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The Gilmer Limestone: Oolite Tidal Bars on the Sabine Uplift
Wayne M. Ahr (1)
Studies of cores, cuttings, and sample logs have shown that the Cotton Valley Limestone, also known as the "Gilmer Limestone," consists of a linear belt of oolitic and pelletal grainstones and packstones along the margin of the Sabine uplift. This lime-sand belt extends from Gilmer field in Upshur County southward at least as far as Overton field in Smith County. The oolite-grainstone sequences with leached, "chalky" porosity are restricted to the north-south trend and are replaced by lime-muddy, nonporous rocks to the east and west.
The high percentage of ooids, the abundant festoon and tabular cross-beds in the grainstone belt, and the linear shoal anatomy of the unit suggest that the Gilmer Limestone is an ancient analogue of the tidal bars in the modern Bahamas. The Gilmer grainstones formed as a series of submarine bars that accumulated in the shallow, agitated water along the flanks of a peninsular shoal (the Sabine uplift). Because the rocks formed as a series of tidal bars, there are local variations in thickness and lithic character along the trend.
The Gilmer lime reservoir exhibits "chalky" porosity that resulted from leaching during regressive cycles in the Cotton Valley. Oomoldic porosity and dolomite are rare to absent, and permeability is low because most of the voids are intragranular rather than intergranular. The leaching appears to have been most extensive along the tops of the thickest oolite shoals.
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