The lower unit is composed primarily of bioturbated limestones, but grades southward into a well developed lake deposit with laminated micrites containing abundant fossil fish.

The middle unit is composed of kerogen-rich laminated-micrite that has been extensively quarried for its abundant fossil fish. Fossil Lake expanded to its maximum size during middle unit time, however it suffered periods of near total evaporation, as evidenced by beds of dolomicrite. The fossil resources of this deposit are spectacular and unique, but may be threatened by multiplied commercial quarrying activities over the past few years.

The upper unit is composed largely of dolomicrite containing salt casts of sodium carbonates. Fossil fish are absent from the upper unit at Fossil Butte. Fossil Lake freshened only briefly for a short period of time before being totally filled in by the encroaching fluvial systems surrounding the lake.

Much remains to be discovered about this unique lake deposit, and no doubt our view of Fossil Lake will change with those new discoveries.

Introduction

Although we do not have time machines to visit the past,
we can come pretty close with the modern tools of geology and paleontology and with the skills to apply them. It is the unknown and mystery of the past that motivates many of us to pursue our professions as geologists and paleontologists. It is like putting the pieces of a puzzle together... the more pieces that we can fit together, the clearer the picture becomes. The Eocene lake deposits (Fossil Lake) of Fossil Butte National Monument and surrounding areas (known in total as Fossil Basin) provide an opportunity to recreate the puzzle and interpret the past in nearly complete way. Most of the puzzle pieces are there because of continuous deposition without periods of erosion, because of incredible preservation of life rarely seen elsewhere, and because of the way nature has
dissected these ancient lake beds into a suite of canyons and buttes that provide a nearly complete three dimensional view of Eocene Fossil Lake. We will now take a walk through time as we walk up through the type section at Fossil Butte National Monument; we shall imagine standing on the ancient shore of this unique lake, and we shall observe the physical lake environment, as well as the life and death of the myriad of creatures that inhabited the lake and it's surroundings. This paper will serve as an introduction to more detailed papers in this volume about Fossil Butte National Monument that provide in depth views of Fossil Lake in regards to specific topics and time periods.

The Type Section Overview

Stratigraphic Nomenclature.—Figure 1 provides a stratigraphic overview of the Green River Formation at Fossil Butte.

Figure 1—The Green River Formation in Fossil Basin at Fossil Butte is divided into three units as indicated. Note the marker beds to the right of the section that are referred to in the text. XRD mineralogy provides a view of the relative amounts of calcite, dolomite, and total silicate minerals (quartz, feldspar, zeolites, clays). Fossil fish occur in units indicated with solid bars.

The basic lacustrine stratigraphy was first described and divided into the Fossil Butte and Angelo Members by Oriel and Tracey (1970) and later studied in more detail by Buchheim (1994a and 1994b) and Buchheim and Eugster (1998). Buchheim (1994a) informally divided the Fossil Butte and Angelo Members of the Green River Formation up into the lower, middle, and upper units. This was necessary in order to accurately incorporate new knowledge obtained about the stratigraphy including the discovery of a thick sequence of fluvial-lacustrine rocks best developed in the Little Muddy Creek and more southern canyons and described in detail by Biaggi (1989). In addition, Buchheim (1994a) found that Fossil Lake underwent three phases: a shallow freshwater phase (lower unit); a relatively deeper freshwater to saline phase (middle unit) and a mostly hyper saline-alkaline phase (upper unit). The boundaries of these units can be accurately located at most locations and are identified by mapable marker beds.

Previous stratigraphic and paleontologic investigations.—Cope (1877 and 1884). Peale (1879, p. 535) first described the geology and fossil fishes of the Green River Formation. The geologic history with emphasis on paleontology was summarized by McGrew and Casilliano (1975). A detailed description of the paleontology of the entire Green River Formation was provided by Grande (1984). Grande and Buchheim (1994) provided a detailed analysis of lateral variations in paleontology and sedimentology in Fossil Lake. A detailed summary of the Green River Formation of Fossil Basin was completed recently by Buchheim and Eugster (1998).

