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Book Chapter

The Precambrian of Central Texas

By
Virgil E. Barnes
Virgil E. Barnes
Bureau of Economic Geology, University of Texas at Austin, University Station, Box X, Austin, Texas 78713-7508
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Published:
January 01, 1988

Abstract

“Precambrian rocks reach the surface in the Llano region of Central Texas in the highest part of a broad domal arch, the Llano uplift, which appears on a regional geologic map as an island of igneous and metamorphic rocks surrounded by Paleozoic and Cretaceus sedimentary rocks. The widest expanse of Precambrian rocks is about 65 mi ( 105 km), extending westward from the valley of the Colorado River through a subdued topographic basin drained by the Llano River. The broad, gentle basin carved into the Precambrian rocks is bordered by a discontinuous rim of flat-topped limestone hills which are the dissected edge of the Edwards Plateau. Within the basin and at its margins are erosional remnants and down-faulted blocks of Paleozoic rocks which form prominent hills, locally referred to as mountains.” Clabaugh and McGehee (1972, p. 9).

The main Precambrian units present in the Llano region are listed in Table 1, and the distribution of some of these units is shown in Figure 1. This figure is derived from the Llano Sheet of the Geologic Atlas of Texas, scale 1:250,000 (Barnes, 1981). On that map (as well as Fig. 1), the Oatman Creek and Sixmile Granites (Table 1) are lumped as younger granitic intrusive rocky locally, in the southeast part of the Llano Sheet, the Packsaddle Schist is subdivided into four formations (McGehee, 1979). Detailed 7½-minute quadrangle maps include most of the area in which the Packsaddle Schist has been subdivided (Barnes and McGehee, 1976; 1977a, b, c).

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Contents

DNAG, Centennial Field Guides

South-Central Section of the Geological Society of America

O. T. Hayward
O. T. Hayward
Geology Department, Baylor University, Waco, Texas 76798
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Geological Society of America
Volume
4
ISBN electronic:
9780813754109
Publication date:
January 01, 1988

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