Contributions to Geology 30.1

Paleoenvironments, lithofacies and varves of the Fossil Butte Member of the Eocene Green River Formation, Southwestern Wyoming

H. PAUL BUCHHEIM Department of Natural Sciences, Section of Paleontology, Loma Linda University, Loma Linda, CA 92350

Pages
3-14

Keywords
Green River, Fossil Butte, Eocene, Wyoming, lacustrine, micrite

Abstract
Lithofacies within the Green River Formation of Fossil Basin, Wyoming can be traced from basin center to margin in as short a distance as 10 kilometers. Five major lacustrine lithofacies within the Fossil Butte Member are recognized including (from mid-lake to margin) kerogen-rich laminated micrite (KRLM or oil shale), kerogen-poor laminated micrite (KPLM), partly-burrowed laminated micrite (PBLM), bioturbated micrite (BM), and dolomicrite (DM).

The lateral distribution of kerogen-rich rocks and associated lithofacies was strongly influenced by a relatively greater amount of sediment deposition towards the margins of Fossil Lake. Periodic variations in inflow are reflected in deposition of carbonate laminae that interrupted an otherwise continuous deposition of organic matter.

A greater number of laminae and thicker laminae towards the lake's margin reflect the dominance of inflow processes on deposition in the lake. Most of the near-shore laminae were deposited in response to periodic non-annual processes, and thus are not true varves or annual deposits. Mid-lake laminae may be annual or a mixture of annual and non-annual laminae. The depositional model described here for Fossil Lake not only explains the observed number of laminae changes within synchronous units, but also changes in observed lateral and vertical lithofacies in the Fossil Butte Member.

Studies of paleoenvironments and historical biogeography in the Fossil Butte and Laney members of the Green River Formation

LANCE GRANDE Department of Geology, Field Museum of Natural History, Chicago, Illinois

Pages
15-32

Keywords
Fossil Butte, Laney Shale, Green River Formation, paleoenvironments, biogeography

Abstract
The Green River Formation contains sediment from one of the world's longest-lived great lake systems. Within these sedimentary rocks, particularly within the early Eocene Fossil Butte Member and the middle Eocene Laney Member deposits of Wyoming, is the best paleontological record of Tertiary aquatic communities in North America, and possibly the world. The well-preserved fossil biotas strongly indicate a freshwater environment, which conflicts with recent sedimentological studies suggesting a saline environment. Possible explanations for the seemingly conflicting data are given. The fossils also indicate tropical or subtropical conditions, and the disappearance of the Green River lake system coincides with the apparent extinction of several groups of fishes in North America that still survive in tropical regions elsewhere in the world.

The entire teleost fauna, as well as other fossils in the Green River Formation, suggest a transpacific origin for much of the modern western North American fish fauna (e.g., teleosts).

Paleontological and sedimentological variation in early Eocene Fossil Lake

LANCE GRANDE Department of Geology, Field Museum of Natural History, Chicago, IL 60605
H. PAUL BUCHHEIM Department of Natural Sciences, Loma Linda University, Loma Linda, CA 92350

Pages
33-56

Keywords
paleontology, sedimentology, Fossil Lake, Green River Formation

Abstract
A number of heavily collected fossil localities within the late early Eocene Fossil Butte Member of the Green River Formation have produced an unprecedented volume of paleontological data for studies of Cenozoic aquatic communities of North America. We discuss and try to quantify the enormous volume of recent fossil collecting activity, map the most active quarry sites of the last two decades, and compare stratigraphic sections taken at three of the main Fossil Butte Member Green River Formation quarries. We also make paleontological and sedimentological comparisons between a particular group of heavily collected near-shore deposits contemporaneous with each other ("F-2" described here) and a group of mid-lake deposits also contemporaneous with each other ("F-l" described here). In general, the mid-lake F-l deposits of the Fossil Butte Member are characterized by a lack of benthic organisms (e.g., absence of decapods and relative scarcity of stingrays), the extreme scarcity of juveniles for many non-clupeomorph fish species, and the presence of adult and very young juvenile specimens of the gonorynchid fish Notogoneus. The near-shore F-2 deposits are nearly contemporaneous with the mid-lake F-l deposits, and are characterized by a greater number of terrestrial vertebrates, a decapod fauna (shrimp and crayfish known by hundreds of specimens), more abundant stingrays, hiodontids and paddlefishes, juveniles of most species, and the absence of Notogoneus of any size. The Warfield Springs site, about 23 km south of the F-2 quarries discussed here, is another nearshore paleoenvironment and is the stratigraphic equivalent of the near-shore F-2 deposits to the north. Because the Warfield Springs deposits are not comparably sampled for fossils, we focus mainly on the F-1 and F-2 deposits for our study here. Based on evidence to date, the fauna of the Warfield Springs quarry is very similar to that of the near-shore F-2 deposits.

The sedimentation rate in the near-shore localities is considerably higher than in the mid-lake deposits. The laminae are not only thicker in the near-shore deposits than in the mid-lake deposits, but the number of laminae per unit of time is greater near shore than in mid-lake. We doubt that the laminae within the F-1 or the F-2 deposits are varves, and we believe that these laminae could have been deposited at a rate of at least several per year.

In addition to major paleofaunal differences between the mid-lake F-1 and near-shore F-2 deposits of Fossil Lake, there are also major fish faunal differences between the Fossil Lake deposits (Fossil Butte Member, Green River Formation) and the Lake Gosiute deposits to the east (Laney Member, Green River Formation). The most notable difference is the lack of catfishes (Ictaluridae and Hypsidoridae) and suckers (Catostomidae) in the Fossil Lake deposits. Catfishes and suckers are abundant at several localities within the Laney Member Lake Gosiute deposits to the east. Although Fossil Lake and Lake Gosiute were both lakes within the Tertiary Green River lake system, the Laney Member deposits are younger than the Fossil Lake deposits (e.g., Grande, 1984: Fig. I.2).

Early Bridgerian (middle Eocene) vertebrate paleontology and paleoecology of the southern Green River Basin, Wyoming

GREGG F. GUNNELL Museum of Paleontology, University of Michigan, Ann Arbor, Michigan 48109-1079
WILLIAM S. BARTELS Department of Geology, Albion College, Albion, Michigan 49224

Pages
57-70

Keywords
Bridgerian, vertebrates, Green River Basin, paleontology, paleoecology, Eocene

Abstract
Recent fieldwork in the southern Green River Basin near Opal, Wyoming has resulted in the discovery of significant new samples of early Bridgerian (Bridger lithostratigraphic zone A) reptiles and mammals. Based on these new discoveries, it is hypothesized that late early Bridgerian is a distinct faunal horizon, separable from earlier Gardnerbuttean and later Bridgerian faunal horizons. Three biostratigraphic zones are recognized for the Bridgerian of the Green River Basin. Cenogram analysis and preliminary study of reptilian and mammalian faunas indicates that this area was a moist, closed forest environment during the Bridgerian. Certain paleontological and geological factors indicate that the early and late Bridgerian may have been more similar to each other paleoecologically than the middle Bridgerian was to either. We hypothesize that fluctuations in the level of the Green River Basin lake system and water table were responsible for these differences.

Multi-scale sedimentary controls on taphonomy of the Paleocene Fort Union Formation, southwestern Bighorn Basin, Wyoming

MICHAEL B. LEITE Department of Geology and Geophysics, The University of Wyoming, Laramie WY 82071-3006

Pages
71-84

Keywords
Paleocene,Fort Union Formation, Bighorn Basin, fossil

Abstract
The Paleocene-age Fort Union Formation in the southwestern Bighorn Basin is the product of meandering river systems. As a whole it contains a low density of vertebrate fossils, but small, dense concentrations contribute important assemblages. Study of the lithological associations of the concentrations reveals that best fossil preservation occurred in swales and proximal overbank settings where low-energy conditions were punctuated by intervals of rapid sedimentation. In addition, activities of crocodilians living in swale ponds had important taphonomic effects on the assemblages. Rarity of preserved deposits from swales and other proximal overbank settings is due to their vulnerability to destruction by meander migration. Paucity of fossils in sandstones may be due to localization of major river courses partly by tectonic effects, resulting in reworking of channel sands and minimizing input of penecontemporaneous floodplain bones into channel systems.

Myrmekomomys, a new genus of Micromomyine (Mammalia, ?Microsyopidae) from the lower Eocene rocks of the Powder River Basin, Wyoming

PETER ROBINSON Museum, University of Colorado, Boulder, Colorado, 80309-0315

Pages
85-90

Keywords
Myrmekomoys loomisi, Wasatchian, Powder River Basin

Abstract
Myrmekomomys loomisi, new genus and species, from the Early Wasatchian of the Powder River Basin, is a micromomyine that differs from Micromomys in having more lingually located trigonids and from Micromomys, Chalicomomys and Tinimomys in being higher crowned and in having greater trigonid/talonid relief. The M/3 of Myrmekomomys has an elongate, tall hypoconulid, unlike that of Chalicomomys, Micromomys and Tinimomys. Myrmekomomys loomisi occurs with both Chalicomomys willwoodensis and Tinimomys graybullianus.

Paromomyidae (?Dermoptera) from the Powder River Basin, Wyoming and a discussion of microevolution in closely related species

PETER ROBINSON Museum, University of Colorado, Boulder, Colorado 80309-0315
LOGAN D. IVY Denver Museum of Natural History, City Park, Denver, Colorado 80205

Pages
91-116

Keywords
Paromomyidae (?Dermoptera), Powder River Basin, microevolution

Abstract
Eight species of paromomyids occur in lower Eocene rocks of the Powder River Basin. Six of these species occur lithosympatrically and no well-sampled locality has fewer than three. The morphology of the teeth of the paromomyids is described and the morphological criteria for defining genera and species are established. The systematics of the early Eocene paromomyids is discussed and revised. Three new genera are described: Simpsonlemur, Dillerlemur and Pulverflumen. Several new species are described. Emphasis is placed on examining the entire dentition, not solely the morphology and size of M/l. If other taxa are comparable to the Paromomyidae, we should expect to find elsewhere similar examples of closely related species occurring in a single locality, particularly during periods of rapid speciation and/or faunal turnover. These species should be similar superficially but differ in significant detail, indicating different diets and, therefore, different niche partitioning.