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Open-File Report 2005–1343
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Conodont Database and Analysis of Conodont Color Alteration patterns in the Las Vegas 30´ x 60´ quadrangle, Clark and Nye Counties, Nevada, and Inyo County, California

By Anita G. Harris, William R. Page, Andrea P. Krumhardt, John E. Repetski, and Kenzie J. Turner

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Conodonts were the primary index fossils used for dating and characterizing the marine depositional environments of the thick section (at least 4,500 meters in the Spring Mountains) of uppermost Cambrian through Lower Triassic rocks in the Las Vegas 30' x 60' quadrangle. The color alteration index (CAI) of conodonts also was used to establish thermal maturation levels in these same rocks. Age and thermal maturation data for 215 productive collections are shown on a generalized geologic map of the Las Vegas quadrangle, and the biostratigraphic position of each sample is plotted on an accompanying stratigraphic column. A database in Microsoft Excel format provides all our significant information for each collection. Nearly 75 percent of the productive collections is from Mississippian and Ordovician rocks.

Late Mesozoic and Cenozoic tectonic, igneous, and geothermal events considerably influenced the distribution of CAI values so that CAIs may vary widely within an area and even within individual samples. The heterogeneity of CAI values observed in many of our conodont collections indicates the rocks were subjected to more than one thermal regime and that the younger thermal imprint(s) is irregularly distributed and likely related to Neogene Basin and Range extension and transform faulting. In general, CAI values in the same age rocks increase upward and westward through the stack of thrust plates and, in some plates, southwestward within a single plate. For example, in the Spring Mountains, latest Cambrian conodonts from the upper level Wheeler Pass plate have a CAI of 4.5 whereas conodonts of latest Cambrian through Devonian age in the structurally lower Keystone plate, to the southwest, have CAIs of only 2.5-3.0. This contrast reflects the much thinner Paleozoic section in the Keystone plate than in the Wheeler Pass plate (300 meters of uppermost Cambrian to uppermost Devonian rocks in the Keystone plate versus 1,700 m in the Wheeler Pass plate). The Las Vegas Valley Shear Zone separates the Wheeler Pass plate from the Gass Peak plate. The stratigraphic succession in these plates indicates they were once contiguous; CAI values confirm this relation.

Version 1.0

Posted November 2005

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