Contributions to Geology 16.1

Independence Mountain thrust fault, North Park Basin, Colorado

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

Pages 1-16

Keywords
Independence Mountain, Colorado, folds, thrust, movement

Abstract
The Independence Mountain thrust fault is located at the north end of North Park basin, Jackson County, Colorado. The north margin of the basin consists of a series of south plunging folds which involve the entire sedimentary column and the crystalline Precambrian basement. The Independence Mountain thrust fault strikes approximately N 75 W, and is oriented approximately normal to the strike of the fold axial surfaces. The fault surface dips 8 degrees to 15 degrees to the northeast. A large reentrant has been eroded into the fault plate exposing one of the marginal folds of the basin in the footwall. Movement on the hanging wall was rotational and to the south with the hinge point located near Kings Canyon, Colorado. A series of high angle faults parallels the thrust trace a short distance to the north.

Rb and Sr concentrations and Sr isotope ratios in the Preacher Creek ultramafic intrusion, Wyoming

PHILIP A. DAVIS, JR. and ALAN M. STUEBER Miami University, Oxford, Ohio 45056
MARK J. POTTS Airborne Geophysics, Texas Instruments Inc., Dallas, Texas 75222

Pages 17-26

Keywords
isotope, Preacher Creek, Wyoming, pluton, Laramie

Abstract
The Preacher Creek peridotite pluton is exposed near the northeastern margin of the Laramie Range, Wyoming. Although the field relationships indicate that it should be classified as an alpine-type intrusion, the term "proto-stratiform" has been applied because of physical and chemical similarities with ultramafic rocks in stratiform intrusions. 87Sr/86Sr ratios and Rb and Sr concentrations in seven whole-rock peridotite samples and in seven mineral separates from four samples were determined by mass spectrometric techniques. Four of the mineral separates along with four whole-rock samples may be interpreted as forming an isochron with an age of 1.72 + 0.33 b.y. and an initial 87Sr/86SR ratio of 0.7036 + 0.0003. This interpreted age falls within a previously recognized period of regional metamorphism which occurred approximately 1.6 to 1.75 b.y. ago in the Laramie Range.

A comparison of the Rb and Sr concentrations and the 87Sr/86Sr ratios of whole rocks and mineral separates from the Preacher Creek intrusion with similar data from other types of ultramafic rocks shows distinct differences between the Preacher Creek pluton and alpine-type intrusions. However, no significant differences were observed with respect to ultramafic rocks from stratiform intrusions or from inclusions in basalts. Thus, the data support a "proto-stratiform" origin for the pluton.

Glacial geology of the Lake Marie area, Medicine Bow Mountains, Wyoming

CHARLES G. OVIATT Department of Geography, University of Utah, Salt Lake City, Utah 84112

Pages 27-38

Keywords
glacial, Lake Marie, Medicine Bow, Wyoming, till, moraine

Abstract
Detailed mapping and relative dating techniques distinguished one early Neoglacial and three late Wisconsin glacial deposits in a five km2 area just southeast of Medicine Bow Peak. French Creek Till ("Pinedale 3?"), primarily forming ground moraine, Lake Marie Till ("Pinedale 4?"), forming a recessional moraine ridge, and Lookout Lake Till-Protalus, tentatively correlated with the type Temple Lake till in the Wind River Mountains, were deposited during latest Pinedale (Wisconsin) glaciation. The probable early-Neoglacial Disaster Peak Till-Protalus consists of two main ridges. The term "till-protalus" indicates that although glacial ice was active, most of the debris fell from the adjacent cliff and moved across the ice surface.

Lichen evidence suggests an extensive Audubon-age snow cover in the 3230 m asl. Lake Marie area, but no glacial deposits of this age (or younger) are present.

Talus, avalanche boulder tongues, and mudflow levees lie below the Snowy Range cliffs; block streams, block fields, active and inactive patterned ground and solifluction terraces are present in favorable locations.

Late Cenozoic grasses, borages, and hackberries from southwestern Nebraska

JOSEPH R. THOMASSON Division of Science and Mathematics, Black Hills State College, Spearfish, SD 57783

Pages 39-44

Keywords
Cenozoic, Nebraska, angiosperm flora

Abstract
Handpicking in the field, as well as chemical preparation of matrix, have produced evidence of the first recorded angiosperm flora from late Cenozoic strata at and in the immediate vicinity of the University of Nebraska State Museum Collecting Locality Ft-40 = Amebelodon frickii Quarry. Taxa collected include Berriochloa s p. and Panicum sp. of the Gramineae, Celtis willistonii (Cockerell) Berry of the Ulmaceae, and Prolithospermum johnstonii Elias and Biorbia fossilia (Berry) Cockerell of the Boraginaceae. Prolithospermum johnstonii has not been reported previously from Cenozoic deposits in Nebraska.

Determining fluvial channel patterns from stratification style: a Triassic example from the Colorado Plateau

ARTHUR S. TREVENA Department of Geology and Ceophysics, University of Utah, Salt Lake City, Utah 84112

Pages 45-54

Keywords
Triassic, Colorado Plateau, braided, meandering, channel pattern

Abstract
Deposits of braided and meandering streams have different abundances of stratification types and different vertical sequences of stratification and grain size. Recent near-source braided streams contain abundant horizontally stratified and structureless coarse-grained sandstone and conglomerate, while suspended-load meandering streams are dominated by trough cross-stratified sandstone that becomes finer-grained upward. Streams of intermediate channel pattern are typified by abundant tabular-planar cross-stratification, trough cross-stratification, and horizontal stratification. Strata representing all three types are recognized in the Sonsela Sandstone Bed, a Triassic fluvial unit within the Chinle Formation of northeast Arizona and northwest New Mexico. Areal variations in the abundance of stratification styles within the Sonsela Sandstone Bed suggest a regional transition from near-source braided streams toward more distally located meandering rivers.

An addition to the known geographic range of the Permian pelecypod Oriocrassatella

DONALD W. BOYD Department of Geology, University of Wyoming, Laramie, WY 82071 NORMAN D. NEWELL Department of Invertebrates, The American Museum of Natural History, Central Park West at 79th St., New York, New York, 10024

Pages 55-57

Keywords
pelecypod, Oriocrassatella, Permian, Spitsbergen, North America

Abstract
Crassatellid pelecypods discovered in a museum collection of Permian fossils from Spitsbergen are assignable to the Russian species Oriocrassatella plana. They are very similar to O. elongata, the only crassatellid known from the Permian of continental North America. The two species differ mainly in posterior outline. The similarity is of special interest considering the restricted distribution of the American species, which is known from only three localities in western Wyoming and eastern Utah.