotted dolomite micrite; note blocky radial dolomite rims. Some peloids are partly or entirely recrystallized to dolomite neomor- 1 A peloid is a particle of cryptocrystalline or microcrystalline material of phic microspar. Note occluded "ghost" rim in sparry grain in left-center of unspecified origin (Bathurst, 1971). photo. Clear interstices are of quartz cement. Scale bar, 0.5 mm.

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Figure 5A. Vertical slab through flat-pebble conglomerate (location F, well below unit D) showing curled pebbles with shrinkage cracks on convex side. Pencil point for scale.

ply currents. During onlap, well-sorted fine sand (intraclast car- bonate and quartz) was washed over the dried mud bottom, filling still-open mud cracks (Figs. 8A and 8B). The sand particles either were tuffaceous and siliciclastic grains transported by wind or water, or were dolomite peloids. The latter represent eroded and Figure SB. Curled desiccation polygons from modern carbonate tidal transported mudchips that originated as fragments of mud-cracked flats of Andros Island, Bahamas. Dark algal mat on surface of supratidal flat. Polygons are ripped up by storm flooding, and shrinkage cracks occur polygons of the adjacent mud flats. as soft moist aragonite mud beneath algal mat dries out. Cross-section During offlap periods, the porous sands were exposed and be- view at top; bottom view in lower left; top view (algal mat) in lower right. came effective traps for interstitial brine generated either by disso- Pencil for scale. lution of efflorescent trona crusts or by capillary evaporation with the sands acting as a wick. In either case, exposure led to precipita- recrystallized to dolomite neomorphic spar and not to calcite. That tion of radiating trona needles within the sand (Figs. 9A and 9B). is, the interstitial brines reacted with the carbonate peloids to form Interstitial precipitation also produced sand crystals with single gaylussite or pirssonite crystals or, at higher temperatures, shortite. long blades enclosing the undisturbed sand host. Hence, trona Gaylussite is a very common authigenic product in the sands and growth was penecontemporaneous with sediment accumulation. muds of present-day alkaline lakes (Haines, 1959; Jones, 1965). We do not fully understand the origin of the calcite rhombs (Fig. 10). They could be interpreted as recrystallized detrital calcite Mudstone Facies grains, as a product of dedolomitization, or as late diagenetic pseudomorphous replacement after an early diagenetic precursor The mudstone facies consists of two rock types, laminated and (such as shortite, pirssonite, or gaylussite). We favor the third in- thin-bedded dolomitic mudstones (Figs. 11A and 11B, respec- terpretation, because we have observed crystal morphologies in- tively). Mud cracking is the outstanding feature of these rocks compatible with normal calcite habit, such as rectangular and (Figs. 11A and 11B). It is significant that the mud cracks are usually square cross sections. Also, dolomite micrite in adjacent peloids is filled with the same material as the overlying lamination or thin bed; this could be sand, silt, or mud. Some cracks are complexly filled, for example, with sand on the inside and mud on the outside,

Figure 6A. Outcrop photo of lime sandstone unit (bed 100, section E); Figure 6B. Vertical slab of thin beds of lime sandstone (from lower Wil- on vertical view, scale is 15 cm. Note lenticular bedding and starved ripples, kins Peak, locality F) showing low-angle ripple cross-lamination. Note ero- Thin partings are of mudstone. sional truncations. Pencil for scale.

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(Figs. 11A and 12), which are obviously scour and depression fills. Where the laminae are graded, the gradation from a silty lenticular base up into a more persistent mud lamina then represents waning currents rather than settle-out from standing water. We believe that soon after flooding, thin algal mats must have established them- selves on the mud flats, as we have observed on the Holocene Baja California sabkha. However, the mats in Baja form after sediment deposition and are soon overwhelmed by efflorescent salt crusts. They do not influence the deposition of the laminae; that is, the laminae are not stromatolitic. We assume this also holds for the Green River laminae. The thin beds were deposited by deeper flood waters or in shal- low depressions on the playa surface and were probably produced by single events. Rippled surfaces (Fig. 11B) indicate that the ma- terial was at least silt size. This is supported by the clotted texture, which suggests that the original particles were soft dolomitic peloids. During diagenesis, compaction and partial recrystalliza- tion obscured many peloid boundaries. Figure 7. Bedding plane view of ripple marks in lime sandstone (cycle The origin of the calcite grains remains obscure. Their anhedral 11, section E). Geologic hammer for scale (30 cm). shape and presence in the coarser laminae and their enclosure in indicating multiple opening and filling (Fig. 11 A). Some fillings are dolomite rhombs make it more likely that they are of detrital laminated, showing that filling took place in steps. These features origin. However, they are single crystals and occur commonly in together with the open sheet cracks and fenestral pores, indicate irregular aggregates. This aspect is difficult to reconcile with a sim- that the mud cracks were formed by desiccation at the sediment ple detrital origin. It is possible that here, too, a precursor mineral surface and that they were neither the result of syneresis nor com- is involved (see Lime Sandstone Facies section, above). paction dewatering. It is possible that some of the isolated cracks seen in vertical slabs could be burrows of insects, but circular cross Oil Shale Facies sections were not found. In some cases, the mud cracking is so in- tense that the thin-bedded mudstones have a chaotic, brecciated, The oil shale facies consists of two rock types, organic-rich marble-cake appearance (Fig. 1 IB) dolomitic laminites and oil shale breccias (Fig. 13). The environment in which the rocks of the mudstone facies were Crucial for the interpretation of the environment in which oil deposited obviously must have been exposed most of the time, in- shales accumulated is the presence of mud cracks and breccias dicating the existence of playa mud flats. These mud flats were sub- noted repeatedly by White (1932, p. 407-413) and Bradley (1931). jected to occasional flooding by storm waters, which brought in de- In the Piceance Creek basin of Colorado, Bradley (1931, p. 28) trital silicates and intraclastic dolomite peloids. These silt-size noted that the "shale breccias" are most common near the margins. peloids have the same origin as the peloids of the lime sandstones. In the Bridger basin we have found abundant shale breccias near Thin sheets of water deposited the fine laminae mainly as a traction load. This is suggested by the lenticular nature of the silt laminae

Figure 8A. Lower bedding plane of lime sandstone (locality F) showing Figure 8B. Lower bedding plane of very thin lime sandstone stringer en- small sand-filled "bath-tile"-size mud cracks of underlying mudstone part- closed in mudstone (bed 11, section D) showing casts of wide, shallow mud ing. Scale in millimeters. cracks of underlying mudstone layer. Pencil point for scale.

