Abstract

Anhydrite cement is common in sedimentary rocks, yet its origin is poorly understood. The high concentration of sulfate in sea water and the lack of appreciable sulfate in most other natural waters suggests that anhydrite cement may have a marine origin, but the relatively late timing of most anhydride cement tends to preclude sea water as the source of sulfate. Anhydrite cement is present in Upper Jurassic sandstones in the Gulf of Mexico as poikilotopic masses in which detrital grains are replaced and as smaller patches that have replaced single detrital grains. Anhydrite is a relatively late cement and postdates all other volumetrically significant authigenic phases, including K-feldspar, dolomite, quartz, and most calcite cements. The expected isotopic composition and trace-element concentration of marine-derived Late Jurassic anhydrite cement is predicted based on analyses of bedded anhydrite and on analyses found in the literature. The expected chemical signature is: delta 34 S ( per thousand CDT) = +16, delta 18 O ( per thousand SMOW) = +14, 87 Sr/ 86 Sr = 0.7069, Sr = 1500-2500 ppm, Ba < 20 ppm, and Mg > 200 ppm. When geochemical analyses of Smackover anhydrite cements are compared to the predicted composition, it is apparent that most of the cement is not of marine origin. Two generations of anhydrite cement have been identified in the East Texas basin on the basis of their strontium isotopic compositions and their strontium concentrations. An early cement may have been derived from slightly modified Late Jurassic sea water, whereas a second group of cements may have precipitated later in the burial history or may represent recrystallization of the first cement. The chemistry of Louisiana and Mississippi basin anhydrite cements indicates that the sulfate was derived from dissolved bedded anhydrite and was reprecipitated in the sandstones from fluids that had undergone extensive water-rock interaction after considerable burial. This study suggests that late anhydrite cements in sedimentary rocks are most likely derived from remobilized calcium sulfate deposits.

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