Basal Claiborne of Texas, Record of Appalachian Tectonism During Eocene

Abstract

The Carrizo formation and the superjacent Newby member of the Reklaw formation (Eocene, Claiborne group) are lithologically similar sandstone units which crop out in Bastrop County, central Texas. The basal Carrizo is a strongly cross-bedded, poorly to moderately sorted, granular medium sandstone; it grades upward into moderately to well sorted fine sandstone, very fine sandy siltstone, and silty carbonaceous shale. The average lower Newby sample is a moderately to well sorted fine sandstone, and the middle Newby section includes clayey siltstone and carbonaceous shale beds which grade upward into cross-bedded, glauconitic sandstone lenticularly laminated with chocolate shale. Both Carrizo and Newby units are texturally submature to mature chert-bearing subgraywackes containing 65-75 per cent quartz, 5-10 per cent potash feldspar, 5-25 per cent phyllite, slate and metaquartzite fragments, and 5-10 per cent chert; there is little or no clay or silt matrix. The non-opaque heavy-mineral fraction consists of subangular kyanite and staurolite, angular and rounded zircon and tourmaline, and smaller amounts of garnet and rutile. Glauconite and authigenic feldspar are common in the upper, more marine part of the section. Most of the Carrizo and Newby detritus in Texas was originally derived from erosion of parts of the Ouachita foldbelt and particularly from uplift of the southern Appalachian Mountains--not from the Laramide orogenic belt as has been previously supposed. Sources north and west contributed reworked sedimentary material, but volcanic quartz and some feldspar were probably transported to the area of Bastrop County from a local source on the south. In Texas the Carrizo formation rests disconformably on the Wilcox (Eocene), and is placed in the Claiborne group. On the Sabine uplift and eastward, the Carrizo reportedly lies conformably on the Wilcox, and is considered the uppermost formation of the Wilcox group. This apparently local disconformity was probably produced by uplift and erosion of an uppermost Wilcox beach sand in south and central Texas. Erosion of this beach produced the subangular, highly polished grains which were reworked to form a large part of the Carrizo and Newby sandstones in Bastrop County. The region of maximum uplift may exist as a subsurface Wilcox high between Bastrop County and the Rio Grande embayment. On the basis of this study, augmented by petrographic examination of scattered samples throughout the rest of Texas Eocene sediments, a large-scale uplift of the southern Appalachian area is believed to have begun during the Midway or Sabine age and culminated during deposition of the great volume of upper Wilcox and lower Claiborne sediments in the western Gulf Coast.