ABSTRACT: Stylolites in the Upper Smackover Formation of North Louisiana

Miles E. Denham, Thomas T. Tieh

Stylolites may be more intimately related to the late stages of diagenesis in carbonate rocks than previously believed. Petrographic and fission track studies of stylolites in subsurface cores of upper Smackover rocks suggest that some of these features may have acted as conduits for late-stage pore fluids and thus have played an active role in the late-stage diagenesis of these rocks.

Stylolites are abundant in the rocks studied, ranging in size from microstylolites--grain to grain sutures--to stylolites a millimeter thick with amplitudes of a centimeter or more. In some of the coarser grainstones, networks of microstylolites have formed that eventually connect to larger stylolites.

Late-stage authigenic phases are present within many of the stylolites. In some stylolites the high abundance of pyrite requires that some was precipitated during or after stylolite formation. Likewise, sphalerite, a common late-stage authigenic mineral, is present in some stylolites. Dolomite rhombs and anhydrite are also associated with stylolites in these samples.

The occurrence of anhydrite is suggestive of late-stage fluid flow along stylolites. Anhydrite replaces the dolomite rhombs, as well as micritized grains within and around stylolites. The degree of this replacement varies from a few anhydrite crystals along a stylolite to large (several centimeters) anhydrite nodules containing traces of dolomite and stylolite-filling organics.

Fission-track studies reveal that uranium is enriched in stylolites compared to their host rocks. Fission-track densities of two stylolites show their minimum uranium concentrations to be 35 and 60 ppm, respectively, compared to host rock concentrations of 1-3 ppm. This enrichment of uranium may result from the migration along stylolites of uranium-rich fluids associated with hydrocarbon maturation. The above observations, though not conclusive, suggest the possibility that fluid migration along existing stylolites occurred during the late stages of diagenesis.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990