Contributions to Geology 27.2

Late Ordovician and Silurian paleogeography of the Great Basin

PETER M. SHEEHAN Department of Geology, Milwaukee Public Museum, 800 West Wells Street, Milwaukee, Wisconsin 53233

Pages
41-54

Keywords
Great Basin, Ordovician, Silurian, glaciation, extinction, brachiopods

Abstract
Two events markedly affected Late Ordovician and Silurian biogeographic and lithofacies patterns in the eastern Great Basin. The first event, which had profound effects on communities inhabiting the carbonate platform, corresponds with a major extinction event. The second event, which had little effect on the carbonate platform, involved regional collapse of the carbonate platform margin. The world-wide extinction event at the end of the Ordovician was associated with a major southern hemisphere glaciation. Physical evidence of the glaciation in the Great Basin is limited to a disconformity associated with a glacio-eustatic drop in sea level on the carbonate platform and shoaling of regions on the carbonate ramp that had been below wave base. After the extinction, biofacies patterns changed dramatically, especially among brachiopods which underwent a complete turnover. The platform margin collapse converted a gradual, westward facing carbonate ramp to an abrupt carbonate shelf margin. Lithofacies and biofacies beyond the platform margin were altered completely.

Surficial geology and geomorphology of the Doublespring Pass area, Lost River Range, east-central Idaho

J. KELLY CLUER Department of Geosciences, University of Arizona, Tucson, AZ 85721

Pages
55-62

Keywords
Doublespring Pass, Idaho, Quaternary, deposits, glacial

Abstract
The Doublespring Pass area of the central Lost River Range contains remarkably well-preserved late Quaternary glacial and alluvial landforms of several ages. These and related deposits are presented in detailed map and cross section form with brief descriptions of individual geomorphic/stratigraphic units. Radiocarbon and tephra data are used to date certain units and events including an hypothesized early Holocene (ca. 8500 to 7000 14C yrs BP) extensive glacial advance and a mid Holocene (near the time of Mt. Mazama tephra fall out, ca. 6850 14C yrs BP) large magnitude seismic event. Surficial effects of this event were paralleled by the October 28, 1983 Borah Peak earthquake. At least seven distinct glacial advance epicycles are represented by mappable deposits, probably correlating to one Bull Lake, three Pinedale, and three post-Pinedale geomorphic complexes. Outwash trains and slope deposits can be traced upstream to terminal moraines. Depositional and erosional landforms are distinct and appear related to glacial advance followed by retreat or stillstand, respectively, creating mappable fill and strath terraces.

Conversion of spinner magnetometers to digital operation

W. BRADLEY ISBELL and PETER N. SHIVE I Department of Geology & Geophysics, University of Wyoming, Laramie, WY 82071

Pages
63-66

Keywords
magnetometer, digital, conversion

Abstract
We converted a Schonstedt SSM-I spinner magnetometer to digital operation. Total cost of this conversion was less than $3,000. Noise tests show that an angular standard deviation of less than 1¡ can be reached for a 10-6 emu/cc sample with data collection time of 4.8 seconds per spin. The same spin time yields an angular standard deviation of about 10¡ for a 10-7emu/cc sample. Although conversion to digital operation did not yield a significant increase in sensitivity, other factors such as convenience, speed of data collection and analysis, and backup capability made the task worthwhile.