Contributions to Geology 23.2

South Fork detachment fault, Park County, Wyoming: geometry-extent-source

D. L. BLACKSTONE, JR. Department of Geology and Geophysics, The University of Wyoming, Laramie, Wyoming 82071-3006

Pages
47-62

Keywords
South Fork, fault, Wyoming, Sundance, anticline

Abstract
The trace of the South Fork detachment fault crops out in the valley of the South Fork of the Shoshone River, Park County, Wyoming. The position of the detachment is controlled by the Jurassic Sundance Formation. The detachment surface cuts up section to the level of the Cretaceous shale to form a ramp with a ramp anticline at the southeast exposure of the detached plate. The strata in the plate are strongly folded and broken by minor faults. The detachment surface southeast of the ramp is antiform in configuration as shown by the trace including a window, and by drill records. Movement is considered to be free-sliding under the force of gravity with resultant tectonic denudation up slope from the present surface trace of the detachment surface. The South Fork detachment occurred prior to the emplacement of the Heart Mountain detachment complex. An additional possible detachment is described in the valley of the North Fork of Shoshone River.

The late Pleistocene (Rancholabrean) avifauna of Little Box Elder Cave, Wyoming

STEVEN D. EMSLIE Department of Zoology, The University of Florida, Gainesville, Florida 32611

Pages
63-82

Keywords
Pleistocene, Wyoming, climate, vertebrate, Little Box Elder Cave, avifauna

Abstract
The late Pleistocene (Rancholabrean) avifauna from Little Box Elder Cave, Converse County, Wyoming, consists of 1,052 identifiable bones representing over 70 species. This collection is the most diverse avifauna yet reported from the Pleistocene of the Rocky Mountain region of western North America and includes the extinct species Neophrontops americanus, Buteogallus sp., Ectopistes migratorius, Euphagus magnirostris, and Anas schneideri, a newly described species of teal known so far only from this locality. Falco rusticolus is reported from this fauna and compared to the extinct species F. swarthi from McKittrick, California. F. swarthi is considered to be inseparable from F. rusticolus and is synonymized with that species. This fossil fauna provides a more thorough understanding of the late Pleistocene vertebrate community in southeastern Wyoming and is congruent with faunas of similar age from the Rocky Mountain region. Most species indicate that steppe-tundra conditions were present in the basins of southeastern Wyoming during the late Pleistocene, but that forested areas were also present and extended to the basin margins. Seasonably equable climate during the late Pleistocene probably accounts for the paleobiogeographic distribution of species in Wyoming that are now largely restricted to arctic and boreal regions to the north, deciduous forests to the east, and subtropical and arid regions to the south.

Stratigraphy of the Eocene Willwood, Aycross, and Wapiti Formations along the North Fork of the Shoshone River, north-central Wyoming

VICTOR TORRES Museum of Paleontology and Department of Geological Sciences The University of Michigan, Ann Arbor, Michigan 48109

Pages
83-98

Keywords
Willwood, Wapiti, Eocene, fossil, Aycross, Wyoming, Sunlight, Trout Peak

Abstract
The Willwood Formation along the North Fork of the Shoshone River forms the base of the Eocene section and was deposited over eroded Cretaceous Cody Shale. On Jim Mountain, it consists of a thin basal wedge of fine lacustrine sediment, followed by some 300 m of fluvial deposits. Fossil mammals indicate that here the Willwood Formation spans from early to middle Eocene (latest Wasatchian to early Bridgerian Land-mammal Age). The Willwood Formation was deposited in a marginal basin whose relationship to the Bighorn Basin proper remains to be clarified. The Aycross Formation, formerly included in the basal part of the Wapiti Formation, overlies the Willwood Formation. This is the most northern exposure of Aycross rocks; an angular unconformity separates these units from the overlying Wapiti Formation. Aycross Formation rocks were formerly included as part of the breccia-rich facies of the lower Wapiti Formation. Such breccia masses are allochthonous gravity slide blocks emplaced on both the Aycross and underlying Willwood Formations, perhaps during the recent past. Unconformable contacts at both the base and the top of the Aycross Formation are the consequence of folding, possibly related to the movement of the South Fork thrust and/or the Heart Mountain fault. Fossil mammals indicate a middle Eocene (early to middle Bridgerian) age for the Aycross Formation in the study area and farther to the south. The activity of a composite volcano in Sunlight Basin produced breccias, lavas, and volcaniclastic sediments of the Wapiti Formation, as well as lavas now exposed as the Trout Peak Trachyandesite. Other stratovolcanoes along the eastern edge of the Absaroka volcanic field gave rise to similar deposits in the region which are not coeval with units exposed along the North Fork of the Shoshone River. Limestone blocks displaced by the Heart Mountain detachment fault on Carter Mountain are overlain by volcaniclastics of early to middle Bridgerian age (Eaton, 1982). Along the North Fork of the Shoshone River the Heart Mountain fault blocks rest on the early-middle Bridgerian Aycross Formation thus providing the first accurate dating (early-middle Bridgerian) for the Heart Mountain fault.

Redescription of Bellerophon bittneri (Gastropoda: Triassic) from Wyoming

ELLIS L. YOCHELSON U.S. Geological Survey, Washington, D.C. 20560
DONALD W. BOYD Department of Geology and Geophysics, The University of Wyoming,
Laramie, Wyoming 82071-3006
BRUCE WARDLAW U.S. Geological Survey, Washington, D.C. 20560

Pages
99-104

Keywords
Bellerophon bittneri, Wyoming, Triassic, Dinwoody, Retispira bittneri

Abstract
Bellerophon bittneri Newell and Kummel is an Early Triassic bellerophontacean from the Dinwoody Formation in the Wind River Mountains. The available type material consists of one fair, but incomplete, external mold, which resembles a Bellerophon but is actually a Retispira. After repeated search, additional specimens were found at one locality in the southern Wind River Range of Wyoming; Retispira bittneri is redescribed from this new material. Like other Triassic bellerophontaceans, there is nothing unusual about the species apart from occurrence in the Mesozoic; it is clearly congeneric with Permian Retispira from underlying rocks.