PALEONTOLOGY AND PALEOECOLOGY OF THE CULMINATING PHASE OF EOCENE FOSSIL LAKE, FOSSIL BUTTE NATIONAL MONUMENT, WYOMING
MARK A. LOEWEN and H. PAUL BUCHHEIM
Geology Section, Department of Natural Sciences, Loma Linda University
Loma Linda, CA 92350
Dolomite and evaporites characterize the upper unit. Desiccation cracks, flat pebble conglomerates and sediments disrupted by evaporites are common, suggesting an arid depositional environment. Sharp bottom contacts of lithologic beds suggest rapid changes in lake levels.
The flora includes prokaryotic algae, stromatolites, algal tufa and vascular plants. Angiosperms are relatively scarce, and the palynoflora is dominated by gymnosperms. The fauna and other organisms include bacteria, protists, invertebrates, fish, birds and reptiles. Fish communities of the upper unit are restricted to facies deposited under relatively fresh conditions, and include Priscacara liops, P. hypsacantha, Lepisosteus, Knightia, Diplomystus and Asineops squamifrons. The shorebird Presbyornis is present in marginal facies.
The latter stages of Fossil Lake exhibit different communities than those present during most of Fossil Lake's history. Stromatolites and tufa become common. The tiny fish Priscacara hypsacantha dominates fish populations, while the major predator was Lepisosteus. These fish adjusted to increasing alkalinity and salinity by retreating to nearshore freshwater environments. Cooler gymnosperm-dominated floras replaced subtropical angiosperm floras.
The flora and faunas of the upper unit represent a unique paleocommunity that represents a response to the changing conditions of upper unit time. These organisms dealt with increasing alkalinity, salinity and fluctuating lake levels.
The purpose of this paper is to present paleontologic data recently collected in Fossil Basin. It will also review the information currently available about the paleontology and paleoecology of the upper part of the Green River Formation in Fossil Basin.
Fossil Butte National Monument lies in the structural
basin formed at the end of the Mesozoic by the ancestral
Wyoming Thrust Belt, Wasatch and Uinta Mountains. This
intermountain basin exhibited distinct depositional and
histories from those of contemporaneous deposits in nearby basins. For a complete review of the structural history and stratigraphic relationships of Fossil Basin, see Lamerson (1982), McGrew and Casilliano (1975) or Oriel and Tracey (1970).
The Wasatch and Green River formations represent the alluvial, fluvial and lacustrine sediments deposited in Fossil Basin during the Eocene. The Wasatch Formation underlies, interfingers with, grades into and overlies the Green River Formation (Figure 1). Oriel and Tracey (1970) divided the Green River Formation in Fossil Basin into the Fossil Butte and Angelo Members. The mudstone tongue of the Wasatch Formation divides the two members.
Buchheim (1994a) informally divided the Green River Formation of Fossil Basin into lower, middle and upper units based on lithologic characteristics. The upper unit is equivalent to the uppermost part of Oriel and Tracey's Fossil Butte Member and the Angelo Member (Figure 1). Its bottom contact is the top of the 8 cm K-spar tuff above the fish bearing layers of the middle unit. The middle and upper units are best represented at the type locality of the Fossil Butte Member on the southeast face of Fossil Butte National Monument.
Siliciclastics, bioturbated micrites and kerogen-poor calcimicrites dominate the lower unit, and kerogen-rich laminated micrites characterize the middle unit (Buchheim and Eugster, 1998). The famous fish deposits such as the "18 inch" and "split fish" layers are located in the middle unit.
The upper unit is characterized by dolomicrites and evaporite minerals (Buchheim 1994b). Several kerogen-rich laminated dolomicrites form the prominent "oil shales" found throughout the basin. Desiccation cracks, flat pebble conglomerates and sediments disrupted by evaporites are common.
