A petrographic study of the Anacacho Limestone (Upper Cretaceous) of Texas



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The Upper Cretaceous Anacacho limestone is exposed in scattered outcrops from eastern Kinney County to western Bexar County. The formation unconformably overlies the Austin chalk and is overlain unconformably by the Escondido clays, occupying the stratigraphic position of the Taylor group. A total of thirty-six rock specimens were taken from seven localities along the outcrop of the Anacacho limestone. Thin sections were made from fourteen of the specimens, Insoluble residues were run on seven of the specimens, and X-ray diffraction patterns were made of three specimens. A detailed study of the fabric, mineralogy, and porosity was made of each specimen. In eastern Kinney County, the limestone members of the formation consist of an alternating succession of strata possessing the fabric of biohermal reef rock and detrital reef rock. The middle, "Milam chalk", member consists mainly of chemically precipitated calcite with some clay and detrital quarts^ suggesting a lagoonal environment. The actively growing portion of the reef probably migrated back and forth, producing an interfingering of biohermal reef rock, detrital reef rock, and chalk. Eastward from Kinney County to western Bexar County the Anacacho limestone possesses the fabric of detrital reef rock, and is overlapped from the east by the Taylor marl. The absence of biohermal reef rock from the eastern portion of the Anacacho limestone suggests that the actively growing portion of the reef existed north of the present outcrop and has been removed by erosion. The principal fossils in the Anacacho reef are pele- cypods and bryozoans. Of the porosity visible under the binocular microscope, that which is definitely secondary is many times as great as that which is possibly primary. It was impossible to demonstrate conclusively that any of the porosity is truly primary. In the abundantly asphaltic portions of the Anacacho, the contact between the asphaltic and nonasphaltic rock was seen to cut across the bedding both in hand specimens and in gross aspect in the asphalt mines. The cutting of the bedding planes by the asphalt contact suggests that the asphalt invaded the limestone and was not deposited contemporaneously with it. The fact that the asphalt does not entirely fill the pore space of the rock, but forms a coating on the limestone fragments suggests that the asphalt is the residuum of a lighter oil which lost its volatile constituents.