- Copyright © 2008, SEPM (Society for Sedimentary Geology)
Tepees are common depositional structures within the shelfal Capitanian Stage Yates and Tansill carbonate strata of the Permian Reef Complex of the Guadalupe Mountains. As part of the tepees, botryoids of radiating aragonite crystals formed essentially contemporaneously with the host pisolitic strata. Although most of the cement in the tepees is composed of diagenetic low-magnesium calcite, X-ray diffraction analyses indicate that aragonite constitutes as much as 27% within centimeter-size patches. Back-scatter electron microprobe analyses demonstrate a highly irregular pattern of alteration of partly preserved aragonite crystals. These patches with the preserved aragonitic fibers have undetectable cathodoluminescence, Sr concentrations that range up to 3,900 ppm, and 87Sr/86Sr ratios from 0.70689 to 0.70695, which are in agreement with worldwide marine values for the Capitanian Stage. Mass-balance calculations show that in some samples the diagenetic low-Mg calcite associated with the preserved aragonite has over 2,300 ppm Sr; thus, the diagenesis to calcite occurred under very low water-to-rock ratios.
The least altered of the aragonitic cements have δ13C values as high as 7.8‰ V-PDB (equivalent to ~ 6‰ V-PDB for calcite) and δ18O values as low as −5.5‰ V-PDB (equivalent to −6.2‰ V-PDB for calcite). The high δ13C values are consistent with worldwide marine values whereas the very low δ18O values probably represent precipitation in hot, equatorial, shallow marine to tidal-flat environments. Contrary to most diagenetic pathways displayed by marine carbonates, increasing alteration has generally resulted in higher δ18O values in these relict Permian aragonites.
These analyses demonstrate conclusively that Paleozoic aragonitic cements are still preserved in some strata.