Fossil Basin has been the focus of many excellent paleontologic studies. Bradley (1929), Grande (1984), McGrew and Casilliano (1975) and Oriel and Tracey (1970) have studied the flora and fauna of Fossil Basin in detail among others. MacGinitie (1969), Cushman (1983) and Cushman et al. (1984) studied the flora of Fossil Basin. The fish of Fossil Basin have been studied in detail by Cope (1874, 1877, 1879, 1884, others); Grande (1979, 1982a, 1982b, 1984, 1985, 1994); Grande and Bemis (1991); Grande and Buchheim (1994); Quo-Qing, Grande and Wilson (1997); Quo-Qing, Wilson and Grande (1997) and Wiley (1976). Ferber and Wells (1995) and McGrew (1975) studied fish taphonomy. Most of these studies have focused on the famous fossil fish bearing layers of the middle unit.
Upper unit studies include Cushman (1983), who
studied the palynoflora, the study of
Presbyornis by Leggitt (1996) and Leggitt and Buchheim (1996, 1997) and
the paleoenvironmental study of the maroon oil shale by
Loewen and Buchheim (1997).
Ambrose et al. (1997) placed the Wasatch Formation underlying the Fossil Butte Member in the Lostcabinian subage of the Wasatchian (NALMA, Wa7). Froehlich and Breithaupt (1997) reported a specimen of Lambdotherium that dates the split fish layers of the Fossil Butte Member as Lostcabinian. The upper unit is Lostcabinian in age or younger.
Sedimentological and stratigraphic data suggest that
Fossil Lake existed in a closed basin with a low topographic
gradient during most of upper unit time (Buchheim, 1994a,
1994b; Loewen and Buchheim, 1997). Saline deposits dominate
the upper unit. They represent a closure of the hydrographic
basin, in contrast to the open basin of middle unit time.
closed nature of the basin produced lake sediments that were dynamically influenced by inflow. When precipitation significantly exceeded evaporation, freshwater sediments were deposited. During times of drought, evaporites were deposited. Rapid transgressions are represented by flat pebble conglomerates of ripped up mud-cracks grading into kerogen-rich purple oil shales with sharp bottom contacts and finally evaporite sequences. Mud-cracks, strandlines of algal encrusted tufa logs, evaporite minerals and Magadi-type chert were deposited during periods of desiccation. Evaporites and hypersaline layers directly overlie relatively fresher deposits.
Although the lake experienced periods of hypersalinity, lateral gradients from relatively fresh at the margin to hypersaline in the center existed in Fossil Lake (Buchheim, 1994a; Loewen and Buchheim, 1997). Facies in these salinity gradient horizons change from partly bioturbated calcimicrite to kerogen rich dolomicrite with evaporites in the center on the lake. The low topographic nature of the basin may have allowed this gradient to exist, similar to the salinity gradient in Bear River Bay where it joins the Great Salt Lake (Buchheim, 1994a).
Marker beds in the upper unit are correlatable basin wide (Figure 2). The maroon oil shale exhibits a lateral change from laminated calcimicrite with fish in marginal facies to dolomicrite with salt casts at the center of the basin. Kerogen-rich oil shales with sharp bottom contacts directly overlie mudstone beds. Mud-cracks, strandlines of algal encrusted tufa logs, evaporite minerals and Magadi-type chert occur at several horizons in the upper unit.
The two major paleoenvironments recognized in Fossil Lake are littoral and limnetic. Siliciclastics, bioturbated facies, algal growth, invertebrate and vertebrate fossils characterize littoral paleoenvironments of the upper unit. Most of the limnetic paleoenvironments of the upper unit lack fossils and are dominated by dolomite and evaporites.
The flora and fauna f the upper stages of Fossil Lake represent communities different than those present throughout the earlier history of Fossil Lake. In general, the communities of the Fossil Basin upper unit reflect a more arid, cooler, less hospitable climate. See Table 1 for a review of known taxa from the upper unit.
Monera, stromatolites, and tufa.Single celled prokaryotes, consisting of bacteria and cyanobacteria, are represented by stromatolites, tufa (Figure 3), and the amorphous kerogen of its numerous oil shales.
Cushman (1983) reports that the kerogen present in the lower and upper purple oil shales is made up of over 90% amorphous kerogen derived from non-structured algal debris. The remaining fraction consists of vitrinite derived from woody plant tissue.
