THE STRATIGRAPHIC SETTING OF THREE PRESBYORNIS NESTING SITES: EOCENE FOSSIL LAKE, LINCOLN COUNTY, WYOMING
V. Leroy Leggitt, H. Paul Buchheim, And Roberto E. Biaggi
Geology Section, Department of Natural Sciences, Loma Linda University, Loma Linda, California 92350
The Green River Formation in Fossil Basin is completely encased in the late early Eocene Wasatch Formation. The "K-spar tuff" at the junction between the upper and middle units of the Fossil Butte Member has been dated at 50.2 +/- 1.9 mya. Palynomorph data from the lower, middle, and upper units of the Fossil Butte Member are consistent with a late early Eocene or an early middle Eocene age.
It is likely that the Bear Divide and Warfield Creek nesting sites are late early Eocene in age, and that the Powerline nesting site may be early middle Eocene in age.
The shorebird-duck mosaic bird Presbyornis, with its duck-like head, and long-legged shorebird-like body, was first described from the Green River Formation of Utah by Wetmore (1926). Since then, Presbyornis has been reported in great concentrations of associated skeletons from all of the Eocene Green River Formation lakes: Lake Gosiute (Simnacher, 1970; McGrew and Feduccia, 1973; Feduccia and McGrew, 1974, 1984; Feduccia, 1978, 1980; McGrew, 1980; Olson and Feduccia, 1980a); Fossil Lake, (Leggitt and Buchheim, 1993); Lake Uinta (Zawiskie et al., 1991); and ?late Paleocene Lake Flagstaff (Zawiskie et al., 1991).
Non-Green River Formation Presbyornis occurrences
include: Eocene rocks in Mongolia (Zawiskie et al., 1991),
the Lower Eocene Casamayor Formation of Patagonia
(Howard, 1955; Cracraft, 1970; Feduccia and McGrew, 1974), and
most recently, the marine late Paleocene Aquia Formation of
land and Virginia (Olson, 1994). Apparently Presbyornis had a worldwide distribution in the Paleocene and Eocene.
In addition to its widespread distribution, Presbyornis is important because its duck like cranial elements and shorebird-like postcranial elements link two modern orders of birds that were never thought to be related before the discovery of its fossils (Olson and Feduccia, 1980b; Olson, 1981; Feduccia, 1996).
Descriptions of avian nesting sites are rare in the paleontologic literature. Mohabey et al. (1993), reported a clutch of small oblong-shaped eggs of avian-like affinity associated with sauropod nesting sites from the Lameta Formation (upper Cretaceous) of India. In the Cretaceous of Mongolia, Gobipterygiform bird eggs are found in clutches and as individual eggs at several stratigraphic levels (Mikhailov et al., 1994). Both occurrences of Indian and Mongolian avian eggs have been interpreted as nesting sites.
Figure 1Presbyornis bones and eggshell from the Powerline nesting site.
There are numerous descriptions of Tertiary avian eggshell in the paleontologic literature (Mikhailov, 1997), but few of the reports emphasize a nesting site hypothesis. A good example of an Eocene avian nesting site was reported by McGrew and Feduccia (1973). This site was located in shoreline facies of the Wilkins Peak Member of the Green River Formation of Lake Gosiute, and contained hundreds of bones of Presbyornis, abundant eggshell fragments, tufa-encrusted logs, and skeletal elements of turtles, crocodiles, and fish. The sedimentology and paleoecology of the site were consistent with a nesting site hypothesis, but the eggshell fragments were not well described and might have been derived from the birds, turtles or crocodiles.
Avian eggshell fragments of prismatic morphotype have been identified by scanning electron microscopy from three Green River Formation localities along the southern shoreline of Eocene Fossil Lake (Leggitt and Buchheim, 1997). The three eggshell sites occur in close association with monospecific, single horizon, bonebeds of adult Presbyornis which are suggestive of mass mortality events (Figure 1).
Because of this close association of the avian eggshell fragments with the monospecific Presbyornis bonebeds, the eggshell has been referred to Presbyornis (Leggitt and Buchheim, 1997). The autochthonous nature of one site (the Powerline site) has been clearly established (Leggitt, 1996), and the remaining sites are believed to be autochthonous because of their proximity to the southern shoreline of Fossil Lake. Autochthonous Presbyornis eggshell is strong evidence in support of the nesting site hypothesis.
