Contributions to Geology 19.1

Paleomagnetism of the Precambrian Lake Owens Complex, Medicine Bow Mountains, Wyoming

PETER N. SHIVE and JOHN A MOREL, Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
1-8

Keywords
Medicine Bow, paleomagnetism, dikes, Lake Owens, Precambrian

Abstract
Study of samples from 30 sites in the Lake Owens Complex, a Precambrian layered mafic intrusion in the Medicine Bow Mountains of Wyoming, shows that a stable magnetization is carried predominantly by single domain magnetite. The paleomagnetic pole for the complex, after alternating field demagnetization, is at 25.8 degrees S, 133.8 degrees W with an alpha 95 of 5.1 degrees; the pole after thermal demagnetization differs by less than one degree. Comparison of this pole with others from North American Precambrian rocks suggests that the complex was completely remagnetized at about 1.4 byBP during an episode of granitic intrusion. Secular variation of the geomagnetic field at the time the complex was magnetized was comparable to that of the recent field.

Depositional environment of the Green River Formation in the vicinity of the Douglas Creek Arch, Colorado and Utah

GEORGE MONCURE and RONALD C. SURDAM, Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
9-24

Keywords
Douglas Creek Arc, Uinta, Uncompahgre, Green River, Mahogany, Parachute

Abstract
The Douglas Creek Arch, a positive structural element connecting the Uinta Mountains and the Uncompahgre Plateau, forms the eastern rim of the Uinta Basin and the western rim of the Piceance Creek Basin. The Douglas Creek Arch was a positive topographic feature of low relief during the majority of time the Green River Formation (Eocene) was being deposited in the two basins. This positive area resulted in the division of Eocene Lake Uinta throughout most of its history into distinctly separate lakes; a smaller lake occupied the western Piceance Creek Basin and the other occupied the eastern Uinta Basin. The arch was transgressed during the deposition of the Mahogany Bed and overlying strata in the Parachute Creek Member of the Green River Formation. Due to the separation, there were differences in the sedimentological, chemical, and biological evolution of the two lakes. The lake in the Uinta Basin received substantial quantities of water and sediment from perennial streams draining the surrounding terrane of high relief. In contrast, the lake in the Piceance Creek Basin was fed primarily by ground-water through the playa surrounding the lake, by spring water that emerged from the flanks of the Douglas Creek Arch, and by sheet flooding. In the Piceance Creek Basin, intense evaporation and little surface recharge resulted in a restricted, shallow, saline, alkaline lake until the deposition of the Mahogany Bed.

Stratigraphy and vertebrate paleontology of the Horse Creek-Trail Creek area, Laramie County, Wyoming

MICHAEL CASSILIANO, Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
25-68

Keywords
Horse Creek, Trail Creek, Wyoming, Harrison, vertebrate, Escarpment

Abstract
The Tertiary strata of extreme southeastern Wyoming can be mapped as three definitive lithostratigraphic units. The lowermost unit is designated as the Harrison Formation. The middle unit is informally referred to as the "middle Miocene formation." This formation may represent a new lithostratigraphic unit, but it is not formally named pending further field studies of its full lateral extent. A vertebrate fossil collection (Horse Creek Quarry fauna) from near the base of the "middle Miocene formation" indicates an early Hemingfordian (middle early Miocene) age for the lower part of the "middle Miocene formation" and a temporal correlation with the Marsland Formation of Nebraska. A latest Hemingfordian (late early Miocene) or earliest Barstovian (middle Miocene) vertebrate fauna, the Joe's Quarry fauna, was collected at the top of the "middle Miocene formation." The upper part of the "middle Miocene formation" is tentatively correlated with the Sheep Creek Formation and/or Olcott Formation of Nebraska. The uppermost formation is a westward extension of Nebraska's Ash Hollow Formation. Two vertebrate fossil collections, the Trail Creek Quarry and Escarpment Quarry faunas, from the lower part of the Wyoming Ash Hollow Formation, are of Valentinian age (middle Miocene). The uppermost local Ash Hollow Formation may be Clarendonian to Hemphillian in age. No new species are described, but an unusual and possibly unique specimen of Menoceras was found.

The late Pleistocene mammalian fauna from the Colby Mammoth Kill site, Wyoming

DANNY N. WALKER and GEORGE C. FRISON, Department of Anthropology, The University of Wyoming, Laramie, Wyoming 82071

Pages
69-79

Keywords
Colby Mammoth, Wyoming, Pleistocene, ungulate, arctic

Abstract
Four seasons of excavation at the Colby Mammoth Kill site near Worland, Wyoming have resulted in the recovery of six ungulate and one small mammal species, herein named the Colby local fauna. Three taxa are extinct (Mammuthus columbi, Equns conversidens, and Camelops sp.). Bison has been extirpated from the Bighorn Basin during the Historic Period. Pronghorn (Antilocapra americana) are still found near Worland. This ungulate fauna substantiates the presence of a biome in northern Wyoming during the late Pleistocene similar to the arctic-steppe of Beringia.