Although there is no direct evidence of bacteria from
the upper unit, Leggitt and Buchheim (1996) suggest
Clostridium botulinum as a cause of death for the shorebird
Presbyornis. Indirect evidence of cyanobacteria is prevalent in the
numerous stromatolites and tufa of the upper unit.
Figure 2Detailed section of the upper unit at Fossil Butte National Monument and lithofacies cross section across the basin. Fossils occur in layers specified or equivalent marginal facies. (one cm K-spar tuff in the maroon oil shale is used as stratigraphic datum.
Table 1Fossil Basin taxa from the latter stages of Fossil Lake.
The upper unit contains several horizons of tufa
encrusted logs. Tufa encrusted logs are porous calcite structures
that preserve faint remnant laminations and molds of the wood
on which they initially grew (Figure 3). They coated
everything from entire logs over a meter long to tiny twigs.
Figure 3A, Tufa encrusted branch. B, Surface texture of stromatolite growing on a tufa encrusted log. C, Stromatolite with rippled surface morphology growing on a tufa log. D, Bark impression on the reverse side of tufa encrusted log.
Interpreted as algal in origin, these layers of tufa represent biogenic calcite deposited on sticks and logs that littered the paleoshorelines of Fossil Lake. Several horizons of tufa logs are several kilometers wide, suggesting low gradient surfaces in Fossil Basin over which the shoreline transgressed and regressed.
Stromatolites occur in three horizons within the
upper unit. These are restricted to marginal environments
around the periphery of the basin. These stromatolites exhibit
fine lamination and represent growth in relatively shallow
lacustrine environments. Both stromatolites and stromatolitic
deposits, absent in the lower and middle units of the Fossil Butte Member, are relatively common in the upper unit.
Protists.The dinoflagellates and acritarchs of the upper unit include Micrhystridium and Baltispheridium (Cushman, 1983). These occur in the lower purple oil shale.
Plants.The flora of the upper unit is poorly known because no detailed paleobotanical studies have focused specifically on the penultimate history of Fossil Lake. Most of our knowledge of the upper unit flora is based on the palynological study of Cushman (1983). He found that the palynoflora of three upper unit oil shales is different from the palynoflora of the middle and lower units. The palynoflora of the middle unit is dominated by subtropical vegetation, suggesting a relatively warm and moist climate. The upper unit has the least diverse palynoflora preserved in the Fossil Butte Member, reflecting a warm temperate ecosystem dominated by spruce, fir, pine, and other gymnosperms. Angiosperms occur in the upper unit, but they are a relatively small part of the flora (Cushman, 1983).
Fern spores from the upper unit make up a tiny fraction of the total palynoflora. They include Laevigatosporites, Cyathidites and Deltoidospora (Cushman, 1983). These occur in the lower and upper purple oil shales and the oil shale directly above the K-spar tuff.
Indicators of a cooler climate such as Abies, Castanea, Picea, Pterocarya and Tsuga are more prevalent in the upper unit. The presence of Ephedra in the basin suggests periodic droughts, because of its leaves with a thick, waxy cuticle that prevent desiccation in times of drought (McGrew and Casilliano, 1975).
Plant macrofossils from the upper unit include deciduous leaf fragments and wood. The blue oil shale contains Sabalites sp. palm fronds and Ailanthus sp. (Tree of Heaven) seedpods Figure 4c).
Cushman (1983) found a variety of microfossils and palynomorphs in the upper unit. Gymnosperm pollen (over 70% relative abundance) dominate the palynoflora of the upper unit and angiosperm pollen occur less frequently than in the lower and middle units (near 15% relative abundance). This gymnosperm to angiosperm ratio of 7:3 is distinctly different than the ratios of the lower and middle units (3:7 for the lower unit and 1:9 in the middle unit).
Invertebrates.The invertebrates of the upper unit include mollusks, arthropods and crustaceans. Oriel and Tracey (1970) reported the gastropods Biommphalaria pseudoammonius, Plesielliptro sp., Physa pleromatis, Elimia nodulifera, and Bellamya paludinaeformis from the upper unit. The gastropod Oreoconus occurs just below the maroon oil shale in the southern part of the basin (Figure 4b). Leggitt (1996) found the freshwater gastropods Goniobasis, Physa, and Gyraulus in marginal facies of the upper unit in the southern part of the basin. Gastropods are restricted to marginal facies in the upper unit.
