Petrology of the Catahoula formation (Miocene) in northeastern Gonzales County, northeastern Lavaca County, and southwestern Fayette County, Texas



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The Miocene Catahoula Formation consists of the Chita Sandstone Member overlain by and frequently interfingering with the Onalaska Clay. Characteristically, the basal unit is an ash gray, conglomeratic, medium- to coarse-grained sandstone, stratified in irregular thin-beds and containing chips of silicified wood. Catahoula Sandstone rests un- conformably on limonitic clays and fine-grained sandstones of the Eocene Whitsett Formation. The Miocene Oakville Sandstone (fluvial, conglomeratic, medium- to coarse-grained calclithite) unconformably overlies the Onalaska Clay (silty to fine-sandy bentonitic clay of ash gray color). Petrographic interpretations were based primarily on a study of forty thin-sections. In decreasing order of abundance the following sandstone types (McBride classification) were found to be present: Sublitharenite (42%), quartzarenite (29%), subarkose (16%), tuffaceous litharenite and clay-ball litharenite (10%), and feldspathic litharenite (3%). The mean phi-diameter of all Catahoula sandstones analyzed is 1.98 phi. The majority of the sandstones are moderately well sorted; average standard deviation is O.63 phi. The mean roundness is 2.53 (subangular). The average sample is submature, although eleven sandstones were found to be immature. Both friable and "quartzitic" highly cemented sandstones were analyzed; the percent of cement averages 12.8%.