The Association of Calcite Veins with Stylolites in the Smackover Formation of North Louisiana

DENHAM, MILES E., and THOMAS T. TIEH, Texas A&M University, College Station, TX

Small calcite veins in the grainstones of the Upper Smackover Formation of north Louisiana provide insight into the role of stylolites in the late stages of diagenesis of these carbonate rocks.

The veins always occur in groups, ranging from a few to swarms. In the two cores examined, vein widths varied from 5 micro m to 1 mm and lengths varied from 100 micro m to several millimeters. Most veins have one end that terminates at a stylolite; some have both ends terminated by stylolites. The same vein never appears on both sides of a large stylolite, although some veins do cut smaller stylolites. Sets of parallel veins occur at angles to each other in the same swarm--one set cutting the other.

The original mineralogy of the veins is calcite. It appears lenticular in thin-section with the long axis of the grains perpendicular to the vein length. Preliminary electron microprobe analyses show the calcite to be relatively pure, containing less than 1% by weight MgCO(3). However, the calcite does contain slightly more iron than the surrounding micritic matrix. Anhydrite and dolomite frequently replace the calcite in the vein. This replacement generally occurs at the terminus of a vein in a stylolite. Dissolution of the calcite in the veins to produce fracture porosity is also common.

The veins are remnants of localized episodic fluid pressure increases that occurred during the latter stages of diagenesis of the Smackover.

variation of slope-ubiquitous species is, perhaps, an even more promising approach to further subdivision of the bathyal realm.

All of these studies utilize modern models of depth relationships but such models become less precise in increasingly older strata. To address this problem, time-specific paleobathymetric models can be derived for fossil foraminiferal assemblages organized along datum planes. Sea-floor topography for a particular datum is reconstructed from the depositional architecture evident on seismic record sections. Paleoenvironmental and/or paleobathymetric zones, based on the foraminiferal assemblages encountered along this profile, can then be defined and compared to models based on modern distribution data.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)