Recent Investigators who have made detailed descriptions of the sedimentology, stratigraphy, and paleontology of Fossil Lake include: Biaggi (1989), who studied an early phase of Fossil Lake during which the lower unit was deposited; Loewen and Buchheim (1997) who reported on the saline to freshwater transitions in Fossil Lake as revealed in the upper unit; Cushman (1983) who conducted the only study on fossil pollen in the lake beds of Fossil Basin; Leggitt and Buchheim (1997) who reported on a significant mass mortality beds of fossil birds in the southern part of Fossil Basin (see also Leggitt, 1996); Trivino (1996) who completed a detailed spatial analysis of unit 5b, an oil shale bed (kerogen-rich laminated-micrite) and recreated the paleogeography of Fossil Lake during one period of Fossil Lake's history.

A Walk Through Time

Lower Unit time.—At Fossil Butte one is impressed with the bold white cliffs of the Green River Formation that contrast markedly with the underlaying slope-forming and variegated red and purple Wasatch Formation. The Wasatch Formation represents a vegetated, fluvial food plain that was crossed by numerous rivers and streams. If you stand at the base of the contact between these two formations and look south and imagine yourself back in early Eocene time, you would actually be standing on the north shore of a lake that was well developed in the southern part of Fossil Basin. However, the lake was usually shallow and was frequently filled in by fluvial sedimentation. It was surrounded by broad mud-flats that were mud-cracked. These mudflats were frequently
swallowed up by a lake that experienced numerous expansions and contractions. Subsidence in Fossil Basin eventually exceeded sedimentation and the lake deepened and flooded the fluvial plain until it stretched north of Fossil Butte another 30 kilometers.

Fossil Lake never attained depths greater than four or five meters at Fossil Butte during lower unit time. It formed a hyper saline lake at least twice and probably totally desiccated. The two meter-plus thick beds of dolomite are evidence of this. The massive white limestones of this unit are heavily bioturbated as indicated by abundant trace fossils with meniscus fillings indicating sediment-feeding organisms, such as insect larvae, worms, or shrimp.

As you near the top of the lower unit you observe a 3-4 meter thick sequence of alternating siliciclastic mudstones and laminated micrites. Close study of the micrites reveal that they are composed of alternating laminae of micrite and clay. This unit grades southward into the sandstone tongue of the Wasatch Formation, that provides a prominent marker bed throughout the southern half of Fossil Basin. This sandstone has been interpreted as a prograding delta (Peterson, 1987). At Fossil Butte, the mudstones of this unit represent a pro-delta deposit. Only a few kilometers north of Fossil Butte, Fossil Lake was replaced by a fluvial flood plain during this time.

Lower unit time ended with a hypersaline lake where dolomite precipitated from a shallow body of water and was clear and free of siliciclastic input. The lake again freshened for a short period of time before drying up again in one last gasp before the onset of a dramatic change marking the beginning of middle unit time.

Cushman (this volume) concludes that the climate during lower unit time was warm temperate as indicated by the palynofloral assemblage.

Middle unit time.—Middle unit time was a time of fascinating events in Fossil Basin. It began with a loud roar as the climate must have changed abruptly, transcending to a wet-humid period of high rainfall. The lower oil shale at the base of the middle unit was deposited as a result of the most expansive phase of Fossil Lake's history. A mudstone unit (coaly mudstone) rich in coalified plant material with a 2-3 cm thick coal at the top can be traced throughout the basin. There are no roots associated. A thin oil shale known as the lower oil shale (kerogen-rich laminated-micrite) about 30 cm thick overlies the coal bed. It contains small Knightia in abundant numbers along with fossil insects and plant fragments. This sequence is interpreted as being deposited in a lake undergoing a rapid transgression. The transgression appears to have been initiated abruptly during a single episodic event that transported huge amounts of plant material into the lake, resulting in the thin coal deposit. The high organic-carbon content and dense fossil fish concentrations are consistent with a lake in it's early stages of expansion. Nutrient levels are high and are inherited from the rich soils recently flooded. The lower oil shale can be traced throughout Fossil Basin wherever the middle unit outcrops and provides an excellent marker bed to the experienced stratigrapher.

