Abstract

The Trinity Stage is subdivided into 4 operational units, referred to herein as formats, each based on the type section of a recognized formation. The formats, in ascending order, are Pearsall, Rodessa, Ferry Lake, and Rusk. Correlations are extended from the type localities by an electrical log network. Isopach-lithofacies maps show the variation in thickness and composition of the Trinity Stage from Mexico to Florida and of each of the formats from central Texas to Alabama. During Comanchean time the Gulf Coast basin was intracratonic with only part of its perimeter flanked by a shelf. Southward the basin graded into the oceanic depths of the Gulf of Mexico. Trinity rocks were deposited under tectonic conditions which varied from stable shelf near the basin margin through various degrees of stability to intracratonic basinal conditions. From W. to E., the major tectonic elements during Trinity deposition were the San Marcos platform, the E. Texas basin and Henderson embayment, the Panola platform, the Louisiana-Mississippi basin and NW. Louisiana embayment, the Appalachian highlands, the Ocala uplift, and the Florida basin. The transgression of the Lower Cretaceous seas onto the craton in the Gulf Coast area occurred during Coahuilan time. Transgression continued during early Comanchean time throughout most of Trinity deposition. The seas regressed during late Trinity deposition because the balance between the supply of clastic detritus and subsidence altered, allowing the basin to fill and marginal and nonmarine environments to advance basinward. Rocks of the Trinity Stage were deposited as a wedge of sediments thickening downdip from the zero edge to more than 4000 ft. Although the zero edge is erosional, it approximates the original extent of Trinity rocks except in the areas of the Llano and Monroe uplifts. The section grades from a predominantly limestone facies in S. Texas, through a limestone and shale association in E. Texas and N. Louisiana, into a sandstone and shale association in Arkansas, Mississippi, and Alabama. The clastics become coarser toward the margin of deposition and the major source area, the Appalachian highlands. Southeastward from this source the percent of clastics in the section decreases abruptly. In the Florida basin the Trinity Stage is predominantly carbonate with significant amounts of anhydrite. The depositional environments were relatively constant in areas of clastic deposition near shore and the areas of predominantly limestone deposition along the tectonic hinge. The greatest lithological variation resulting from environmental control occurs in E. Texas and N. Louisiana. In this area of intermediate clastic-ratio values early Trinity environments alternated from lagoonal conditions, with varying degrees of restriction in which shales were deposited, to epineritic biostromal conditions. Rocks deposited in these various environments are present in the Pearsall Formation. Extensive biohermal development began along the tectonic hinge during deposition of the Rodessa. Shoreward from the biohermal development the Ferry Lake was deposited in a back-reef lagoonal environment which allowed sufficient concentration of calcium sulfate for the deposition of gypsum. Subsequently the gypsum was altered to anhydrite by pressure of the overlying sediments. The factors restricting the lagoonal environment gradually diminished and normal marine circulation returned to the shelf during Rusk deposition. The alternating beds of dense limestone and shale of the Rusk were deposited in a quiet, off-shore, neritic environment with uniform variation of depositional conditions over a wide area. Clastic material was deposited farther basinward, and biohermal development along the hinge ended as the seas became shallower during late Rusk deposition. The 2 most important zones for the production of hydrocarbons from the Trinity Group are the sands in the upper part of the Rusk and the sands and the porous, oolitic, or fossiliferous-fragmental limestones in the Rodessa. The trend of hydrocarbon production from each of these zones and from others less important, appears to be related to the facies patterns of the respective units. No important production has been found in rocks of the Trinity time-stratigraphic unit beyond the limits of anhydrite in the Ferry Lake format.

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