Distribution, Petrography and Geochemical Characterization of Radiaxial Calcite and Associated Diagenetic Events in the Capitan Formation, West Texas and New Mexico
M. A. Rahnis, B. L. Kirkland, 1999. "Distribution, Petrography and Geochemical Characterization of Radiaxial Calcite and Associated Diagenetic Events in the Capitan Formation, West 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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The Capitan Formation represents a Permian (Guadalupian) reef complex deposited on the margin of the Delaware Basin. The reef was cemented by aragonite botryoids, radiaxial-fibrous (RFC) and fascicular-optic calcite (FOC), radiaxial-prismatic calcite (RPC), dolomite, and late calcite spar. Variability of fibrous cement textures correlates with the presence or absence of dolomite overgrowths. Zones that contain subequal amounts of RFC and FOC are characterized by radiaxial prismatic terminations lacking dolomite overgrowths, but contain <10 pm diameter dolomite microinclusions, In contrast, zones with <20% FOC and >80% RFC have dolomite cement overgrowths and contain dolomite microinclusions 10-100 pm in diameter.
Trace element concentrations from ICP and stable isotope compositions correlate with stratigraphic depth. Mn concentrations in RFC and FOC increase from an average of 25±2.9 ppm in the upper Capitan Formation to 62±31 ppm in the lower Capitan, whereas average Mg concentration increases from 1.45±0.81 to 2.40±1.37 mole % MgC03 and average Sr concentration decreases from 210±72 ppm to 63±22 ppm. RFC and FOC also contain covariant carbon and oxygen isotope ratios, which trend towards isotopically lighter values from compositions of 5180=-2.8%c, 813C=5.2%o in the Middle Capitan. Microprobe analyses on individual RFC and FOC crystals include the <10 pm dolomite microinclusions. Point analyses traversing crystals from substrate to termination define apparent trends of increase from 1 to 4 mole % MgCOj, and reflect relative abundance of dolomite microinclusions. Strontium and MgC03 do not covary, although both generally increase.
Combinations of RFC, FOC, and RPC form complex cement stratigraphies. Anhedral dolomite microinclusions may be either primary (penecon- temporaneous with host calcite), or secondary (formed during later diagenesis). Euhedral dolomite inclusions within calcite resemble cement overgrowths. Changes in dolomite and kaolinite microinclusion abundance in these cements may reflect porewater changes during early diagenesis. Early diagenesis was probably driven by reef growth (aggradational and progradational), and sea-level change.
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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.