It is the purpose of this paper to document the precise stratigraphic position of the three nesting sites so that temporal, spatial and morphologic comparisons can be made between these Fossil Lake avian nesting sites.
Structrue And Stratigraphy Of Fossil Basin
Figure 2Map showing the location of measured sections in
Fossil Basin. Location 217 is the type section of the Fossil Butte
Member of the Green River Formation and is located within the
boundaries of Fossil Butte National Monument. The solid
correlation line represents middle unit correlations with the type section.
The dashed correlation line represents lower unit correlations
between locations 1525 and 1530.
Fossil Basin is a small north-south trending structural basin in southwestern Wyoming (Figure 2). Both the western and eastern margins of Fossil Basin were topographically elevated at the beginning of Wasatch and Green River deposition by reactivated thrust uplift and footwall uplift (Coogan, 1992). Basin margin uplift continued throughout Wasatch deposition as evidenced by a belt of the Tunp Conglomerate Member of the Wasatch Formation, which interfingers with other Fossil Basin sediments on the west, north and east basin margins (Oriel and Tracey, 1970; Hurst and Steidtmann, 1986; Coogan, 1992). This basin margin uplift resulted in symmetrical (west to east) Wasatch and Green River facies deposition (Coogan, 1992).
The Tertiary stratigraphy of Fossil Basin has been described by several authors (Oriel and Tracey, 1970; Rubey et al., 1975; Hurst and Steidtmann, 1986). Oriel and Tracey (1970) described more than 7,000 feet of uppermost Cretaceous and Tertiary continental strata in northern Fossil Basin. These sediments include the Adaville, Evanston, Wasatch, Green River, and Fowkes Formations.
The Green River Formation in Fossil Basin can be characterized as a large lens of heterogeneous lacustrine sediments which includes well laminated limestone, siltstone, marlstone, and claystone with layers of oil shale (Rubey et al., 1975). This lens of lacustrine sediments is encased in the red, detrital, coarse grained strata of the Wasatch Formation (Rubey et al., 1975).
The red Wasatch sediments are roughly 3,000 feet thick and have been divided into seven mapped members by Oriel and Tracey (1970): 1) the basal conglomerate member, 2) the lower unnamed member, 3) the main body, which underlies the Green River Formation, 4) the sandstone tongue, which divides the Fossil Butte Member of the Green River Formation, 5) the mudstone tongue, which divides the Fossil Butte and Angelo Members of the Green River Formation, and is characterized by abundant tufa-encrusted logs, 6) the Bullpen Member, which overlies the Green River Formation, and 7) the Tunp Member, which is a peripheral member which interfingers with units 2-6 (Figure 3).
The Green River Formation in Fossil Basin has been divided into the Fossil Butte Member and the overlying Angelo Member by Oriel and Tracey (1970). The two members of the Green River Formation are separated by the time-transgressive mudstone tongue of the Wasatch Formation.
Buchheim (1994), and Buchheim and Eugster (1998), noticed that the Fossil Butte Member could be divided into three units on the basis of major facies changes in Fossil Lake history: 1) a lower unit, recently studied by Biaggi (1989), which contains a record of the early development of Fossil Lake; 2) a middle unit, which is characterized by high-stand freshwater calcimicrite and which contains the famous fossil fish deposits; and 3) an upper unit, which is characterized by low-stand saline minerals and dolomicrite. This upper unit includes most of Oriel and Tracey's Angelo Member.
Buchheim and Eugster (1998) used time synchronous horizons to divide the Fossil Butte Member into units. The division between the lower and middle units was set at the lower oil shale and the overlying lower sandwich horizon (a pair of thin tuff beds about 50 cm apart). The middle and upper units were divided by the "K-spar tuff". Both marker horizons can be widely mapped in Fossil Basin and are used in this paper to correlate sections.
The stratigraphy of the lower unit was described in detail by Biaggi (1989). The lower unit was subdivided into the following ascending stratigraphic subunits: 1) the lower shale horizon (LSH) subunit, characterized by its brown-greenish-gray color; 2) the lower white marker (LWM) subunit, characterized by its distinctive white color and bench forming units; 3) the upper limestone (ULS) subunit, characterized by its golden or buff color; and 4) the sandstone (SS) subunit, characterized by its brown color. This sandstone subunit is equivalent to the sandstone tongue of the Wasatch as described by Oriel and Tracey (1970).