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.) v. / jpl . llM^vf | jfi&mÊ fi yT/laUvi .''.¿S^S L V 1 ,t 1I'AFipvv^^^Be ht v - Figure 9B. Thin-section photo of lime sandstone (cycle 12, section J) * 'Wff^"tllk ^ 'ir showing rounded dolomite micrite peloids (dark grains). Large blades are molds after trona, now partly filled with calcite (which in turn is probably pseudomorphous after shortite; compare with Fig. 36 in Fahey, 1962). Figure 9A. Bedding plane view of radiating crystal molds in lime sand- Trona mold is 850 /u wide. stone (bed 49, section D). Crystals likely were trona. Scale in millimeters. than 5 /x) filamented algae of the oscillatoracia type are able to the center of the basin (section A in Fig. 3) associated with trona thrive. beds. Therefore, we concur with Bradley's original concept that the In our view, then, the oil shale environment was characterized by oil shales accumulated in a shallow body of standing water that shallowness of the water body, periodic desiccation, high organic periodically dried up. Bradley (1973) reconfirmed this interpreta- productivity, and exceptionally low sediment influx. The most ac- tion and has suggested heat fixation associated with subaerial ex- posure as the principal mechanism for the remarkable preservation of organic matter. During exposure, the organic-rich ooze cracked into desiccation polygons, as is common in many modern marine carbonate tidal-flat environments (see, for example, Davies, 1970; Logan and others, 1974). During the next storm or rise of lake level, loose polygons of dried algal ooze and carbonate mud were washed into depressions, forming the lenses of oil shale breccias that are found scattered throughout the section. The dolomite present in the oil shale is a diagenetic microspar crystallized from a micrite. There is no evidence of the original form of the dolomite, which could have been either a settle-out from chemical precipitation or detrital silt-size peloids. We favor the latter interpretation because the associated quartz and feldspar grains are exclusively silt size and because the lenticular laminae of dolomite suggest deposition by gentle bottom currents rather than by settle-out of mud. If we accept the premise that the oil shale formed in a shallow lake environment and that the dolomite or its precursor was de- rived from adjacent exposed mud flats, the concept of annual dep- osition of a double layer or varve from a permanently stratified lake must be abandoned. The mode of accumulation, then, is not gov- erned by seasonal blooms of planktonic algae alternating with "rains" of carbonate precipitates, but rather with the presence of a gelatinous, bottom-dwelling, flocculent ooze consisting of blue- green algae and fungi, and with periodic floods that brought in car- bonate and silicate grains. These floods could well have been sea- sonal, but not necessarily so. Judging from our experience with modern carbonate environ- ments, protected, ponded subenvironments that receive sediment only on rare occasions (from hurricane floods, for instance) are colonized by thick mats of coccoid blue-green algae (for example, Figure 10. Thin-section photo of lime sandstone (bed 100, section E). see Logan and others, 1974). In contrast, the organic-poor, Dark ovoids are dolomite micrite peloids. Clear euhedral rhombs are cal- sediment-rich laminites, such as those of the mudstone facies, ac- cite. Irregular to rounded clear grains are quartz. Fine-grained clear patches cumulate in an exposed subenvironment, in which only small (less are dolomite neomorphic microspar. Scale bar is 200 fi.

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ceptable model is that of a shallow, central lake surrounded by wide mud flats, which acted as a very effective sediment trap. Thus, the oil shales that are so characteristic for this time period not only in Wyoming, but also in Utah and Colorado, are not so much a testimony to unusually high organic activity as they are to the exis- tence of broad, shallow basins with centers protected from sedi- ment influx.

Trona-Halite Facies

The Wilkins Peak Member is famous for the presence of bedded

trona (Na2C03 • NaHC03 • 2H20) deposits. These deposits have been described most recently by Bradley and Eugster (1969), Cul- bertson (1966, 1971), and Deardorff and Mannion (1971). Cul- bertson (1971) counted at least 42 individual trona beds covering a total area of 3,400 km2, with 25 of these beds attaining a thickness of more than 1 m; the most massive bed is 11 m thick. Halite is present in some of the beds, particularly the lower portion of the Wilkins Peak Member, either mixed with trona or as a pure bed up to 6 m thick. Culbertson (1971) noted that trona beds are usually underlain by oil shale of varying thickness, whereas Deardorff and Mannion (1971) recognized the association with oil shale and marlstone. Nearly all trona beds contain thin partings of dolomitic mudstone. Because of our reliance on outcrops, we did not study trona- halite units in detail, but we have used our experience with active trona-precipitating lakes, particularly Lake Magadi, in Kenya (Baker, 1958; Eugster, 1970), to interpret the environment of trona Figure 11A. Vertical slab of laminated mudstone (cycle 3, section J). deposition. During trona accumulation, the lake must be visualized Light-colored lenticular layers are silt-fine sand and quartz rich. Note dis- as a trona-brine body with a normally dry surface. Solutes are con- continuous scoured contacts and complexly filled mud cracks. Pencil point tributed primarily by alkaline springs or ground water, or by occa- for scale. sional flood waters that dissolve efflorescent crusts. Larger floods bring in dolomitic mud from the surrounding playa and are respon- and oil shales. Culbertson (1961) has given the exact stratigraphic sible for the partings in the trona beds. These partings could record position of six tuff beds that he was able to correlate across the a single event, or they could signal a temporary expansion of the Bridger basin for considerable distances. These beds are 1 to 50 cm lake. thick and consist of highly altered volcanic ash. The alteration products have been described by Surdam and Parker (1972). Siliciclastic Sandstone Facies

Culbertson (1961) has described the presence of nine siliciclastic sand tongues in the Wilkins Peak Member that extend northward from the Uinta Mountains. He has given their stratigraphic posi- tions and has correlated them in the southern part of the Bridger basin. The thickest of these bodies is unit D of Culbertson (1961), which we have used as our main stratigraphic marker. We have correlated the main sand body of this unit over a distance of about 100 km (see Fig. 15). In the Firehole Canyon, this body is 10 m thick, but it thins drastically to the north. The siliciclastic sandstones consist of well-sorted quartz- feldspar-biotite sands that typically are cross-bedded. We have not studied these sandstones in detail but have made the following ob- servations: The sandstones are fining-upward sequences with trough-cross-bedded coarse sands at the base, fining up to climbing ripples made of fine sand and silt at the top (Fig. 14). The fining-upward sequences do not persist laterally, but they are essen- tially channel fills that truncate each other. The ripple cross- laminations indicate transport from the southeast and southwest, that is, from the direction of the Uinta Mountains. These observa- tions are consistent with deposition in a braided stream flood-plain complex rather than by meandering stream accumulation. The sed- iment load was probably derived by reworking of alluvial fans ini- tiated by tectonic events. Figure 11B. Vertical slab of thin-bedded mudstone (cycle 3, section J). Volcanic Tuff Units Disruption by mud cracking is outstanding feature here producing marble cake-like appearance. Light-colored areas are quartz-feldspar-biotite rich Volcanic ash is common in the Wilkins Peak Member and is pres- and dark areas are dolomite micrite rich. Upper surface of prominent dark ent either as distinct tuff beds or as an admixture in the mudstones layer is actually rippled. Pencil point for scale.

