Petrology of the Presidio formation and equivalent units; Presidio County, Texas



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The Lower Cretaceous (Albian-Aptian) Presidio Formation and its equivalents, the Yucca and part of the Bluff Formations, consist of 150 to 285 m of interbedded carbonate and terrigenous strata. These units were deposited in the Chihuahua Trough on the western flank of the Diablo Platform. The Presidio and equivalent formations consist of seven lithofacies; 1) conglomerate; 2) dolomite; 3) micrite, including biomicrite, intramicrite, pelmicrite, and oomicrite sublithofacies; h) sparite, including oosparite, biosparite, and. intrasparite sublithofacies; 5) poorly-washed sparite, including poorly-washed biosparite, poorly-washed intrasparite, and poorly-washed oosparite sublithofacies; 6) oncolite-bearing sparite and poorly-washed oosparite; and 7) sandstone and siltstone, including units that are horizontally or near horizontally laminated and rippled cross-stratified, units predominantly trough cross-stratified, and units exhibiting a sequence of sedimentary structures. The strata of the Presidio Formation represent sediments that were deposited in several paralic envirounents. The conglomerate units accumulated as a basal lag on the underlying Paleozoic strata. As Cretaceous seas transgressed the Diablo Platform, the upper part of the conglomerate was reworked. Carbonate rocks overlying the basal conglomerate represent three major nearshore environments; tidal flat, lagoonal, and shoal. Sandstone and siltstone units were deposited as point bars in fluvial channels that migrated across the carbonate tidal flat and as deltaic distributary mouth bar and distal bar sediments within the lagoon. Beach and shoreface deposits were developed adjacent to the delta complex through the reworking of deltaic sands. The Presidio and equivalent formations were deposited during a major Cretaceous transgression. However, the transgressive nature of the basin and the progradation of carbonate and deltaic environments during sea level stillstands resulted in a complexly interbedded sequence of rocks that comprise the Presidio and equivalent formations.