Ostracods are tiny crustaceans enclosed in a bivalve shell. They are interpreted as indicators of marginal or shallow facies (Bradley, 1926). The ostracod genera Hemicyprinotus, Procyprois and Pseudocypris occur in the upper unit (Oriel and Tracey, 1970). Ostracods occur in several horizons near the margins of the lake, and some horizons extend far out into the basin. Both larval and adult insects also occur in upper unit sediments.
Fishes.Unlike the famous fish bearing deposits of
the middle unit, fish are relatively scarce in the upper unit.
Buchheim (1994a) reported Knightia from some of the
freshwater shales near the top of the upper unit. Recent studies
of the maroon oil shale demonstrated that, at times, large
populations of fish lived along the fringes of Fossil Lake
(Loewen and Buchheim, 1997). The blue oil shale of the upper
contains fossil fish at many locations throughout the basin. Other layers have also produced a few fish fossils.
Immediately above the K-spar tuff is a laminated calcimicrite horizon that contains Knightia, Diplomystus and Priscacara. It was deposited in shallow, freshwater lake conditions similar to those of the middle unit. The K-spar tuff does not represent a depositional hiatus. Therefore, the fish bearing bed above it represents a continuation of the sedimentation patterns of the fossiliferous middle unit.
The blue oil shale (about 2 meters above the K-spar tuff) contains fish, plant material, insects and feathers. This kerogen-rich, finely laminated calcimicrite, exhibits fossil preservation different from any other in the basin. The partial outlines of the body and bones are preserved as dark films on the surface of the limestone (Figure 4f).
The maroon oil shale exhibits a distinct fish fauna near the southwestern shores of Fossil Lake. Priscacara hypsacantha (Figure 4e) make up the majority of the fish fauna, while Lepisosteus sp. and Asineops squamifrons (Figure 4d) are minor components. Priscacara hypsacantha were first recognized by Cope (1886). Grande (1984) refers to these fish as Percoid genus A (since a revision of the family Priscacaridae has not been made to date, we will refer to this fish as P. hypsacantha). A single Priscacara liops and Lepisosteus (gar) have been found in marginal facies of the maroon oil shale. Fry and juvenile P. hypsacantha are relatively common near the margin (Figure 4a).
Near the top of the upper unit, just above the chert horizon, two small limestone units contain Knightia (Buchheim, 1994a). They represent brief freshwater lacustrine intervals within dominantly alluvial-fluvial conditions.
Birds.Bird tracks occur in marginal facies of the upper unit at the southern end of the basin. Leggitt (1996) reported a mass mortality of the shorebird-duck mosaic Presbyornis in the upper part of the upper unit. These are the same birds described by Olson (1994) as Presbyornis pervetus. Disarticulated bird bones, eggshell and gastropods form strand line deposits along the southern end of the basin (Leggitt and Buchheim, 1997). Presbyornis bones and eggshell occur in at least three horizons in the upper unit.
Tetrapods.Crocodile remains and large coprolites occur in marginal facies of the "maroon oil shale." The soft-shell turtle Trionyx sp. occurs in marginal facies just above the K-spar tuff. Fragments of turtle shell, crocodile teeth and mammal bones occur in sandstones that interfinger with the upper unit at the southern end of the basin.
The deposits at the bottom of the upper unit (from the K-spar tuff through the blue oil shale) exhibit similar paleocommunities to those of the middle unit. These communities are consistent with deposition in a freshwater lake, and probably represent a continuation of the conditions present during middle unit time.
The prevalence of stromatolites and tufa, often associated with saline-alkaline lakes in physiographically low-gradient basins, reflects a change in the lake conditions from those of lower and middle unit time.
Figure 4A, twelve mm long juvenile Priscacara hypsacantha from maroon oil shale. B, Oreoconus sp. from below the maroon oil shale. C, Ailanthus sp. (Tree of Heaven) seed pod from the lower blue oil shale. D, Tail of Asineops squamifrons from maroon oil shale. E, Priscacara hypsacantha from maroon oil shale. F, "Ghost" outline of Knightia from the lower blue oil shale.
