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Abstract: Trace Fossil Analysis of the Depositional Environment of a Portion of the Anacacho Limestone, White's Mines Quarry, Uvalde County, Texas
Robert Curtis (1), Walter W. Coppinger (2)
Pholadid borings preserved as internal molds were collected from a narrow stratigraphic interval within the Cretaceous Anacacho Limestone in the White's Mines asphalt quarry near Dabney, in Uvalde County, Texas. Specimens were gathered from an abandoned bench in the quarry where they weathered from an enclosing shaley matrix.
The borings have a sac-like morphology, circular in cross section, with a wide base and narrow neck. The long axis is straight. Maximum diameter is 2 cm; maximum length is 4 cm. Shape is constant throughout the sample population. Many of the molds display a systematic concentric external pattern which extends from the base to midway up the boring. The borings may or may not show the development of an inner lining, which if present, is visible in the texture of the chamber wall. Most of the borings are vacant. However, a small number contain structures which may be nestling organisms or possible original shell material. All are filled with clastic debris dominated by abraided fossil fragments and lithic clasts.
These fossils are indicative of both a low-energy, relatively shallow-water marine environment and the development of a hard substrate. Deposition of this portion of the Anacacho Limestone, therefore, was not uniform and continuous, but was characterized by alternating periods of sedimentation and partial lithification, followed by rapid burial under coarse clastic debris. We suggest a general stable, shallow-water biostromal environment subject to periodic sudden influx of sediment, perhaps the result of storm tides.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Department of Geology, Texas A&M University, College Station, Texas 77843
(2) Department of Geology, Trinity University, San Antonio, Texas 78284
Copyright © 1999 by The Gulf Coast Association of Geological Societies