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Sediments of the Claiborne Group in central Texas were deposited in a variety of nearshore-marine and coastal-plain environments. Fluctuations of the shoreline during middle Eocene time resulted in vertical stacking of 2,600 ft of sediments from contrasting depositional environments. Shoreline fluctuations probably did not result from simultaneous eustatic changes of sea level. They were caused by migrations of large fluvial, fluvio-deltaic, and interdeltaic complexes on the margins of a gently and perhaps uniformly subsiding basin. As a result of migrations of major sediment depocenters, the Claiborne rock record is characterized by high lateral and vertical variability.
Each formation of the Claiborne Group commonly is considered to be the product of a single depositional environment which extended laterally for hundreds of miles. For example, the basal formation of the Claiborne, the Carrizo Sandstone, is generally accepted as having been deposited under fluvial conditions close to the shoreline. This assumption is not borne out by detailed field work. An examination of the Carrizo Formation from Bastrop to Freestone Counties reveals that Carrizo sediments were deposited in fluvial, deltaic, and marine environments. In a single outcrop the Carrizo Sandstone may be shown as having been deposited in both barrier bars and delta distributary channels. Such variability also holds true for sediments from each formation comprising the Claiborne Group.
Petrographic analysis indicates that the sediment source remained constant during deposition of the Claiborne Group. The presence of phyllite and schist fragments together with the heavy minerals kyanite, staurolite, garnet, and zircon indicates that the source terrane was dominantly metamorphic. Evidence for volcanic activity is particularly strong in the Carrizo Formation and the uppermost Claiborne Yegua Formation. Volcanic minerals in these formations include bipyramidal quartz, euhedral apatite, and bentonite, indicating that the Claiborne coastline periodically was subjected to wind-transported detritus.
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