The palynology suggests that the climate during upper unit time became warm temperate, in contrast to the more subtropical climate of lower and middle unit time. Indicators of a cooler climate are more prevalent in the upper unit than in the middle and lower units. Unlike the lush tropical angiosperm flora of the middle unit, the upper unit reflects a warm temperate paleocommunity of conifers. These gymnosperms probably lived on the uplands and mountains surrounding Fossil Lake. Angiosperms and ferns may have lived at the base of these mountains, but they are drastically reduced in relative abundance. McGrew and Casilliano (1975) suggested that the upland flora of gymnosperms existed at elevations of 6000 _ 8000 feet above sea level.
As hydrologic conditions changed from open to closed, the salinity and alkalinity of the lake increased. Small communities of fish living on the fringes of the lake replaced the diverse freshwater fish faunas of the lower and middle units. Fish faunas are less diverse than those of the lower and middle units. Previously rare fish are the major constituent of the fauna. Priscacara hypsacantha may have been able to survive salinity fluctuations better than other species. While the affinities of Priscacara are poorly known, gar are known to occur in fresh to brackish water. They are considered secondary freshwater fish and cannot survive in saline environments. All fish occurring in the upper unit are restricted to freshwater facies.
Grande (1984) states that all of the aquatic mollusks of the Green River Formation indicate freshwater conditions. Gastropods are uncommon in the upper unit and occur only in marginal facies consistent with freshwater deposition.
The paleontologic data allow reconstructions of trophic relationships of the maroon salt cast layer. Ostracods are detritivores, which feed on decaying organic matter from plants and animals. Algal blooms resulted in high fractions of organic material in the sediment. The presence of P. hypsacantha associated with abundant ostracods suggests that P. hypsacantha may have preyed on these tiny arthropods. Asineops squamifrons and gar may have preyed on the large P. hypsacantha population. Crocodiles were the top carnivores in the ecosystem.
Communities of the wading shorebird lived and died along the southern end of the lake. Presbyornis eggshell material suggests possible nesting sites. The presence of Presbyornis from the basal Member of the Wasatch Formation, and evidence in several layers of the upper unit, suggest that Presbyornis inhabited the region for a long time. Presbyornis may imply saline conditions in the lake. The frontal and nasal bones of Presbyornis have depressions for very large salt glands (Feduccia, 1980). This suggests that the birds had an affinity to more saline waters.
Sedimentary structures, mineralogy, evaporites and the paleontology of the upper unit all suggest that the flora and fauna of the basin endured periods of drought. The picture emerges of a community dramatically influenced by climatic changes. These fluctuations between periods of drought and more moist climates may have led to the final infilling of Fossil Lake. The relatively cool arid interval of the upper unit ends with a return to freshwater deposits at the top of the upper unit. These lake deposits were ultimately drowned by sedimentation from the ancestral Wasatch and Uinta Mountains.
Grants from the National Park Service and Loma Linda University significantly contributed to the success of the research that provided the background database for this paper. Both the logistical support and encouragement provided by the personnel of Fossil Butte National Monument helped to make this work successful. In particular we would like to thank David McGinnis, Peter Ambrose, Rachel Benton (now at Badlands National Monument), Vince Santucci, Arvid Aase and the rangers and staff of Fossil Butte National Monument. The excellent taphonomic study of Leggitt and the palynological study of Cushman provided valuable information about upper unit paleontology. Field assistants who assisted with aspects of this research (in chronological order of the year of contribution) include Hans and Ivan Buchheim, Aimee Wyrick, Meredith Church and John Loewen. Quarry operators who allowed significant access to their excavation sites for sample collection include Dick Dayvault, Rick Hebdon, Dennis Kingery, Tom Lindgren, Jimmy Tynsky and Pete Severns. Other quarry operators too numerous to list here also allowed us to visit their sites. Permission to conduct research on private ranch land was provided by Richard and Roland Lewis; Truman, Don and Josephine Julian; George Cooper; the Gunthers; and Don Failoni. The Bureau of Land Management provided collection permits for public lands.
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