Fossil Lake remained very productive throughout middle unit time resulting in the deposition of the fish beds that the Green River Formation is so famous for. One of these beds, known as the lower sandwich bed, has been studied intensively throughout Fossil Basin at over 70 locations (Trivino, 1996). The two tuff beds delineating the top and bottom of this unit allow precise stratigraphic correlations and spatial analysis studies. Studies conducted of bed thickness, laminae number, stable isotopes, mineralogy, and paleontology have provided a detailed recreation of the paleogeography of Fossil Lake during this time. Most of the inflow areas were concentrated in the southern half of the lake; however a significant inflow point was present in the northeastern part of the lake as is indicated by a dramatic thickening of laminae in the vicinity. The laminae thickening is thought to be a result of a high rate of calcite precipitation in this area. In addition, a fluvial channel filled with conglomerate replaces most of the middle unit a few kilometers east of the thick laminae anomaly. Buchheim and Biaggi (1988) and Buchheim (1994b) concluded that the laminae of this unit are not true varves because laminae number between the two tuff beds increases from about 1100 to 1600 from lake center to margin. Deposition of a greater number of laminae near the lake-margins occurred where calcium-rich inflow water first mixed with the bicarbonate-rich (alkaline) lake water (Buchheim, 1994a).

Similar results were obtained from a paleogeographic study (Buchheim, 1993) of the "18-inch layer". This unit has been extensively excavated for it's well-preserved fossil fish fauna since the late 1800's. Grande and Buchheim (1994) provide a detailed analysis of the lateral variation of paleontological and sedimentological characteristics of this unit. The 18 inch layer (as a kerogen-rich laminated micrite containing abundant fossil fishes) is spatially limited to only about a hundred kilometer area from about the Fossil Butte National Monument Visitor's center on the north to about 10 kilometers south; and from about the middle of R119W to the middle of R118W. About 60% of this area has been eroded out, leaving only about 40 square kilometers of area where this incredible paleontologic resource is still preserved. The 18 inch layer only crops out over a linear distance of about 30 kilometers. Of this only about 10 kilometers is accessible to major paleontologic excavation activities, because of steep slopes over most of it that require excessive overburden removal before collection can begin. These are rough estimates, but it is clear that the preservation of this unique paleontological heritage is at risk. Of the 10 accessible kilometers, up to half may be under private or state lease to commercial quarry operators! The so called "split fish" layers at more marginal geographical localities is not within the 18 inch layer, but within the lower sandwich beds. The sandwich beds extend over a 40 by 20 kilometer square area centered around location 1306, about 6 kilometers south of Fossil Butte National Monument.

The glory period of Fossil Lake finally faded with the temporary onset of arrid conditions leading to nearly total dessication of the lake during middle dolomicrite bed time. Fossil Lake during this period was clear with little siliciclastic input. Although this bed is dolomicrite at the center of the
basin, it grades laterally into calcimicrite. This observation suggests that fresher conditions exited at the margins of the lake, a trend that dominates the lake system throughout it's history. This relationship suggests that the dolomite was precipitated either directly from the water column or formed syndepositionally (Buchheim, 1994a), rather than generated on marginal mudflats. The ostracodal dolomicrite is a prominent marker bed in the southern half of the basin, where it forms a hard, dense and blocky unit that leaves "bricks" of gray limestone scattered over an otherwise soft weathered slope.

Fossil Lake did not experience a major expansion again until k-spar tuff time (from the top of the ostracodal dolostone to one meter above the k-spar tuff: a major marker bed that is composed of authigenic potassium feldspar), but remained brackish to saline. Abundant fish fossils are presently being quarried from this unit at some the basin center quarries. Along with fossil fish occur abundant Goniobasis gastropods. The significance of this relationship is not well understood because gastropods generally signal very shallow water and well oxygenated conditions (Surdam and Stanley, 1979). Shallow water conditions are indicated by the nearly basin-wide distribution of kerogen-poor laminated-micrite, a facies deposited in shallower near-shore water (Buchheim, 1994b) along with the occurrence of gastropods and abundant burrow traces. The potassium feldspar mineralogy of the k-spar tuff suggests hyper saline conditions during this period. Authigenic feldspar is produced by the reaction of hypersaline-alkaline water (rich in potassium) with volcanic ash (Sheppard and Gude, 1968, 1969; Surdam and Stanley, 1979). However, the relationship of feldspar and zeolite mineralogy to salinity is not as clean as expected. Although authigenic potassium feldspar tuff beds (rather than zeolite or clay beds) nearly always occur in dolomite sequences, they occasionally occur in calcitic sequences as well (Buchheim ,1998). The calcite mineralogy of the laminated micrite, occurrence of gastropods, and abundant fossil fishes suggests the lake was fresh to saline, but not hyper saline.

