Contributions to Geology 23.1

Vestigial shell structure in silicified pectinacean pelecypods

DONALD W. BOYD Department of Geology and Geophysics, The University of Wyoming, Laramie, Wyoming 82071-3006
NORMAN D. NEWELL Department of Invertebrates, American Museum of Natural History, Central Park West at 79th St., New York, New York 10024

Pages
1-8

Keywords
shell, pelecypod, Permian, microstructure, silicification

Abstract
The microstructure and layering of the pelecypod shell usually are not preserved in silicified examples. But vestiges of original shell structure are occasionally encountered in Permian scallops from West Texas. Simple and fibrous prismatic microstructures are represented in outer layers by either silicified prisms or molds of prisms. The former presence of crossed lamellar microstructure in inner layers can be inferred from preferred orientation and surface morphology of quartz crystals. Evidence of original wall layering is related to differences in stability of aragonite and calcite. At some localities, dissolution of an inner aragonitic layer before silicification of an outer calcitic one resulted in thin valves whose interior surfaces show growth lines and details of ornamentation. In valves from other localities, space formerly occupied by the aragonitic layer was filled by silica distinctively different in diagenetic fabric from that representing the calcitic layer.

A three-dimensional map of a paleontological quarry

WILLIAM L. ABLER 3350 S. Michigan Ave., Chicago, Illinois 60616

Pages
9-14

Keywords
fossil, map, three-dimensional, quarry

Abstract
Complete taphonomic interpretation of a fossil deposit involves the use of three-dimensional information. A three-dimensional map of a paleontological quarry can be produced quickly and easily using inexpensive and highly portable equipment. Using cords nailed to poles or to the quarry wall, an imaginary x-y reference plane is established parallel to the quarry bedding planes, whose orientation is measured using Brunton pocket transit, or compass and protractor with plumb line. The distance of fossil materials 'below' the reference plane is measured using a square grid measuring box and a measuring rod. A serial section map is provided as an example.

The northern termination of the Crawford thrust, western Wyoming

JAMES P. EVANS and JOHN H. SPANG Center for Tectonophysics, Department of Geology, Texas A&M University, College Station, Texas 77843-3113

Pages
15-32

Keywords
syncline, thrust, Crawford, Wyoming, anticline, Twin Creek

Abstract
Displacement on the Crawford thrust in western Wyoming decreases rapidly as measured from south to north in the area where the surface trace of the thrust terminates. Structures in the thrust sheet developed to accommodate this loss of displacement. Folds in the Jurassic Twin Creek Limestone formed above a prominent decollement horizon in the Lower Twin Creek Limestone. The Sublette anticline, a large, overturned asymmetric anticline with Jurassic through Permian units on the remaining east limb shortens the rocks in the Crawford sheet north of the folds in the Twin Creek Limestone. Further shortening of the hanging wall was accomplished by the Sublette anticline thrust, a forelimb thrust which may splay off the Crawford thrust at depth. Evidence for the Cokeville normal fault shows that the normal fault breached the Sublette anticline in the post-Eocene, and the spatial relationship between the normal fault and the Sublette anticline is used to infer the presence of a ramp in the Crawford thrust at depth.

The Crawford thrust cores a cylindrical, overturned anticline which plunges to the north. This anticline is flanked by a conical syncline which plunges and opens to the north. Displacement is transferred locally and upsection to the thrust-cored anticline. North of the termination of the surface trace of the Crawford thrust, displacement continues to decrease, the thrust drops to a lower structural level, and folds formed in the Cretaceous and Jurassic section above the Twin Creek decollement. These folds are below the Meade thrust and above the Crawford thrust, and show that the Crawford thrust is at a lower structural level than the Meade thrust in the area.

Magnetostratigraphy of the early Oligocene Pipestone Springs locality, Jefferson County, Montana

DONALD R. PROTHERO Department of Geology, Knox College, Galesburg, Illinois 61401

Pages
33-36

Keywords
Oligocene, Montana, Pipestone Springs, magnetostratigraphy, mammal, Chadronian

Abstract
A classic early Oligocene (Chadronian) mammal locality, the Pipestone Springs local fauna of Jefferson County, Montana, was paleomagnetically sampled. 24 sites, spanning 43 m of section, were treated by thermal demagnetization. The lower 25 m showed a predominantly normal polarity; the upper portion of the section showed reversed polarity. Faunal correlations and magnetostratigraphic work on other Chadronian sections suggest that the Pipestone Springs sequence correlates with Chron 13 and Chron 12r of the magnetic polarity timescale, or approximately 35 to 36 mybp.

Residual strain measurements in selected materials from the Black Hills, South Dakota

JON M. WHITE 4326 Marsh Road, DeLand, Florida

Pages
37-44

Keywords
Tin Mountain, Black Hills, Dakota, strain, stress

Abstract
Residual strains in selected materials from the Black Hills, South Dakota, were determined by overcoring. Residual strains in single crystals of quartz and orthoclase from the discordant intrusive at the Tin Mountain Mine, Custer County, South Dakota are consistent within each crystal but vary widely from one crystal to another. Residual strains in a plagioclase crystal are related to crystal geometry and to exsolution features. Residual strains in and about a tabular concordant pegmatite mass at the Skyrocket Mine, Custer County, South Dakota are related to the geometry of the body. These facts are thought to indicate that a "fabric" of small residual stress domains exists within each mass of material examined. Similarity, but not exact reproducibility, of results is explained by variations of stress from one domain to another.