These subunit bands of color indicate facies changes within the lower unit and are present across most lower unit outcrops (Biaggi, 1989). The color bands are useful for stratigraphic correlation even though they represent time-transgressive facies.
The Age Of Fossil Basin Sediments
Most of the Wasatch Formation is of early Eocene age on the basis of pollen, invertebrate and vertebrate fossils (Gazin, 1952, 1956; Oriel and Tracey, 1970), but the basal conglomerate member may be partly Paleocene (Hurst and Steidtmann, 1986), and the Bullpen member may be partly early middle Eocene (Oriel and Tracey, 1970). The age of the Wasatch formation is important because it completely encases the Green River Formation and therefore constrains Fossil Basin Green River time. The Green River Formation in turn encases the Presbyornis Nesting Sites.
The late early Eocene Green River Formation age, as determined using Wasatch fossils, is independently corroborated by palynomorph analysis of the lower, middle, and upper units of the Fossil Butte Member of the Green River Formation (Cushman, 1983). The "K-spar tuff" which divides the middle and upper units of the Fossil Butte Member has been dated at 50.2 +/- 1.9 mya (Buchheim and Eugster, 1998). This date suggests that the upper unit may be early middle Eocene in age.
Figure 5Bear Divide section and the stratigraphic position of the Bear Divide nesting site within the Green River Formation.
The Nesting Site Stratigraphy
Stratigraphic sections, which include the lower
sandwich horizon tuff beds and the lower oil shale, were measured
the fossil quarries and compared with the type section of the Fossil Butte Member of the Green River Formation (Figure 2). The locations of the three Presbyornis nesting sites are as follows: 1) the Powerline site (Figure 4), section number 1525, quarry location NE 1/4 Sec. 36, T. 20 N., R. 118 W., Warfield Creek Quadrangle, Lincoln County, Wyoming; 2) the Warfield Creek site (Figure 4), section number 1525, quarry location SW 1/4 Sec. 6, T. 19 N., R. 117 W., Warfield Creek Quadrangle, Lincoln County, Wyoming; and 3) the Bear Divide site (Figure 5), section number 1530, quarry location NW 1/4 Sec. 29, T. 20 N., R. 119 W., Windy Point Quadrangle, Lincoln County Wyoming.
Figure 6Fence diagram showing the middle unit correlations of the study site sections with the Fossil Butte type section (solid lines), and the lower unit correlations of the study site sections with each other (dashed lines). The distance between location 1530 and location 1525 is 17 kilometers.
The correlations between the two measured sections at the fossil sites and the type section of the Fossil Butte Member are shown in Figure 6. The lower sandwich horizon tuff beds and the lower oil shale, separate the lower and middle unit of the Fossil Butte Member and were used as a datum for correlation. Correlations were made by the marker units described by Buchheim and Eugster (1998), and Biaggi (1989).
Discussion and Conclusions
Although fossil avian nesting sites are rarely described, Presbyornis nesting sites appear to be common in the shoreline facies of the Green River Formation (Leggitt and Buchheim, 1997; McGrew, 1980). Within Fossil Basin, the stratigraphic position of three of these nesting sites was described using standard stratigraphic technique. We found that more than 160 meters of sediment separated the lowest and highest Presbyornis occurrence.
This stratigraphic interval includes the complete Fossil Butte Member of the Green River Formation and most of the overlying Angelo Member. The lowest Presbyornis nesting sites (Bear Divide and Warfield Creek) are found at the base of the Green River Formation at a time when Fossil Lake was developing. The highest Presbyornis nesting site (Powerline) is found at the top of the Green River Formation at a time when Fossil Lake was shrinking in size.
Most lines of evidence suggest that these Presbyornis nesting sites are late early Eocene in age. It is possible that the Powerline nesting site is early middle Eocene in Age.
We are grateful to Storrs L. Olson of the National Museum, who identified the Presbyornis bones at the Powerline quarry. The staff at Fossil Butte National Monument provided useful support, and Laurie Bryant and the Wyoming Bureau of Land Management are acknowledged for arranging the permits for this use of public lands.
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