Influence of Sea-Level Changes on Development, Community Structure, and Quantitative Composition of the Upper Capitan Massive (Permian), Guadalupe Mountains, Texas and New Mexico
Oliver Weidlich, J. A. Fagerstrom, 1999. "Influence of Sea-Level Changes on Development, Community Structure, and Quantitative Composition of the Upper Capitan Massive (Permian), Guadalupe Mountains, Texas and New Mexico", Geologic Framework of the Capitan Reef, Arthur H. Saller, Paul M. (Mitch) Harris, Brenda L. Kirkland, S. J. Mazzullo
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Facies relations determined from outcrop studies and photomosaics have been used in conjunction with reef maps, polished slabs, and wicrofacies data to better understand the depositional facies, quantitative composition, and control mechanisms of the upper Capitan massive, which differs from the outer shelf in its biotic composition and unbedded nature. A three-stage model of the seaward shelf is established comprising (I) a sponge reef/algal cement/phylloid algal stage, (2) a Tubiphytes stage, and (3) prograding cyclic outer shelf beds with isolated reefbuilders Stage 1 is characterized by progressive shallowing as evidenced by a shift from aggradation to progradation, by changes in boundstone composition, and finally by disintegration of the biogenic framework. Stage 2. dominated by Tubiphytes obscurtis. bryozoans, microbes, and small reefbuilders. exhibits a lateral conation triggered by the seaward dipping outer shelf. Stage 3 is composed of cyclic outer shelf grainstones; scattered sponges, and Tuhtphytes microbial-level bottom communities are restricted to few horizons.
The entire depositional sequence is controlled by a third-order sea-level fall as evidenced by exposure horizons. This sequence is in turn superimposed by three high-frequency cyclcs. Water was only deep enough for typical Permian reef types during stage I, which produced the thickest depo- silional unit Relative water depth decreased significantly during stage 2, leading to a small, tabular reef constructed by an impoverished fauna. During stage 3, water was too shallow for reef communities. In contrast to modem reefs, shallowing beyond a distinct level in subtidal depth limited reef growth. In addition, salinity fluctuations indicated by cyclic cementation caused the demise of the Tubiphytes microbial-level bottom communities.
Our quantitative data from the reef maps suggest a reinterpretation of existing models. The most important constructional clement is the microframework (76.7% average coverage), a consortium of low-growing reelhuilders and manne-phreatic cements. In contrast to previous interpretations, neither macro-sponges nor syndepositional cement predominated, making the upper Capitan massive a poor analog for both modern well- skeletonized metazoan and Prccambrian cement reefs.
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The Capitan Formation of southeast New Mexico and west Texas contains one of the world's best exposed and most famous reefs. Depositional and diagenetic models derived from the Capitan have been used to interpret carbonate strata throughout the world. This volume contains 12 state-of the- art papers summarizing major new research on the Capitan, putting the Capitan into a modern statrigraphic, depositional, paleontologic, and diagenetic framework.