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Abstract

Miller, James F., James D. Loch, and John F. Taylor, 2012, Biostratigraphy of Cambrian and Lower Ordovician strata in the Llano uplift, central Texas, in J. R. Derby, R. D. Fritz, S. A. Longacre, W. A. Morgan, and C. A. Sternbach, eds., The great American carbonate bank: The geology and economic resources of the Cambrian–Ordovician Sauk megasequence of Laurentia: AAPG Memoir 98, p. 187202.

DOI:10.1306/13331494M983498

Copyright copy2012. The American Association of Petroleum Geologists.

Biostratigraphy of Cambrian and Lower Ordovician Strata in the Llano Uplift, Central Texas

James F. Miller,1 James D. Loch,2 John F. Taylor3

1Department of Geography, Geology, and Planning, Missouri State University, Springfield, Missouri, U.S.A.
2Department of Biology and Earth Sciences, University of Central Missouri, Warrensburg, Missouri, U.S.A.
3Geoscience Department, Indiana University of Pennsylvania, Indiana, Pennsylvania, U.S.A.

ACKNOWLEDGMENTS

We acknowledge, foremost, the generous access to the Threadgill Creek and Lange Ranch sections that the Welge and Lange families continue to provide. We acknowledge the influence of Charlie Bell, Virgil Barnes, James Lee Wilson, and Preston Cloud on our work in the uppermost Cambrian and Lower Ordovician. James F. Miller has received financial support from the National Science Foundation through grants EAR 8108621, EAR 8407281, and EAR 8804352, as well as several faculty research grants from Missouri State University. Numerous students from Missouri State University aided in the collection of trilobites during the last several years. J. R. Derby and S. E. Longacre provided valuable reviews for the manuscript.

ABSTRACT

Decimeter-scale sampling of the Cambrian and the lowermost Ordovician (Sauk megasequence) rocks of the Llano uplift, Texas, has produced a finely resolved biostratigraphic framework based primarily on trilobites and conodonts. Systematically collected trilobites of the Llano Uplift allow recognition of 13 biozones that extend from the Bolaspidella Biozone (Cambrian System, Marjuman Stage) through the Symphysurina Zone (Ordovician System, upper Skullrockian Stage). Systematic collection of conodonts has produced specimens assignable to 13 zones that range from the Proconodontus tenuiserratus Zone (Cambrian System, Sunwaptan Stage) through the Rossodus manitouensis Zone (Ordovician System, upper Skullrockian Stage). The base of the Ordovician System in the Llano uplift, as elsewhere, has been identified by the lowest occurrence of the conodont Iapetognathus fluctivagus and is closely approximated by the lowest occurrences of the cosmopolitan trilobite Juyjuyaspis and the Laurentian trilobite Symphysurinabulbosa.” Although the overlying Ordovician strata of the Ellenburger Group have not been systematically sampled, scattered trilobite collections do establish the approximate positions of the base of the Stairsian Stage (based on Paraplethopeltis) and the base of the Jeffersonian Stage (based on Rananasus and Jeffersonia) in the Tanyard and Honeycut Formations, respectively.

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