Paleoenvironments, Cyclicity, and Diagenesis in the Outer Shelf Tansill Formation in the Carlsbad Embayment (Dark Canyon), Northern Guadalupe Mountains, New Mexico
S. J. Mazzullo, 1999. "Paleoenvironments, Cyclicity, and Diagenesis in the Outer Shelf Tansill Formation in the Carlsbad Embayment (Dark Canyon), Northern Guadalupe Mountains, 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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Outer shelf carbonates in the Tansill Formation in Dark Canyon include the following inferred depositional environments landward of the Capitan reef: (1) subtidal: bioclastic wackestones, packstones, and graded and locally cross-stratified grainstones; and biostromes; (2) peritidal flats: fenestral and locally desiccated mudstone; and (3) shoreface: admixed subtidal and peritidal deposits. Two sequences are recognized in the section based on biotic diversity and parasequence thickness and facies-stacking patterns, and the boundary between them is at or near the base of the Ocotillo Member. Maximum flooding of the platform occurred during deposition of sequence 1 in the lower part of the middle Tansill. Environments were biostromes, mainly high energy shallow subtidal packstones and grainstones, and associated peritidal islands in the early HST in this sequence. Precipitation of abundant marine cements in these rocks may have been promoted by active seawater pumping through the sediments on this wide and shallow shelf. Microbial activity in the grainstones may have been promoted by restricted circulation around associated peritidal islands. In contrast, environments in the late HST of this sequence and in sequence 2 were more restricted in terms of shelf width and energy, and the rocks are dominantly micritic and contain little marine cement. The principal difference between depositional facies in the Carlsbad Embayment and along the Reef Escarpment appears to be the presence of patch reefs in the former area, which were deposited on an outer shelf that was wider and of more open circulation than to the south.
Inferred marine cements are dominated by prismatic calcites, with microdolomite inclusions and some recognizable radiaxial-fibrous habit, interpreted as former high-Mg calcite. The 8I80 and 813C compositions of the least-altered of these cements (−1.6 %c and 5.8 %o PDB, respectively) suggest precipitation from marine pore fluids. Former aragonitic cement of similar isotopic composition is a volumetrically minor, first-generation marine cement in these rocks. Dominance of inferred former high-Mg calcite cement contrasts that in coeval platform-margin patch reef facies, the Capitan reef, and in pisolites on the shelf crest in which former aragonite marine cement dominates. Mean 8180 composition of the dolomite that replaced peritidal deposits (0.1 %c PDB) suggests that it precipitated from ambient marine fluids of elevated salinity. Mineralogic stabilization of earlier diagenetic phases likely attended precipitation of equant calcites (spar 1) in the rocks, which is interpreted to have occurred in a meteoric phreatic system during the latest Guadalupian and/or early Ochoan. A second generation of equant cements (spar 0) is inferred to have precipitated later, during the Ochoan or later, in a deeper meteoric-dominated groundwater system. Replacement of syndepositionally precipitated evaporites by poikilotopic calcite is the last diagenetic phase present and, based on stable oxygen-carbon isotope data, appears to have attended meteoric dedolomi- tization during the Tertiary.
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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.