PALYNOSTRATIGRAPHY AND AGE OF THE GREEN RIVER FORMATION IN FOSSIL BASIN, WYOMING

Robert A. Cushman, Jr.

Geology Section, Department of Natural Sciences, Loma Linda University
Loma Linda, California 92350, bcushman@ccmail.llu.edu


Abstract—The palynoflora of the Green River Formation in Fossil Basin, Wyoming, provides a method for estimating the age of the formation. Outcrop samples were collected and processed for plant microfossils from three measured sections representing the center and marginal areas of Fossil Lake.

A reported occurrence of Lambdotherium in the middle unit suggests a Lostcabinian (Ypresian) age for most of the Green River Formation in Fossil Basin. A potassium-argon age determination of 50.2 + 1.9 Ma on a potassium-feldspar tuff near the top of the middle unit of the Fossil Butte Member suggests a late early to early middle Eocene age. A late early Eocene (Ypresian) to early middle Eocene (Lutetian) age range for the Fossil Butte palynoflora is suggested by the co-occurrence of Bombacacidites, Eucommia, Pistillipollenites mcgregorii, Platycarya platycaryoides, and Momipites triradiatus. Consequently, the palynoflora does not rule out the possibility that the upper part of the Green River Formation in Fossil Basin may be early middle Eocene (early Lutetian or Bridgerian) in age.

The late early to early middle Eocene age of the Green River Formation in Fossil Basin correlates with deposition of the Wilkins Peak Member of the Green River Formation in the Green River Basin, Wyoming.



Introduction

The purpose of this study was to use plant microfossils to estimate the age of the lacustrine Eocene Green River Formation in Fossil Basin, Wyoming. Fossil Lake was one of three major Eocene lakes whose sediments form the Green River Formation(Figure 1). Fossil Lake lay to the west of the much larger Lake Gosiute, which covered most of southern and central Wyoming. Fossil Lake formed along the eastern edge of the Idaho-Wyoming thrust belt in a small, structurally controlled basin. The Crawford Mountains and Tunp Range form the western boundary; Oyster Ridge forms the eastern boundary; and the Uinta Mountains form the southern boundary. The lacustrine Green River Formation consists of buff colored, laminated calci- and dolomicrite, brown to black, kerogen-rich, laminated calci- and dolomicrite, siltstone, mudstone, and claystone with several thin tuff beds. Laterally, these lithologies grade into algal, ostracodal, gastropodal, and bioturbated calcimicrites deposited in shallow water near the shore of ancient Fossil Lake (Rubey, Oriel, and Tracey, 1975; Buchheim, 1994).

Buchheim (1994) divided the Green River Formation in Fossil Basin into three informal units (Figure 2). Each of these units represents a distinct depositional phase of Fossil Lake. The lower unit is a lacustrine sequence characterized by siliciclastic mudstone and sandstone, bioturbated calci- and dolomicrite, and kerogen-rich and kerogen-poor laminated micrite (see Biaggi and Buchheim, this volume). Some fossil fish and gastropods occur in the lower unit. Toward the margin of the lake the sandstone tongue of the Wasatch Formation separates the lower unit from the middle unit (Oriel and Tracey, 1970).


Figure 1—Geographic and geologic features in the vicinity of Fossil Basin, Wyoming. Study localities are FBM, CCS, and LMC in the left-hand figure. (After Biaggi and Buchheim, this volume).


The middle unit is a well-developed lacustrine sequence characterized by laminated calci- and dolomicrite with high kerogen content. Toward the margin the laminated micrite becomes bioturbated. The middle unit contains most of the fossils that occur in the basin. The upper unit represents the waning stages of the lake. It is characterized by poorly laminated dolomite-rich carbonates, many of which contain calcite pseudomorphs after saline minerals, and some kerogen-rich, laminated dolomicrite. Fossils are rare in the upper unit (see Loewen and Buchheim, this volume). The lower, middle, and lower part of the upper units form the Fossil Butte Member and the upper part of the upper unit forms the Angelo Member (see Figure 2 and Buchheim, 1994).