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perhaps the most significant clue to the interpretation of the deposi- tional environment, and all portions of the sections examined ex- hibit these cycles. We have distinguished four basic types of cycles, which are summarized in Figure 17. In Type I, the basal member is a thin (<20 cm) flat-pebble conglomerate (A) that contains chips eroded from the underlying mud-cracked mudstone unit. Next fol- lows an oil shale unit (B) of variable thickness (10 to 280 cm). Toward the top of this unit, the organic content decreases, and the oil shale passes gradually into a heavily mud-cracked, laminated mudstone unit (C), with a thickness of as much as 2 to 3 m, which represents the cycle cap. Thin stringers of lime sandstone are com- monly present in increasing numbers toward the top of the mud- stone unit until this unit grades into a bench of thinly bedded rip- pled lime sandstone with thin mudstone partings (D) up to 2 m thick. This lime sandstone marks the base of a new cycle of Type II (a D-B-C sequence), or Type III, which is a D-C alternation (Fig. 17). Note that a flat-pebble conglomerate is never in contact with a lime sandstone unit. The maximum thickness of individual cycles is on the order of 4 to 5 m, and in one section (section E, Figs. 3 and 15), we have counted 40 cycles in a 120-m section of the middle and upper Wil- kins Peak Member. Cycles can be correlated laterally with little thickness variation over a distance of 24 km (Fig. 18). However, individual components of a particular cycle may change; for exam- ple, a Type II cycle may be replaced by a Type I or Type III cycle. The Type IV cycle (trona-oil shale) is restricted to the central por- tion of the basin. This cycle was recognized by Culbertson (1971) and is clearly illustrated in his Figure 2.

INTERPRETATION OF THE DEPOSITIONAL CYCLES

Reconstruction of the depositional environment is based on the meaning of each individual rock unit and its position within the depositional cycle. We consider the flat-pebble conglomerate, oil shale, and lime sandstone units to be transgressive deposits, whereas the mudstone units are regressive. Figure 12. Vertical slab of thickly laminated to thinly bedded mudstone (15 m above base of Wilkins Peak, localities F-G). Light-colored layers are The mudstones formed on normally exposed mud flats fringing a sandy and quartz rich. Dark layers are dolomite-mud rich. Note especially permanent, closed lake; they always represent the top of a cycle. lenticular nature of sandy laminae. Pencil point for scale. The main mechanical processes of sedimentation on the flats were deposition and reworking by sheet wash, which produced lami- nated units during periodic storm flooding. Desiccation caused ex- We have not studied these tuff beds in detail, but we have noticed tensive and multiple mud cracking. Evaporation was intense during the presence of a large amount of material in the mudstones and oil periods of exposure, and the water table was probably near the sur- shales that could well be of pyroclastic origin. This includes par- face. Efflorescent crusts formed, and the growth of these crusts, to- ticularly biotite, plagioclase, and quartz. The tuff units do not rep- gether with the mud cracking, was responsible for the disruption of resent a single and unique subenvironment with respect to deposi- the laminae. Rapid transgression of the playa flats by the expand- tional processes, and we will not consider them further. ing lake led to the formation of the basal flat-pebble conglomerate units, which are interpreted as transgressive lag deposits with the TIME AND SPACE DISTRIBUTION OF pebbles derived in situ by wave erosion of mud-crack polygons. ROCK UNITS: DEPOSITIONAL CYCLES The transgression represents the beginning of the next cycle and in- itiates a relatively long period of a shallow and fairly fresh to saline To arrive at a satisfactory understanding of the depositional en- lake in which oil shale accumulated. Culbertson (1971) recognized vironment of the Wilkins Peak rocks, the relationships among indi- that the flat-pebble conglomerates are "the first deposits of a trans- vidual subenvironments in space and time must be examined. gressive lake" (p. 22) and that the immediately overlying oil shale Toward this goal we have measured six partial sections in the mid- "is the first deposit of the expanded lake and that it was probably a dle and upper Wilkins Peak Member. Their locations are shown in shallow-water deposit" (p. 22). Figure 3. We have correlated these sections with the more complete Oil shale is the major transgressive deposit and forms the middle sections reported by Bradley (1964) and Stuart (1963), with the of the cycle. Shallowness and periodic desiccation of the lake is in- main correlation based on the large siliciclastic sand bodies (Fig. dicated not only by the internal structures discussed above but also 15; Bradley, 1964; Culbertson, 1971). More detailed bed-for-bed by the position of the oil shale in the cycle. Oil shales grade upward correlation was achieved by using lime sandstones, oil through lean oil shale into the laminated mudstone units, with the shale-flat-pebble conglomerate units, and tuffs. Farther north, the boundaries usually being transitional. This transition is clearly re- section thins drastically, making correlation more difficult. gressive in nature, produced by a slowly shrinking lake combined Subenvironments do not follow each other randomly in vertical with increased sediment load. sequences, but they are arranged in well-defined cycles that are eas- Many transgressions are not so rapid and complete so as to form ily observed in the field (Fig. 16). Recognition of these cycles is flat-pebble conglomerates, but are more gradual and oscillatory,

