--> ABSTRACT: Time Constraints on Sulfate-Related Diagenesis, Capitan Reef Complex, West Texas and New Mexico, by Gill Darke, Gill Harwood; #91003 (1990).

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ABSTRACT: Time Constraints on Sulfate-Related Diagenesis, Capitan Reef Complex, West Texas and New Mexico

Gill Darke, Gill Harwood

Petrographic, paleomagnetic, and outcrop studies of the middle Capitan Reef Complex in the Guadalupe Mountains have provided time constraints on diagenetic events and demonstrated the crucial role of calcium sulfate. Sulfate emplacement occurred at an early stage. The sulfate emplacement post-dated and replaced syndepositional marine cements and grains and also occurred along early fractures. Castellated margins and lath forms demonstrate replacement was by anhydrite rather then gypsum. Fabric-selective dolomitization and kaolinite precipitation accompanied this initial hypersaline brine migration. The brine was derived from reworking shelf evaporite sequences moving downdip during early stages of drawdown within the Delaware basin.

A second period of brine migration, causing sulfate are emplacement and dolomitization, occurred as the Delaware basin gradually filled with the Castile evaporites, when all remaining porosity within the Capitan shelf margin became indurated by calcium sulfate-laden fluids. This caused pervasive dolomitization, particularly in the lower foreslope, with formation of dolomite rhombs and overgrowths on earlier dolomitized marine cements, coeval with replacive clusters of anhydrite.

Most porosity was plugged, some with syndepositional marine cements, but the greater proportion with evaporites until uplift in the Tertiary. Then a meteoric groundwater system became established with subsequent sulfate dissolution. Minor sulfate reduction formed iron sulfides. This oxidized to hematite, which was enclosed within a first generation of zoned calcite spar along some pore margins. Most hematite has a paleomagnetic age of 20 Ma, although minor hematite formation continues to the present. A second, also zoned, coarser calcite spar generation was followed by the latest, nonluminesent calcite spar. These calcite spars form the vast bulk of that visible at outcrop.

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