Previous Studies

The Green River Formation in Fossil Basin contains a rich assemblage of fossils. Fossils reported from the Fossil Butte Member include fish (Cope, 1877, 1884; Thorpe, 1938; Hesse, 1939; Grande, 1984), sting rays (Schaeffer and Mangus, 1965), birds (Wetmore, 1933; Leggitt, 1996; Leggitt and Buchheim, 1997), a bat (Jepsen, 1966), a snake (Schaeffer and Mangus, 1965), insects (Scudder, 1890; Cockerell, 1920), plants (Lesquereux, 1883; Brown, 1929, 1934), and freshwater mollusks, ostracods, and algal limestones (Bradley, 1926). Until recently, the age of the Green River Formation in Fossil Basin was based on mammal fossils from the intertonguing Wasatch Formation (Gazin, 1959). The Green River Formation was interpreted as Lostcabinian (North American Land Mammal Age), although no mammals of that age were known from Fossil Basin (Gazin, 1959; Schaeffer and Mangus, 1965). Two recent vertebrate fossil discoveries have shed more light on the age of the Fossil Butte Member. Breithaupt (1990) reported the occurrence of Orohippus in the middle unit. Because of this find, he questioned the Lostcabinian age assignment for the Fossil Butte Member. However, the more recent discovery of Lambdotherium (Froehlich and Breithaupt, 1997) in the F2 or "split fish" zone of the Fossil Butte Member (within the middle unit of Buchheim, 1994), supports the Lostcabinian Land Mammal Age for at least some of the Green River Formation in Fossil Basin. Plant microfossil studies by Wodehouse (1933) from the Parachute Creek Member (specifically the Mahogany Ledge) of the Green River Formation in Colorado and Utah led him to conclude that the Green River Formation was Middle Eocene in age. In later studies of the Piceance Creek and Uinta basins, Newman (1974, 1980) developed palynomorph range zones for the Green River Formation (Douglas Creek, Garden Gulch, Anvil Points, Parachute Creek, and Evacuation Creek members). Newman assigned Early and Middle Eocene age estimates and land mammal ages to each of the plant microfossil biozones.



Figure 2—Stratigraphic chart illustrating stratigraphic units, facies relationships and relationships of measured sections. Locality 217 = FBM, 122 = CCS, and 740 = LMC. (Modified from Buchheim and Eugster, 1998).

Methods

Forty-nine outcrop samples of the Green River Formation in Fossil Basin were collected and processed for plant microfossils. The samples were collected from three measured stratigraphic sections corresponding to localities 217, 122 and 740 (Figure 2). These three sections are equivalent to localities FBM, CCS, and LMC, respectively (Figure 1). The Fossil Butte section (FBM, locality 217, SW ¼ NW ¼ sec. 5, T. 21 N., R. 117 W.) represents an intermediate area of the lake, just north of the lake depocenter. The Clear Creek section (CCS, locality 122, NW ¼ SE ¼ sec. 35 and NE ¼ SE ¼ sec. 34, T. 21 N., R. 117 W.) represents the depocenter of Fossil Lake. The Little Muddy Creek section (LMC, locality 740, SE ¼ SE ¼ sec. 24, T. 20 N., R. 118 W.) represents an environment more proximal to the lake margin. Rock samples were collected from each of the major lithologies at each section. Phillips Petroleum Company processed the samples using standard palynological techniques. Twelve of the 49 samples produced palynomorphs. Analysis of the palynoflora included pollen counts of all 12 samples. Ten of the 12 productive samples contain statistically adequate numbers of palynomorphs.

The Palynofloral Assemblage

The outcrop samples from the Fossil Butte Member and lower Angelo Member of the Green River Formation yielded a diverse palynoflora. The assemblage consists of 176 forms representing 38 families, 54 genera, and 7 identifiable species. Approximately 2270 pollen, spores, dinoflagellates, and acritarchs are identified from the 12 productive samples. Of the 2270 plant microfossils, 1.5% represent non-bladdered conifers, 23% bladdered conifers, 37.5% angiosperms, 14% ferns and lower plants, 1% dinoflagellates, 22.5% acritarchs, and 0.5% of unknown affinity.

