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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 4. (April)

First Page: 491

Last Page: 491

Title: Emplacement of Nonevaporitic Sulfates in McKnight Formation, Maverick Basin, and Associated Complex Diagenesis: ABSTRACT

Author(s): Alonzo D. Jacka


In the Maverick basin the so-called upper and lower anhydrites of the McKnight Formation (Edwards Group) predominantly consist of nonevaporitic anhydrite which was emplaced within limestones, dolostones, and dedolostones. Most nonevaporitic anhydrite has been emplaced within fossiliferous limestones, which had been subjected to freshwater diagenesis, leaching, and lithification, and in dedolostones.

Anhydrite was emplaced in many grainstone intervals, first as cement and then as replacement of grains. Nodular mosaics of felted-lath anhydrite, emplaced within limestones and dedolostones, closely resemble those formed as evaporites within dolomitic sediments of modern Persian Gulf sabkhas. The most abundantly represented morphology of replacive anhydrite is the blocky porphyroblast with stair-step outlines. Molds formed by dissolution of these porphyroblasts have been misidentified as molds of halite cube aggregates.

Much anhydrite in the McKnight represents a second or third generation, emplaced after dissolution of previous generations. The McKnight contains many dedolostone intervals which probably were produced during episodes of sulfate dissolution. McKnight diagenesis records multicyclic influxes of meteoric groundwater, dolomitizing, anhydritizing, and dedolomitizing fluids. Some McKnight intervals record the sequence of dolomitization, dedolomitization, and partial redolomitization of the dedolostones. Some examples of rededolomitization of redolostones have been noted.

Nonevaporitic anhydrite layers do not occur below true evaporite deposits, and so downward reflux of sulfate enriched brine cannot be invoked as a mechanism of anhydritization. The fact that anhydrite-bearing layers of the upper and lower McKnight alternate with anhydrite-free layers suggests that nonevaporitic anhydrites were emplaced by lateral discharge of calcium sulfate-enriched brines.

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