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with shallow-water conditions alternating with exposure for a pro- longed period of time. We consider the lime sandstone units to have formed in the waters of such a slowly expanding lake. Energy was high enough to winnow out mud and to prevent cohesive algal mats from forming. The amount of sand-size sediment was small, as the many starved ripples show (Fig. 6A). The periods of expo- sure and mud-flat accumulation in this facies are signaled by mud- cracked mudstone interbeds, as well as by the many trona casts oc- curring in the sandstone units. If transgression was complete, an oil shale developed on top of the sandstone; if it was not, a mud flat was re-established. As Figure 18 shows, the position occupied by a flat-pebble con- glomerate in a particular cycle of one section may be occupied by a lime sandstone in the same cycle of another section. Regional pat- terns of transgressive types perhaps could be established by more detailed stratigraphic measurements, using cycles as time lines. A schematic view of the cycles within the complete depositional framework is shown in Figure 19. We must now briefly consider the probable causes responsible for the cyclical sedimentation. Because the successions of subenvi- ronments are linked to the position of the shoreline of the central lake, climatic effects — particularly the balance between precipita- tion and evaporation — must be involved. During more pluvial Figure 13. Vertical slab of brecciated oil shale (bed 60, section E) show- periods the lake expanded, and with it the area of oil shale deposi- ing scattered pebbles (light colored) and pockets of pebbles of dolomite tion. Conversely, most of the basin area became exposed mud flats mudstone. Note compaction of dark organic-rich laminae around pebbles during dry periods, while the center was occupied by a salt lake. and pebble lenses. Sample is 2.5 cm thick. During this time the higher altitudes probably remained heavily vegetated. The hydrologic changes could have been initiated by changes in precipitation or by changes in the basin floor elevation evaporative environment is necessary. The most favorable setting is due to tectonic events, or both. a low basin floor surrounded by high mountains that collect pre- Transgressive and regressive cycles based on an alternation of cipitation but at the same time shield the arid floor from rain. The wet and dry periods occurred during Wilkins Peak time at least 50 sharp relief caused by the faulting leads to the development of allu- times. Bradley (1964) estimated 1 m.y. for this period, but because vial fans that trap the coarse detritus at the edges of the basin. we have abandoned the idea that each lamina represents an annual Build-up of the fans is primarily caused by very brief catastrophic accumulation, the time span occupied by the Green River Forma- storms. The sharp break in slope at the toes of the fans insures that tion must be re-examined. However, Bradley's estimate cannot be only the finer material will be carried beyond the fans. The main short by more than a factor of two or three, if we accept the prem- part of the basin is very nearly horizontal, consists mainly of silt ise that the whole Eocene Period did not last more than 16 m.y. In and mud, and is dominated by sheet-flow processes associated with other words, each wet-dry cycle must have lasted at least 20,000 yr, storms. Throughout most of the existence of the playa, however, and perhaps as long as 50,000 yr. the hydrologic processes are quite different from those that prevail The nine siliciclastic sand bodies are not included in the deposi- during the infrequent periodic flooding. The perennial freshwater tional cycles. As mentioned previously, they are presumably as- springs and streams issuing from the mountains disappear into the sociated with special events, such as tectonic rejuvenation of allu- alluvial fans, and circulation from there on into the basin is primar- vial fans. The depositional cycles described by Picard and High (1968) from the Uinta basin are different from the cycles described in this paper.

GEOCHEMICAL AND SEDIMENTARY PROCESSES IN THE PLAYA ENVIRONMENT

A substantial portion of the Green River rocks are chemical sed- iments, including the saline minerals and the alkaline-earth car- bonates. Sedimentary structures and depositional cycles indicate that Wilkins Peak time was predominantly a time of exposed playa mud flats. Thus, it is helpful to reconstruct the piaya environment and the associated hydrologic, sedimentary, and chemical proces- ses, using our experience with Holocene playas and playa-lake complexes (Jones, 1965; Hardie, 1968; Eugster, 1970; Hardie and Eugster, 1970) as a guide.

General Hydrochemical Processes

The playa environment must be understood in terms of the basic tectonic and hydrologic setting. Normally, a closed basin is ini- tiated by block faulting. In order to produce chemical sediments in Figure 14. Weathered vertical surface of siliciclastic sandstone (unit D, the basin, a continuous supply of water bringing solutes into an section J) showing climbing ripples. Thickness of sample is 10 cm.

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CYCLES IN THE WILKINS PEAK MEMBER

TYPE /

® RIPPLED 3 D LIME SANDSTONE

MUDCRACKED R LAMINATED LIME

MUDSTONE

B OIL SHALE C-VTVÜI A FLAT PEBBLE CONGLOMERATE

TYPE II TYPE III TYPE IV

Figure 16. View, looking west, of upper part of Wilkins Peak Member in White Mountain scarp between Rock Springs and Green River (section E). This is white band above unit D (Fig. 15), so conspicuous from a dis- tance. Prominent ledges are lime sandstone units. Dolomite mudstone (and, S3 in some cases, oil shale) units make up softer white zones between the sand- ma stone ledges. Rather uniformly spaced repetitions of mudstone-sandstone units making cycles are well displayed. About 50 m of vertical section shown. c

ily subsurface. Chemical weathering processes active in the moun- ^TTTT-» D tains provide the waters with solutes. From the fans to the center of ESTTZTiRiif the basin (actually, the hydrographic low), evaporation is intense. The center may be occupied by a perennial body of shallow water, which is fed directly by diffuse discharge of ground water or by springs. This body is also the final collecting place of the storm Figure 17. Four types of depositional cycles in Wilkins Peak Member. waters which traveled overland as sheetwash. The most important process operating during subsurface flow of infiltration into the ground water, increasing the ground-water the waters is evaporation in the capillary zone (Hardie, 1968; Eug- concentration considerably and modifying the chemical pathways. ster, 1970). Evaporative concentration increases in intensity Many of the brine springs found at the perimeters of saline lakes toward the center of the basin. Saturation with respect to alkaline- may have acquired most of their solutes in this manner. earth carbonates is reached during the early stages of the concen- Near the center of the basin, the ponded brines derived from tration history, regardless of inflow composition (Hardie and Eug- ground water or overland flow continue to be exposed to evapora- ster, 1970). Precipitation may occur in the subsurface of the alluvi- tion, and the saline minerals are precipitated, producing a bedded al fans and the playa fringes or at the surface in connection with crystal-brine accumulate. peripheral springs (Jones, 1965; Hardie, 1968). Precipitation of the alkaline-earth carbonates profoundly affects Evolution of Alkaline Brines the composition of the subsurface waters and hence the sequence and nature of the saline minerals formed in the central part of the In applying these general principles of playa processes to the basin. The net effect is the creation of a mineral zonation with the Green River Formation, we must first consider the composition of most soluble minerals occupying the center and segregated from the inflow waters. The extensive trona deposits present in the Wil- the less soluble phases (Hunt, 1960; Jones, 1965; Hardie, 1968). kins Peak Member severely restrict the possible compositional var- A more efficient process for separating soluble from less soluble iations. The parent waters must have been rich in HC03~, the ++ 14 constituents and accumulating them in the center is based on HC03~/Ca + Mg molar ratio must have been distinctly greater efflorescent crusts. Such crusts form on the playa wherever the than unity, and other anions such as Cl~ and S04 must have water table is near the surface; they are the residues of evaporation been minor. To produce a carbonate-rich brine, the following con- to dryness. Because of the rapid evaporation, many of the mineral dition must hold (for details, see Hardie and Eugster, 1970): assemblages found in such crusts are metastable, but they do reflect (m f + 2 m ,—) < (w + + m +). the major constituents of the brines after alkaline-earth carbonate c so Na K removal. Typical inflow waters of this nature are produced by weathering Efflorescent crusts are ephemeral. Rain and storm runoff prefer- reactions of silicates, mainly feldspars, with carbonic acid (Garrels entially dissolve the more soluble minerals of the crusts, producing and Mackenzie, 1967). Sources for sulfate and chloride must be a concentrated brine by fractional solution. This brine ultimately minor; that is, sulfides and saline minerals must be nearly absent in will reach the center of the basin either by surface flow or by the watershed. This implies that the rocks weathered must be pre-

