Geological and geophysical studies of a portion of the Little Llano River Valley, Llano and San Saba counties, Texas
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- Geology of the Babyhead - Wilberns Glen Area LLano County, Texas
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- Magnetic survey of the Little Llano River Valley, Lone Grove to Cherokee, Llano and San Saba Counties, Texas
Almy, Charles Coit
Rogers, John J. W.
Master of Arts
A geologic study of the Precambrian Valley Spring gneiss was made in the Babyhead-Wilberns Glen area in northern Llano County, Texas, Magnetic and gravity surveys were made over this and the surrounding areas to determine the relationships of the major rock units: Valley Spring gneiss, Packsaddle schist, associated granite intrusions, and Paleozoic sediments. The area studied lies on the northeast flank of the northwest-trending Babyhead anticline, which forms the major structural control. The Valley Spring gneiss was mapped as four phases: felsic gneiss, mafic gneiss, feldspar-quartz-amphibole-pyroxene gneiss, and biotiteamphibole schist. Two meta-diabase dikes were also mapped. The composition of the Valley Spring gneiss, the constancy of its parallel bands, and its conformity and gradation into the metasedimentary Packsaddle schist indicate a parent rock composed primarily of impure sand stone. Preservation of original structures indicates a lack of mobilized constituents during metamorphism. The Valley Spring gneiss belongs to the quartzo-feldspathic mineral assemblage in the staurolite-quartz subfacies of the almandine-amphibolite facies. After metamorphism the area studied was folded and then intruded by granitic material. From middle Precambrian time to the present, the area studied has under gone only cycles of erosion, deposition, and mild uplift except for faulting at the end of the Paleozoic. The gravity survey indicates a synclinal body of schist trending northwest under the sediments and lying over the Valley Spring gneiss on the northeast flank of the Babyhead anticline. The magnetic survey indicates that the Packsaddle schist does not extend under the sediments to the east, and that mineralization occurred along the llanite dike.