Cushman (this volume) suggests that the abundance of hardwood, riparian and conifer taxa provide a picture of moist lowlands and flood plains around Fossil Lake with upland forests on the surrounding ridges and mountains. Pine and other upland taxa grew in the highlands surrounded the lake. The palynofloral assemblages of the lower and middle units and the lower part of the upper unit indicate that a mixed mesophytic forest grew near Fossil Lake. The climate during middle unit time was probably more subtropical than during lower unit time.

Upper Unit Time.—Fossil Lake developed into a large, but shallow, hyper saline lake during upper unit time, generally devoid of fossils in central basin areas (Buchheim, 1994a). Salt casts of sodium carbonate minerals are abundant beginning about four meters above the k-spar tuff, indicating hypersalinity. A series of 3-4 oil shales (kerogen-rich laminated-micrite) within the lower half of the upper unit provide marker beds that can be traced throughout the basin. The lenticular nature of the laminae suggest these were deposited in shallow water.
<
Did fish entirely disappear from Fossil Lake during upper unit time? Loewen and Buchheim (1997) recently reported the first occurrence of fossil fish including very abundant numbers of Priscacara hypsacantha (perch) and Lepisosteus (gar) in the upper unit at a near-shore locality south of Elk Mountain in the southwest part of Fossil Butte (see Loewen and Buchheim, this volume, for further details). Their preliminary study indicated that a relatively fresher-water apron existed on the margins of Fossil Lake. In addition a lateral trend (lake-center to margin) in mineralogy (dolomite to calcite), oxygen stable isotopes (heavy to light), petrology (kerogen-rich laminated-micrite, highly disrupted by salt casts to laminated calcimicrite to bioturbated calcimicrite) all provide collaborating evidence of a salinity gradient in Fossil Lake during upper unit time.

About 16 meters above the k-spar tuff in the upper unit a four meter thick sequence of chert nodules and bedded chert occurs in a dolomicrite (chert horizon). The chert horizon can be correlated throughout Fossil Basin and provides an excellent marker unit. Lake water pH probably exceeded 10 during this time driving most silicates into solution. The dissolved silica precipitated out as silica gel and eventually converted to chert nodules or bedded chert (Buchheim, 1994b).

Abundant fossil birds of the genus Presbyornis frequented the shorelines of Fossil Lake during upper unit time (Leggitt and Buchheim, 1997), as indicated by a mass mortality bird bed that occurs in the vicinity of Warfield Creek about 40 meters above the k-spar tuff. Egg shell fragments, gastropods and ostracods are commonly associated. The reader is referred to Leggitt and Buchheim (this volume) for further details.

Volcanism increased significantly during upper unit time as indicated by a higher percentage of silicate minerals in the carbonate rocks (see XRD mineralogy in Figure 1). Tuff beds tend to be thicker as well. The k-spar tuff, that marks the lower boundary of the upper unit, averages 13 cm and increases to a maximum of 25 cm in the northwest corner of the basin at the head of Watercress Canyon. Most of the tuff beds in the underlying units are thinner (1-3 cm) and the carbonates contain generally less than 10% silicate minerals. The local thickening of the k-spar tuff suggests that a local volcanic sources existed nearby. If the source was hundreds of kilometers away, the ash would be more evenly distributed across the basin.

Pollen collected and studied by Cushman (this volume) suggest that the climate gradually cooled during upper unit time, but may have returned to a warmer, subtropical climate during its final phase. This conclusion is supportedg by the occurrence of Reevesia (a tropical to subtropical element) at the top of the upper unit (Cushman, this volume) and is corroborated by the presence of about a meter of laminated micrite containing fossil fish. This essentially marked the end of Fossil Lake's history. Subsidence within the basin apparently slowed and the fluvial Wasatch Formation the lake. A few limestones in the Wasatch Formation above the upper unit indicate that a few short-lived ponds or less extensive shallow lakes attempted a come back.

We have now completed our walk through time. Our view is still somewhat hazy, but as research continues in Fossil Basin it will sharpen and maybe even change. New discoveries await the motivated researcher; new fossils, new views of processes occurring in Fossil Lake. This walk will be made again... and with each walk we will see Fossil Lake with a clearer perspective!

References

Biaggi, R. E. 1989. Paleogeography and paleoenvironments of the lower unit, Fossil Butte Member, Eocene Green River Formation, Southwestern Wyoming. Unpublished M.S. thesis, Loma Linda University, 134 p.