palyostratigraphy and age

Biostratigraphically significant plant microfossil taxa from the Green River Formation in Fossil Basin include Bombacacidites, Eucommia, Pistillipollenites mcgregorii, Platycarya platycaryoides, and Momipites triradiatus. Figure 3 shows the vertical distribution of significant taxa present in the lower, middle, and upper units of the Green River Formation in Fossil Basin, Wyoming. The Fossil Butte palynofloral assemblage is compared with other palynological studies of the Wasatch and Green River formations (Leopold and MacGinitie, 1972; Newman, 1974, 1980; and Nichols, 1987) to determine the stratigraphic ranges of the significant taxa. The results of this comparison confirm that most of the Fossil Butte palynoflora is late early Eocene (late Ypresian or Lostcabinian) in age as suggested by the occurrence of Lambdotherium in the middle unit (Froehlich and Breithaupt, 1997). However, the stratigraphic ranges of Bombacacidites, Eucommia, Pistillipollenites mcgregorii, Platycarya platycaryoides, and Momipites triradiatus extend into the lower middle Eocene (lower Lutetian or Bridgerian). Consequently, the palynoflora does not rule out the possibility that the upper part of the Green River Formation in Fossil Basin may be early middle Eocene (early Lutetian or Bridgerian) in age.

The early middle Eocene age is suggested by a series of K-Ar age dates on a tuff (known as the "K-spar tuff" by Buchheim, 1994) in the uppermost part of the middle unit. In the early 1980's, four samples of this tuff from four different localities were age dated for Buchheim by Geochron Laboratories. The results were as follows: locality 242 (in the northern part of Fossil Basin) yielded a K-Ar age of 43.6 + 1.6 Ma; locality 217 (FBM in Figure 1) yielded a K-Ar age of 49.1 + 1.8 Ma; locality 252 (252 in Figure 1) yielded a K-Ar age of 50.2 + 1.9 Ma; and locality 122 (CCS in Figure 1) yielded a K-Ar age of 52.2 + 2.0 Ma. The date from locality 242 is clearly anomalous with the other three dates. Buchheim and Eugster (1998) reported the 50.2 + 1.9 Ma date because it represents the median of the three more consistent age dates. Although the variability among the three age dates needs to be resolved, the ages suggest that the upper part of the Green River Formation may be early middle Eocene (early Lutetian or Bridgerian) in age.



Figure 3—Stratigraphic distribution of biostratigraphically significant plant microfossils of the Green River Formation in Fossil Basin, Wyoming.


However, that scenario is complicated by the uncertainty surrounding the absolute ages for the Wasatchian-Bridgerian Land Mammal Age boundary and the Ypresian-Lutetian boundary. The accepted age of the Ypresian-Lutetian (Early-Middle Eocene) boundary is 49 Ma (Berggren et al., 1995) and is placed at the top of Chron C22n on the paleomagnetic polarity scale. In the same volume, Prothero (1995) placed the age of the Wasatchian-Bridgerian Land Mammal Age boundary at 50.4 Ma and within Chron C22r. In the past, the Wasatchian-Bridgerian boundary was considered correlative with the Early-Middle Eocene (and hence the Ypresian-Lutetian) boundary. The results of Prothero's (1995) work suggest that the Wasatchian-Bridgerian boundary is slightly older than the Early-Middle Eocene boundary. Further taxonomic and stratigraphic resolution of the Fossil Butte palynoflora, new tuff samples for age dating utilizing new analytical techniques, and additional study of the mammal fossils in Fossil Basin is needed to resolve this confusing situation.

Conclusions

The reported occurrence of Lambdotherium (Froehlich and Breithaupt, 1997) in the middle unit suggests a Lostcabinian (Ypresian) age for most of the Green River Formation in Fossil Basin. Potassium-argon age determinations of 49.1 + 1.8, 50.2 + 1.9, and 52.2 + 2.0 Ma on a potassium-feldspar tuff near the top of the middle unit suggest that the upper part of the Green River Formation in Fossil Basin may be early middle Eocene in age. The co-occurrence of Bombacacidites, Eucommia, Pistillipollenites mcgregorii, Platycarya platycaryoides, and Momipites triradiatus support a late early Eocene (Ypresian) to early middle Eocene (Lutetian) age range for the Fossil Butte palynoflora. Consequently, the palynoflora does not rule out the possibility that the upper part of the Green River Formation in Fossil Basin may be early middle Eocene (early Lutetian or Bridgerian) in age. A late early (to possibly early middle) Eocene age for deposition of the Green River Formation in Fossil Basin correlates with deposition of the Wilkins Peak Member of the Green River Formation in the Green River Basin, Wyoming.

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