Figure 15. Measured sections of the upper Wilkins Peak Member. Locations of sections are given in Figure 3. Major markers are silicidastic sandstones, units D and I of Culbertson (1971). Numbers give position of samples referred to in text. Section A is in Big Island mine and has also been measured by Deardorff and Mannion (1971); section B is Tollgate Rock west of town of Green River; section C is taken from Bradley (1964, p. A71-A73); section D is located in T. 18 N., R. 106 W., sec. 17 and 18; E is near Kanda, T. 18 N., R. 106 W., sec. 3 and 10; section F is along the road leading up to the Rock Springs radio-TV tower and is taken from Stuart (1963); section G is 0.5 km northeast of section F; sections H and I are taken from Stuart (1963).

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Deposition of Alkaline-Earth Carbonates

Bradley and Eugster (1969) have estimated that 20.5 x 1016gof Ca and 9.6 X 1016 g of Mg are present in the Wilkins Peak sedi- ments, corresponding with 11.7 X 1016 g of calcite and 72.8 X 1016 g of dolomite and a molar ratio in the parent waters of Ca/Mg of 1.30. Compared to waters known to produce alkaline brines (for example, see Hardie and Eugster, 1970, Table 1), this ratio is low by perhaps as much as a factor of two, reflecting the great prepon- derance of dolomite. If it is assumed that most of the calcite precipi- tated during' evaporation is located in the alluvial fans as cement and hence is not found in the playa sediments, the discrepancy is removed. In this case, the springs issuing at the toes of the alluvial fans and flowing onto the playa flats precipitate predominantly high-magnesian calcite and protodolomite. Judging from examination of recent playas, the alkaline-earth carbonates so produced form a soft micron-size mud rather than lithified caliche, because the mud is normally soaked in solution, whereas formation of caliche seems to require many cycles of dry- ing and wetting. The micritic mud is subject to reworking by sheetwash and transport as detrital material. -0 Together with detrital quartz and silicates, some of which may 24 Km well be of volcanic origin, the carbonate mud and eroded mud Figure 18. Example of correlation of cycles in Wilkins Peak Member. aggregates (peloids) are transported toward the center of the playa See Figures 3 and 15 for location and vertical position of sections. Key: where lamination and cross-lamination of traction or bed-load cross-lined = siliciclastic sandstones (units are those of Culbertson, 1971); origin are produced. In other words, the laminae are not produced black = oil shale; white = dolomitic mudstone; dotted = lime sandstone; by precipitation and settle-out in situ; they are not annual deposits black and white bars = flat-pebble conglomerate. but are associated with individual, erratic storms. dominantly igneous or metamorphic or be sediments other than The precursor of the well-ordered dolomite now present in the pyritic shales and evaporites, for example. mudstones and oil shales cannot be established unequivocally. Pre- The most crucial step in the evolution of an alkaline brine is the sumably, most of it comes from the high-magnesium calcite and early precipitation of alkaline-earth carbonates. Saturation occurs protodolomite micritic mud that recrystallizes to dolomite soon first with respect to CaC03, usually calcite. Continuing precipita- after deposition, as happens at Deep Springs Lake (Peterson and tion of calcite during evaporative concentration is governed by the others, 1963; Clayton and others, 1968). It is, of course, entirely following restrictions: possible that dolomite also formed by penecontemporaneous dolomitization of low-magnesium calcite mud peloids by intersti- 2- "1. Ca* and C03 must be lost from solution in equal molar propor- tial brines. There is no direct textural evidence for such a conver- tions, and sion, however; instead, there is clear indication that early in situ 2 2. The IAP • iJr0, ~) of the solution must remain constant at con- diagenesis of the peloids involved only the production of a stant P (total) and T. neomorphic dolomite microspar. Early recrystallization rather than Now, because the initial molar proportions in general will not be equal, the 2+ 2 dolomitization is also supported by petrographic observation of a first restriction implies that the Ca to C03 ~ proportions in the solution must change as calcite precipitates. The second restriction allows only few dolomite rhomb overgrowths around calcite rhomb nuclei. 2+ 2 2+ antipathetic changes in Ca and C03 ~ concentrations, that is, if Ca 2- increases then C03 must decrease, and vice versa." (Hardie and Eugster, Deposition of Trona and Halite 1970, p. 277) The bulk of the calcite is precipitated during the early stages of After completion of alkaline-earth carbonate precipitation, the evaporation. This leads to rapidly increasing values of brines have reached a pH of >9, and Ca""" and Mg""" have been re-

wCOa—lmCa++ and wMB++/mCa^, which causes precipitation of mag- moved nearly quantitatively. Evaporation continues, Pt02 builds nesian calcites, and ultimately protodolomite, as has been observed up, and CO 2 is lost to the atmosphere, causing pH to increase. The by Nesbitt (1974) in the Basque Lakes of British Columbia. Prob- brines may still move by subsurface paths, or they may now emerge lems with nucleation may delay precipitation of magnesian phases, as springs at the perimeter of the central water body. The latter is so that by the time protodolomite forms, the brines may be quite the case for Lake Magadi, an active trona-producing lake (Baker,

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events is compatible with the irregular spacing and thicknesses of the dolomitic mudstone laminae that occur in the trona beds (see Fig. 14 of Bradley and Eugster, 1969). The deposition of halite during Wilkins Peak time can be under- stood, if one accepts the recycling processes outlined earlier. Halite is present in many trona beds. Deardorff and Mannion (1971, p. 28) have characterized the trona-halite relationship as follows: "The salt-trona mixture is normally found within the thicker part of the trona bed, which was probably deposited in a low-lying por- tion of the basin. . . ." They have also given a detailed account of the trona-halite relationship in bed 17.