Buchheim, H. P. 1993. Final report: Paleo-historical fluctuations in paleogeography, depositional environment, and chemistry of Eocene Fossil Lake. National Park Service, Fossil Butte National Monument. 116 p.

———. 1994a. Eocene Fossil Lake: a history of fluctuating salinity, p. 239-247. In R. Renaut, and W. Last (eds.), Sedimentology and geochemistry of modern and ancient saline lakes. SEPM Special Publication, 50.

———. 1994b. Paleoenvironments, lithofacies and varves of the Fossil Butte member of the Eocene Green River Formation, Southwestern Wyoming. Contributions to Geology, 30(1):3-14.

———. And R. Biaggi. 1988. Laminae counts within a synchronous oil shale unit: a challenge to the "varve" concept (abs.): GSA Abstracts with Programs, v. 20(7), p. A317.

———. And H. P. Eugster. 1998. Eocene Fossil Lake: The Green River Formation of Fossil Basin, Southwestern Wyoming, p. 1-17. In J. Pittman, and A. Carrol (eds.), Modern and Ancient Lacustrine Depositional Systems. Utah Geological Association Guidebook 26.

Cope, E. D. 1884. The Vertebrata of the Tertiary Formations of the West: U.S. Geological and Geographical Survey of the Territories. Annual Report no. 3, 1009 p.

———. 1877. A contribution to the knowledge of the ichthyological fauna of the Green River Shales. U.S. Geological and Geographical Survey Bulletin, 3: 807-819.

Cushman, R. A. 1983. Palynology and paleoecology of the Fossil Butte member of the Eocene Green River Formation in Fossil Basin, Lincoln County, Wyoming. Unpublished M.S. thesis, Loma Linda University, California, 88 p.

Grande, L. 1984. Paleontology of the Green River Formation, with a review of the fish fauna. Second edition. The Geological Survey of Wyoming Bulletin, 63, 333 p.

———, and H. P. Buchheim. 1994. Paleontological and sedimentological variation in early Eocene Fossil Lake. Contributions to Geology, 30(1):33-56.

Leggitt, V. L. 1996. An avian botulism epizootic affecting a nesting site population of Presbyornis on a carbonate mudflat shoreline of Eocene Fossil Lake. Unpublished M.S. thesis, Loma Linda University, 114 p.

———, and H. P. Buchheim. 1997. Presbyornis (Aves: Anseriformes) eggshell from three avian mass mortality sites: Eocene Fossil Lake, Lincoln County, Wyoming. Journal of Vertebrate Paleontology, Abstracts of Papers, 17 (supplement to Number 3):60A.

Loewen, M., and H. P. Buchheim. 1997. Freshwater fish in a hypersaline lake: evidences of a salinity gradient in Eocene Fossil Lake (Green River Formation). Geological Society of America, Abstract with Programs, 29(6)

Mcgrew, P. O. , and M. Casilliano. 1975. The Geological History of Fossil Butte National Monument and Fossil Basin: National Park Service Occasional Paper Number 3. National Park Service Occasional Paper Number 3, 37 p.

Oriel, S. S., and J. I. Tracey, Jr. 1970. Uppermost Cretaceous and Tertiary stratigraphy of Fossil Basin, southwestern Wyoming: USGS Professional Paper 635, 53 p.

Peale, A. C. (ed.). 1879. Report on the geology of the Green River district. 11th Annual Report, U.S. Geological Survey of the Territories, 509-646 p.

Petersen, F. S. 1987. Lacustrine deltaic deposits of the sandstone tongue of the Wasatch Formation, Fossil Basin, Wyoming. M.S., Thesis, Loma Linda University, 217 p.

Sheppard, R. A., and A. J. Gude, 3rd. 1968. Distribution and Genesis of Authigenic Silicate Minerals in Tuffs of Pleistocene Lake Tecopa, Inyo County, California. United States Geological Survey Professional Paper 157: 36 p.

Surdam, R. C., and K. O. Stanley. 1979. Lacustrine sedimentation during the culminating phase of Eocene Lake Gosiute, Wyoming (Green River Formation). GSA Bulletin 90:93-110.

Trivino, E. 1996. Freshwater inflow and carbonate depositional patterns in Eocene Fossil Lake. Unpublished M.S. thesis, Loma Linda University.