Contributions to Geology 18.2

The occurrence and chemistry of epidote-clinozeisites in mafic gneisses from the Ruby Range, southwestern Montana

P. S. DAHL Department of Geology, Kent State University, Kent, Ohio 44242
L. M. FRIBERG Department of Geology, University of Akron, Akron, Ohio 44325

Pages
77-82

Keywords
Ruby, gneiss, microprobe, epidote, clinozoisite, zoned

Abstract
Optically and chemically zoned epidote-clinozoisite occurs as a retrograde mineral in Precambrian mafic gneisses from the Ruby Range, southwestern Montana. Computer-corrected electron microprobe analyses place the chemical boundary between epidote and clinozoisite at 15.4541.55 mole percent pistacite (Fe+3/ (Fe+3+AlVI)). Zoning profiles indicate that epidote-clinozoisite crystals formed during a single episode in which: (1) temperatures declined gradually (in the range from 745¡C to 550¡C); and/or (2) oxygen fugacity increased. Superimposed on these changes were local and relatively minor T-fO2 fluctuations, as evidenced by oscillatory AI-Fe+3 zonation in single crystals. These physico-chemical variations probably characterize conditions that prevailed in mafic gneisses during the post-2700 m.y. uplift event inferred by Dahl (1979a, 1979b) for the Ruby Range.

Foreland deformation: compression as a cause

D. L. BLACKSTONE, JR. Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
83-100

Keywords
foreland, Elk, Wyoming, Pryor, Arlington, faulted

Abstract
Three examples of foreland deformation in which compressional stress played the dominant role are discussed. The examples include the Elk Mountain anticline and the Arlington thrust and related folding of southeastern Wyoming, and East Pryor Mountain, south central Montana. All three examples exhibit faulted margins, overturned strata associated with the faulting, and crustal shortening. Preservation of original bed length and original volume are honored in all cross sections.

The writer concludes that the most rational explanation for deformation described is response of a rigid basement by rupture, lateral movement and shortening of the crust, and displacement of the overlying sedimentary rocks without change in volume or thickness. Compressional stress was the activating force in all three cases.

Early Cenozeic mammalian paleontology, geologic structure, and tectonic history in the overthrust belt near LaBarge, western Wyoming

JOHN A. DORR, JR. Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan 48109
PHILIP D. GINGERICH Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan 48109

Pages
101-116

Keywords
Cenozoic, Wyoming, Chappo, Wasatch, fossil, Darby Thrust

Abstract
Three fossil mammalian faunules from the Chappo Member of the Wasatch Formation west of LaBarge, Wyoming, range from middle Tiffanian to early Wasatchian in age. Thus the minimum age range of the Chappo Member is middle late Paleocene to early Eocene. Other synorogenic deposits in the overthrust belt have been dated previously. The uplifted Darby (= Hogsback) Thrust yielded synorogenic conglomerates beginning in Torrejonian time and it was overlapped by the Chappo Member by middle Tiffanian tirne. Therefore, the time of movement of the Darby Thrust was middle Paleocene. The Prospect (=Cliff Creek=Jackson) Thrust cuts the Darby Thrust. It has been dated as post-Tiffanian and pre-Wasatchian. The LaBarge (='Hilliard") Thrust deformed the Chappo Member, and was overlapped by the LaBarge Member of the Wasatch Formation of latest Wasatchian (Lostcabinian) age. Therefore, the LaBarge Thrust is Wasatchian in age and younger than the Prospect Thrust. The progression of datable west-dipping thrusts in the Wyoming-ldaho Overthrust Belt can now be shown to be, from old on the west to young on the east, the Paris, Crawford, Absaroka, Darby, Prospect, and LaBarge thrusts in that order. The concept of Royse and others (1975) that the Prospect Thrust ramped up into the Darby (=Hogsback) Thrust, producing simultaneous late movement along a single plane in a linked system consisting of all three thrusts, is contradicted by both geometrical and temporal evidence.

Paleobiology and depositional setting of a Late Cretaceous vertebrate locality, Hell Creek Formation, McCone County, Montana

CARTER LUPTON Department of Anthropology, Milwaukee Public Museum, Milwaukee, Wisconsin 53233
DIANE GABRIEL and ROBERT M. WEST, Department of Geology, Milwaukee Public Museum, Milwaukee, Wisconsin 53233

Pages
117-126

Keywords
mammals, fossil, Montana, Hell Creek, dinosaurs, logs, Cretaceous

Abstract
Milwaukee Public Museum fossil locality 2256 (Chris's Bonebed) is on an isolated hill in the SW1/4 of sec. 10, T. 24 N., R. 43 E., McCone County, Montana. It is high in the upper Cretaceous Hell Creek Formation, 35 m below the "Z" coal bed which locally marks the base of the Paleocene Tullock Formation. The locality, discovered in 1977, was excavated in 1978. The 4 m by 10 m site was gridded into 36 oriented meter squares. The surface rubble within each square was collected, and each was excavated to the base of the bone level. Fossils were collected from a medium-grained sandstone and clay-pebble conglomerate sequence (up to 35 cm thick) overlying a drab siltstone.

The fauna (19 genera) from locality 2256 includes dinosaurs (hadrosaurs, ankylosaurs, hypsilophodonts, ceratopsians, and theropods), turtles, champsosaurs, crocodilians, fish, amphibians, and mammals. In addition, carbonized impressions of several logs were exposed during the excavation; these were not collectible.

The deposit is interpreted to have been the result of abrupt sheet wash in the Cretaceous across the relatively flat surface. This produced a biologically and mechanically heterogenous accumulation of fossil remains. No strong size-sorting is evident, nor does one taxon or individual dominate the assemblage. Orientation of 93 specimens was measured. They show a poorly-defined preferred orientation, even where polarity is considered.

A titanothere from the Green River Formation, central Utah: Teleodus uintensis (Perissodactyla: Brontotherlidae)

MICHAEL E. NELSON Department of Earth Sciences and Sternberg Memorial Museum, Fort Hays State University, Hays, Kansas 67601
JAMES H. MADSEN, JR. Antiquities Section, Division of State History, Salt Lake City, Utah, 84101
W. LEE STOKES Department of Ge

Pages
127-134

Keywords
Eocene, titanothere, Teleodus uintensis, skull, Utah, Green River

Abstract
The assignment of a late Eocene age to part of the Greer River Formation in central Utah is based upon the identification of a partial skull of the titanothere Teleodus uintensis, characteristically a latest Eocene (Duchesnean "land mammal age") mammal. The specimen was collected from the Green River Formation in Sanpete County, Utah. The overlying Crazy Hollow, Bald Knoll, and Dipping Vat formations are interpreted to be of Oligocene age.

Eocene perissodactyls from the type section of the Tepee Trail Formation of northwestern Wyoming

BRUCE J. MacFADDEN Florida State Museum, University of Florida, Gainesville, Florida 32611

Pages
135-143

Keywords
perissodactyls, Eocene, Tepee Trail, Wyoming, horse, Epihippus uintensis

Abstract
At least four taxa of perissodactyls are present in unit 24 (bone bed A) of the type section of the medial Eocene (early Uintan) Tepee Trail Formation from northwestern Wyoming. The most abundant perissodactyl in the faunule is the horse Epihippus uintensis. The taxonomy of the genus Epihippus is reviewed; based on discrete differences in size, two species should be recognized, the smaller E. gracilis and the larger E. uintensis. The other perissodactyls, represented by only a few specimens, include the tapiroid Dilophodon minusculus, the rhino-like Amynodon sp., and a brontotherioid.