SYNOPSIS OF THE DEPOSITIONAL ENVIRONMENT

The combination of sedimentological and geochemical con- Figure 19. Schematic block-diagram showing general depositional straints makes the playa-lake model very attractive as the deposi- framework envisaged for Wilkins Peak Member. tional framework for the Wilkins Peak Member. The need for a permanently stratified lake is eliminated. The main aspects of the 1958; Eugster, 1970), which is fed by alkaline springs with a pH of playa-lake model can be summarized as follows: 9 to 10 and a salinity of 20,000 to 30,000 ppm. Evaporation of 1. Block faulting produced a closed basin with an arid floor, sur- such waters in contact with air eventually leads to precipitation of rounded by high mountains that trapped precipitation and pro- trona. During the dry season, the main body of the trona deposit is vided substantial perennial inflow into the basin. exposed to the air. The deposit of interlocking trona needles has a 2. Alluvial fans (normally mapped in the Bridger basin as part of high porosity (up to 50 percent), and the interstices between crys- the Wasatch Formation) built up by outwash from the mountains tals are occupied by a saturated sodium carbonate brine. Even dur- and rimmed the basin, while the floor consisted of a very wide ex- ing the driest periods, the brine level rarely drops more than a few panse of mud flats with a central body of shallow water that meters below the surface, because surface crusts strongly inhibit periodically expanded to a larger lake and shrank to a saline al- further evaporative loss. The crystal framework is hard and very kaline lake (Fig. 19). Records of these transgressions and regres- tough, and the surface of the lake can support heavy weights. The sions are beautifully preserved in the depositional cycles that are surface is not smooth but is dissected by polygonal thrust plates. the characteristic feature of the Wilkins Peak Member. During the dry season, a considerable amount of airborne dust may 3. Fluviatile processes were restricted to the edges of the basin, be trapped on this rough surface. If inflow is by springs or seeps, except for the nine long tongues of siliciclastic sand bodies that ex- perennially marshy and wet areas exist at the perimeter of the salt tend northward over the playa. These bodies were produced by lake. These areas usually have considerable organic productivity braided streams issuing from the Uinta Mountains and represent and are colonized by a fauna such as tilapia fish and flamingoes alluvial fan material reworked during tectonic rejuvenation. that thrive on alkaline waters. Waters flowing from these marshes 4. Soft waters with a low sulfate and chloride content were pro- toward the dry lake are exposed to intense evaporation and reach duced by weathering of silicates in the mountains. Perennial trona saturation near the lake border, thus continually adding streams disappeared into the alluvial fans and recharged the trona to the deposit. ground-water system. Capillary evaporation from the water table At the onset of the rainy season, the lake surface becomes caused concentration to increase, leading to precipitation of calcite, flooded by storm waters. At Magadi, which lies in a narrow, steep- high-magnesian calcite, and protodolomite in that succession. sided trough, flooding occurs during every normal rainy season, 5. Alkaline-earth carbonates were precipitated as a soft, micritic and the lake level may rise by 2 to 3 m. The fresher waters dissolve mud from springs and surface inflow at the outer perimeter of the trona from the lake surface and also bring in clastic material. After mud flats. The desiccated and mud-cracked micrite was washed the rainy season, the surface waters evaporate again and a new toward the center of the basin as sand and silt-size mudclasts by layer of trona is deposited. Obviously, this trona does not represent sheet wash during periodic storms. Siliciclastic material, both a completely new annual accumulation, as most of it is recycled. At weathering residues and pyroclastic debris, was added during these Magadi, the trona layering is on a 2- to 5-cm scale, and it has been storms or by wind. Deposition produced laminated mudstones of estimated (Eugster, 1970) that 90 percent of the salts of each layer intraclastic detrital nature. The laminae of many mudstones were have resided in the basin previously, with two-thirds derived from a disrupted by multiple mud cracking and by growth and dissolution brine recirculation path and the remaining one-third derived by of efflorescent crusts. dissolution of surface trona. The layers may be separated by mud 6. Precipitation of alkaline-earth carbonates produced high pH partings formed by a combination of storm water muds and the sodium-carbonate brines that migrated toward the hydrographic windborne material that had accumulated on the surface during the low of the playa. During dry periods, the central lake was con- previous dry season. Accumulation of trona can be quite rapid, tracted and trona precipitated from the brine. The lake was proba- with the vast mud flats acting as a continuous and very effective bly seasonally dry. The mud of dolomite partings in the trona se- brine factory. Estimated deposition rates are on the order of 300 to quences was washed in during storms. These storm waters also 1,000 yr/m for Lake Magadi trona. brought in solutes acquired by dissolution of efflorescent crusts. Because of the differences in topography and hydrology, Lake 7. At the onset of a pluvial period, rapid transgression of the Magadi probably is not a good analog for the Wilkins Peak rocks shoreline produced a flat mudstone-pebble lag deposit. A more with respect to flooding and drying cycles. Also, the largest area of gradual and oscillatory transgression produced rippled lime sand trona precipitation during Wilkins Peak time was about 30 times bodies alternating with mud-cracked dolomitic mudstones. that of Lake Magadi. Lake Gosiute's shores rose extremely gradu- 8. Oil shale accumulated in the expanded, fresher lakes, which ally, and the surrounding mud flats were tens of kilometers wide. were still surrounded by wide mud flats. Protection from sediment Overland storm waters may not have reached the lake annually but influx allowed thick, bottom-dwelling, coccoid, blue-green algal only under exceptional circumstances, and if they did, the lake ex- mats to thrive. Infrequent storms provided dolomitic laminae with panded dramatically. The catastrophic nature of the flooding the dolomite derived from the mud flats. The lakes were shallow

