Leonardian (Permian) bioherms of the Wilkie Ranch formation, Finlay Mountains, Hudspeth county, Texas

James David Rathjen, University of Texas at El Paso

Abstract

Leonardian bioherms occur in the Wilkie Ranch Formation in the Finlay Mountains of West Texas. These bioherms are exposed at different levels within the 533-m-thick Wilkie Ranch Formation and are the only autochthonous Leonardian exposures of the western Diablo platform/eastern Marfa basin. Bioherms were sampled at three different stratigraphic horizons on the outcrop. Bioherm “F” crops out at 183 m, Bioherm “D” at 297 m, and Bioherm “C” at 357 m; all are measured from the subsurface base of the section. ^ Bioherm “F” was the lowest bioherm sampled in the section. The “F” bioherm is characterized as being the most diverse in terms of numbers and types of marine organisms, has the highest percentage of sponges, and is dominated by the problematic encrustor Tubiphytes . ^ Bioherm “D” was sampled approximately mid-section and is dominated by Tubiphytes encrustations associated with a stressed faunal assemblage. ^ The highest bioherm in the section sampled was the “C” bioherm. It is Tubiphytes-encrusted with a “restricted” faunal assemblage. The dominant animals in this highest bioherm are crinoids. ^ At all three stratigraphic horizons investigated, initial bioherm development began on piles of bioclastic debris which were stabilized by the problematic encrusting organisms Tubiphytes and Archaeolithoporella . Once the substrate was stabilized, subsequent ecological niches were established that were inhabited by a normal marine fauna including sponges, brachiopods, echinoderms, and bryozoa. ^ Differences in the biotic make-up of the bioherms at the different stratigraphic levels reflect differing and changing conditions within the Marfa basin. Optimal conditions existed low in the section as is reflected by the diverse make-up of the fauna. Fluctuations in sea level (possibly third-order sequences) led to deposition of conglomeratic units followed by an overall deepening of the basin in which bioherm development was re-established but with different environmental conditions which resulted in a “stressed” fauna midsection (“D”). Repeated changes of sea level again resulted in an influx of siliciclastics followed by another deepening and renewal of bioherm growth, but now in even less hospitable conditions which resulted in the “restricted” fauna of the uppermost bioherm (“C”). (Abstract shortened by UMI.)^

Subject Area

Geology|Paleontology|Paleoecology

Recommended Citation

Rathjen, James David, "Leonardian (Permian) bioherms of the Wilkie Ranch formation, Finlay Mountains, Hudspeth county, Texas" (1998). ETD Collection for University of Texas, El Paso. AAI9919357.
http://digitalcommons.utep.edu/dissertations/AAI9919357

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