Jay N. Vogt
Normal-marine, subtidal carbonate facies of the San Andres Formation on the Central basin platform in west Texas are completely dolomitized and contain several types of accessory anhydrite. Oxygen isotopes indicate pervasive dolomitization occurred by reflux of CaSO4-saturated brines. Strontium isotopes and electron microprobe analysis suggest that anhydrite precipitated early, probably as a by-product of dolomitization, and has undergone subsequent alteration by strontium-depleted, radiogenic fluids.
Unaltered dolomite is generally a gray, nonporous wackestone containing anhydrite-filled fusulinid molds. Altered dolomite is light brown, porous, and contains fusulinids that are generally well preserved. Around anhydrite nodules, however, the altered dolomite is fabric destructive. Porosity is mostly intercrystalline and intraparticle.
X-ray diffraction data indicate that both dolomite types are well ordered. Electron microprobe analysis of the unaltered dolomite indicates that it is stoichiometric. Oxygen isotopes for the unaltered dolomite average ^dgr18O = +4.7 ^pmil (PDB). Assuming near-surface conditions, the unaltered dolomite formed from a fluid enriched about 4 ^pmil in 18O relative to Permian (Guadalupian) seawater, suggesting a hypersaline origin for the dolomitizing fluid. CaSO4-saturated brines originating from an overlying anhydrite/dolomite sequence were the likely dolomitizing fluids. The altered dolomite is depleted by about 1.3 ^pmil in 18O relative to the unaltered dolomite, indicating stabilization at higher temperatures and/or by an isotopically deplete fluid.
Anhydrite probably precipitated as calcium was released to CaSO4-saturated brines during dolomitization. Anhydrite occurs as replacement nodules, interparticle cement, and as a replacement cement. 87Sr/86Sr values for anhydrite nodules are 0.7071, coinciding with values for the Late Permian. Anhydrite nodules have low average strontium contents and may have originally been gypsum. Other types of anhydrite have radiogenic strontium values that range from 0.7072 to 0.7076 and have a wide distribution of strontium contents (1,000 ppm to over 7,000 ppm by electron microprobe analysis), indicating precipitation from, or recrystallization in, a radiogenic fluid.
Late calcite (average ^dgr18O = -10.4 ^pmil, ^dgr13C = -19.7 ^pmil) replaces some of the anhydrite and is a by-product of bacterial sulfate reduction. Replacement calcite contains less than 500 ppm strontium (ICP analysis) and yields 87Sr/86Sr values of 0.7075 and 0.7080, indicating the influence of a low-strontium, radiogenic fluid during late diagenesis. The origin of the late diagenetic fluid could be either the dewatering of basinal shales or the influx of meteoric waters which have interacted with clastics.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.