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and must at times have dried up in part to produce mud cracks and as the other extreme need not invalidate the basic concepts. This intraclast breccias. assertion must, of course, be tested in the field. 9. The Wilkins Peak fauna and flora are compatible with a Playa sediments of other geologic periods can now be better in- playa-lake complex. McGrew (1971) mentioned a flamingo nesting terpreted. The best-known example is the Triassic Lockatong For- area in the northern part of the Bridger basin where the Cathedral mation (Van Houten, 1965), but there are also a large number of Bluffs tongue of the Wasatch Formation interfingers with the Wil- closed-basin deposits present in the Tertiary formations of the kins Peak Member. Judging from Lakes Natron and Magadi in western United States. Furthermore, detailed study of marine tidal Tanzania and Kenya, this exactly fits our proposed setting. The flat areas, such as those of the Persian Gulf and the Gulf of Califor- adult flies and fly larvae and the aquatic insects mentioned by Brad- nia, is now revealing that these areas are not exclusively under ley (1964, p. 43) also point to shallow-water bodies and wet mud marine influence, but that contributions from continental processes flats. The subtropical climate visualized by MacGinitie (1969) and are substantial. These contributions and processes are identical McGrew (1971) does not conflict with a highly evaporative playa, with those of a normal playa environment. as long as it is kept in mind that the higher portions of the alluvial fans and the mountains were forested and that in the lower parts ACKNOWLEDGMENTS of the fans, wherever water was available, a luxuriant flora could establish itself, just as it now has on the Rift Valley slopes above The work was supported by National Science Foundation Grant Lake Magadi. GA-31076 and Petroleum Research Fund Grant 2093C. We thank 10. Modification by diagenetic processes was extensive. This in- R. C. Surdam for introducing us to some of the critical outcrops cludes reactions of interstitial brines with carbonates and silicates and W. C. Culbertson for a helpful review and for stratigraphic ad- to produce authigenic minerals. vice. The senior author thanks L. E. Mannion for the opportunity to participate in the 1971 Society of Economic Geologists' field CONCLUSIONS AND IMPLICATIONS conference on Wyoming trona deposits and W. H. Bradley for con- tinued inspiration. Our interpretation of the depositional environment of the Wil- kins Peak Member is based on sedimentary structures and on the APPENDIX 1. SEDIMENTARY FEATURES IN chemical constraints imposed by the presence of a thick sequence of LITHOFACIES OF WILKINS PEAK FORMATION carbonate rocks including trona. A particular rock unit is the pro- duct of a specific combination of hydrologic, sedimentary, chemi- FLAT-PEBBLE CONGLOMERATE FACIES cal, and biologic processes; it is not normally possible or desirable to focus attention upon each kind of process separately. We have Sedimentary Structures. Thin to medium parallel-sided beds 5 to 20 cm, learned, for example, that all of the dolomite we have examined is traceable for more than 30 km along strike. Flat, abraded pebbles (up to 5 of intraclastic-detrital derivation, so that this dolomite or its pre- cm) of dolomitic peloidal mudstone (Fig. 4A), some curled with desiccation cursor must have been precipitated chemically elsewhere in the cracks on the underside (Fig. 5 A), set in a sand-silt "matrix" that commonly predominates. basin, presumably at the outer playa fringes, thus involving chemi- Textures in Thin Section. Pebbles and peloids consist of dolomite mi- cal and hydrologic considerations as well. We have used our ex- crite (1 to 4 /j.) aggregates; micrite of larger grains itself pelleted (Fig. 4B). perience with Holocene playas to interpret observations made on Many grains are "superficial oolite" type, coated with 20pt rim of radial the Wilkins Peak rocks and have achieved a more complete and in- blocky dolomite. Some larger coated grains contain smaller coated grains. tegrated understanding of this particular environment. We have No muddy matrix; quartz cement fills voids. abandoned the concept of a stratified lake and have replaced it with Diagenetic Features. Coated grains are predepositional, as evidenced by the idea of a dynamic playa-lake complex. We are not familiar with lack of meniscus junctions where grains touch, by inclusion of coated grains any individual observations that cannot be explained as well or bet- within pebbles, and by presence of abraded remnants of coatings. Early ter by this new interpretation. postdepositional diagenesis resulted mainly in recrystallization of peloids to dolomite microspar (4 to 20n), commonly seen as patchy mosaic in interior Many questions are unresolved, particularly those connected of peloids or as single rhombs enclosing "ghosts" of rimmed peloids (Fig. with biologic processes. We have not found the areas of primary 4B). Final diagenetic event was emplacement of void-filling quartz cement. chemical precipitation of carbonates that must exist in the mud Depositional Subenvironment. Shallow wave-agitated lake rapidly flats closer to the alluvial fans, and we do not understand the origin transgressing over wide mud flats, leaving a coarse coated-grain intraclast of the calcite found in the lime sandstones and mudstones. Evi- lag. dence for diagenetic processes is widespread, and these processes have profoundly modified the unconsolidated sediment that we im- LIME SANDSTONE FACIES agine to have accumulated in various parts of the playa-lake envi- ronment. Discussion of these processes has been omitted for lack of Sedimentary Structures. Makes prominent ledges 10 cm to 2 m thick, traceable over 35 km. Consists of interbeds of two lithologjes: (1) thin beds, space. 1 to 10 cm of calcite-dolomite sandstone; and (2) very thin beds, 1 to 2 cm From a cursory examination of other Green River rocks and of mud-cracked dolomitic mudstone. Lime sandstone typically ripple from a careful reading of Bradley's papers on the Uinta and cross-laminated, low-angle foresets 5° to 20°, amplitudes 0.5 cm, Piceance Creek basins (Bradley, 1929, 1931), we are convinced wavelengths 5 to 10 cm (Fig. 6B). Scours and starved ripples common (Fig. that the Wilkins Peak model also applies to the other members of 6A). On bedding planes, ripple marks are predominantly straight crested the Green River Formation. We are aware, of course, that oil shales (Fig. 7). Spectacular radiating trona molds typical of sandstones (Fig. 9A). are much more abundant and richer in pyrobitumins in the Tipton, Mudstones as described for Mudstone Facies. Sandstone fills mudcracks in Laney Shale, Garden Gulch, Parachute Creek, and Evacuation mudstones (Fig. 8). Textures in Thin Section. Laminae in sandstones 0.1 to 5 mm, with Creek Members, for instance, indicating wetter periods and thicker laminae coarser and quartz-rich, thinner laminae rich in dolomite perhaps diminished mud flats, but we see no need to invoke a fun- peloids. Most prominent markers are wispy dolomite micrite partings (0.1 damentally different mode for their formation. Wilkins Peak time to 0.2 mm). Particles are 20 to 150;u., principally ovoid to irregular dolo- simply represents the most extreme evaporative condition of the mite peloids (Figs. 9B and 10), subangular quartz grains, and euhedral cal- spectrum with the widest expanses of mud flats and the largest ac- cite rhombs (Fig. 10). Typical composition is 45 percent well-ordered cumulation of saline minerals. Even an overflowing Lake Gosiute stoichiometric dolomite, 30 percent calcite, 25 percent quartz, minor

