Abstract: Deposition and Diagenesis of Fort Terrett Formation (Edwards Group) in Vicinity of Junction, Texas

Alonzo D. Jacka

Excellent exposures of the carbonate rocks of the Fort Terrett Formation (Edwards Group) are present along new roadcuts of Interstate 10 in the vicinity of Junction, Texas. The Fort Terrett contains well-developed depositional cycles, most of which consist of supratidal, intertidal, and subtidal facies. The Fort Terrett also exhibits complex diagenetic cycles that include calcitization, dolomitization, sulfate emplacement and dissolution, silicification, and some dedolomitization. Depositional and diagenetic patterns reflect superimposition of the following processes: (1) eustatic changes in sea level and associated climatic fluctuations; (2) seaward progradations of supratidal, intertidal, and subtidal facies; and (3) subsidence.

Limestone intervals exhibit abundant evidence of having stabilized in freshwater diagenetic environments. Supratidal deposits represent sabkhas and were penecontemporaneously dolomitized according to the Persian Gulf model. In many cycles, portions of subtidal facies also have been dolomitized, possibly in zones of mixing at the bases of freshwater lenses.

Primary porosity was formed in grainstones, but very little was preserved. Much secondary porosity was formed in limestone intervals through selective dissolution of crystalline aragonitic shells and ooids. Most secondary porosity in limestones has been occluded by calcite cements, but some has been selectively preserved within hollow micrite envelopes, on micritized foundations, and in large micrite-walled solution vugs. Secondary intercrystalline and moldic porosities were formed and extensively preserved in dolostone intervals.

Much porosity was created within collapse breccias, which were formed through dissolution of sulfates. In thin collapse breccias, most porosity has been occluded by deposition of internal sediment and meteoric cements. Considerable porosity still exists in a thick collapse zone in the upper Fort Terrett.

Tertiary porosity is very well developed and extensively preserved in most dolostone intervals and in many limestone units. Tertiary porosity was created through dissolution of sparry anhydrite which extensively replaced limestones and dolostones after secondary voids had been formed and filled by cements. Pulverulent limestones and dolostones represent soft, powdery materials that once contained exceedingly high concentrations of sparry replacement anhydrite.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas