PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS ON THE FORMATION AND DIAGENESIS OF ANHYDRITE CEMENTS, SMACKOVER SANDSTONES, GULF-OF-MEXICO

Anhydrite cement is common in sedimentary rocks, yet its origin is poo rly understood. The high concentration of sulfate in sea water and the lack of appreciable sulfate in most other natural waters suggests tha t anhydrite cement may have a marine origin, but the relatively late t iming of most anhydrite cement tends to preclude sea water as the sour ce of sulfate. Anhydrite cement is present in Upper Jurassic sandstone s in the Gulf of Mexico as poikilotopic masses in which detrital grain s are replaced and as smaller patches that have replaced single detrit al grains. Anhydrite is a relatively late cement and postdates all oth er volumetrically significant authigenic phases, including K-feldspar, dolomite, quartz, and most calcite cements. The expected isotopic com position and trace-element concentration of marine-derived Late Jurass ic anhydrite cement is predicted based on analyses of bedded anhydrite and on analyses found in the literature. The expected chemical signat ure is: deltaS-34 (parts per thousand CDT) = +16, deltaO-18 (parts per thousand SMOW) = +14, Sr-87/Sr-86 = 0.7069, Sr = 1500-2500 ppm, Ba < 20 ppm, and Mg > 200 ppm. When geochemical analyses of Smackover anhyd rite cements are compared to the predicted composition, it is apparent that most of the cement is not of marine origin. Two generations of a nhydrite cement have been identified in the East Texas basin on the ba sis of their strontium isotopic compositions and their strontium conce ntrations. An early cement may have been derived from slightly modifie d Late Jurassic sea water, whereas a second group of cements may have precipitated later in the burial history or may represent recrystalliz ation of the first cement. The chemistry of Louisiana and Mississippi basin anhydrite cements indicates that the sulfate was derived from di ssolved bedded anhydrite and was reprecipitated in the sandstones from fluids that had undergone extensive water-rock interaction after cons iderable burial. This study suggests that late anhydrite cements in se dimentary rocks are most likely derived from remobilized calcium sulfa te deposits.