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amounts of biotite, and feldspar. Mudstone textures as described below for Shortite and calcite casts after shortite are common, especially in central Mudstone Facies. parts of basin. Diagenetic Features. Outer edges of dolomite peloids recrystallized to Textures in Thin Section. Organic laminae 5 to 60 fx, clear with brown dolomite microspar (4 to 20 fx). Difficult to identify cement because sharp selvages; they commonly pinch and swell and bifurcate; carry scattered calcite rhombs commonly penetrate dolomite peloids (Fig. 10). Calcite coarse silt grains of dolomite, quartz, feldspar. Draping of laminae around rhombs probably pseudomorphs after early diagenetic reaction product these grains extensive. Dolomite laminae up to 200 fx, markedly lenticular, such as pirssonite, gaylussite, or shortite. made of microspar 4 to 20¡x. Penecontemporaneous intrasediment trona needles have been dissolved Rare fragments of fish(?) bones. Algal, bacterial, fungal remains and in- and molds partly filled with calcite (Fig. 9B). sects and insect larvae reported by Bradley (1931). Depositional Subenvironment. Very shallow lake shore zone, domi- Diagenetic Features. Compaction of organic layers major feature; drap- nated by wind-wave agitation, oscillating back and forth over playa mud ing of laminae over mineral grains and pebbles indicates organic layers flats. Slow oscillatory transgression. thinned as much as three orders of magnitude by compaction (and chemical reorganization). MUDSTONE FACIES Dolomite entirely 4 to 20\x microspar, but association with detrital quartz silt suggests dolomite may originally have been peloids. Sedimentary Structures. Units range in thickness from 10 cm to 15 m; Some shortite crystals (or their precursors) formed before compaction most are commonly 1 to 3 m. Consists of two types of green-gray dolomitic because organic laminae are draped around crystals both top and bottom mud rocks: (1) laminated, and (2) thin bedded; both extensively mud (see Fahey, 1962, Fig. 14). Other shortite crystals are postcompaction be- cracked. Laminae, 0.1 to 3 mm, discontinuous, with coarser laminae mark- cause sharp edges penetrate laminae. edly lenticular (Figs. 11A and 12); finer laminae more persistent laterally. Depositional Subenvironment. Very shallow fresh to brackish lake, Shallow (3 mm) scours are common and usually filled with fine sand or silt; bottom covered by thick flocculent gelatinous algal-fungal ooze. Occasion- some scours show inclined lamination. Thin beds (1 to 10 cm) show traces ally dried out and mud cracked; polygons gently reworked into small gravel of internal lamination but mainly are completely disrupted with a brec- pockets by next influx of water. ciated appearance due to multiple mud cracking (Fig. 11B). Mud cracking is the major feature. Sinuous V-shaped sediment-filled TRONA-HALITE FACIES cracks range from being a few millimeters deep, 1 mm wide, and 1 cm apart, to 5 cm deep, several millimeters wide, and 15 cm apart (Figs. 8 and Sedimentary Structures. Units up to 11 m thick; confined to southwest 11). Many mud cracks filled with internally laminated sediment and pene- part of basin (see Fig. 2). Consists of three lithologies: (1) trona (some units trated by younger sediment-filled mud cracks (Fig. 11). carry admixture of halite, (2) halite (minor), and (3) dolomitic mudstone Shortite and calcite casts after shortite common in central part of basin. partings between trona-halite beds. Trona occurs as thin to thick beds of Textures in Thin Section. In laminated mudstones, coarser laminae con- "massive" to radiating crystals. Detailed morphology of trona-halite beds sist of silt to fine sand (40 to 120 fx) of quartz, dolomite peloids, biotite, and given by Deardorff and Mannion (1971). feldspar. Calcite in these layers found as larger anhedral aggregates up to Textures in Thin Section. Trona crystals 0.5 mm to 10 cm long. De- 500 fi. Fine-grained laminae consist mainly of dolomite micrite with clotted tailed description given by Deardorff and Mannion (1971). texture, with scattered silt grains of quartz and biotite. Coarse layers have Diagenetic Features. Extensive dissolution and recrystallization typical, sharp lower boundary and may grade upward into micrite layer, but coarse as described by Deardorff and Mannion (1971). layers pinch out laterally while mud layers persist. Depositional Subenviroment. Seasonally(?) dry salt lake (at times sev- Thin-bedded mudstone consists of clotted dolomite micrite with scat- eral isolated salt lakes coexisting). Recharge by ground waters and surface tered silt grains of quartz, feldspar, biotite, and calcite. Some calcite grains flood waters that dissolved trona efflorescent crusts on adjacent mud flats. are enclosed in dolomite microspar rhombs. Many wisps, lenses, and con- torted veinlets of quartz-feldspar silt. Highly brecciated zones appear as SILICICLASTIC SANDSTONE FACIES pockets of dolomite mudstone granule intraclasts in quartz-rich silt matrix. Microdesiccation features in mudstones are horizontal sheet cracks (1 cm Sedimentary Structures. Units up to 10 m thick, traceable in north-south long) and smaller irregular fenestral pores; some of these features are filled direction for 100 km but thin drastically northward (Fig. 15). Essentially with sparry calcite. narrow wedges extending north into basin from Uinta Mountains. Diagenetic Features. Much of dolomite micrite recrystallized to dolo- Trough cross-bedded coarse sandstones fining upward to fine sandstones mite microspar (4 to 20fx) with a scattering of larger dolomite rhombs. with climbing ripples (Fig. 14) and siltstones. Channeling and deep scouring Clotted micrite texture is suggestive of lightly compacted peloids. typical; individual beds not continuous in strike sections but are essentially Shortite crystals appear to have grown within soft sediment as a "late" lenticular channel fills that truncate each other. diagenetic product, perhaps by reaction of dolomite with interstitial brines Textures in Thin Section. Well-sorted quartz-feldspar-biotite sands and at elevated temperature. Calcite fenestral pore filling also "late"(?). silts, very tightly packed so that it is difficult to identify cement. Depositional Subenvironment. Exposed playa mud flats subject to ex- Diagenetic Features. Not studied. tensive and repeated desiccation. Erosion and deposition by sheetwash dur- Depositional Subenvironment. Tongue-like braided stream flood plain. ing infrequent storm flooding. Trona efflorescent crust likely covered Nine discrete episodes only. surface during driest periods. Ephemeral algal mat cover after flooding(?). REFERENCES CITED OIL SHALE FACIES Baker, B. H., 1958, Geology of the Magadi area: Geol. Survey Kenya Rept. Sedimentary Structures. Mostly thin units <1 m but may be up to 3 m. 42, 81 p. Some persistent along strike for 50 km. Bathurst, R.G.C., 1971, Carbonate sediments and their diagenesis: De- Consists of two lithologies: (1) fine parallel-laminated oil shale; brown velopments in sedimentology, Vol. 12: New York, Elsevier Pub. Co., organic laminae alternating with dolomite mud laminae, with individual 620 p. layers <1 mm and may. pinch out over distance of several centimeters; (2) Bradley, W. 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