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In the central Texas coastal plain, the Catahoula Formation consists of a basal Chita Sandstone Member (fluvial-channel facies) and an overlying and/or laterally contiguous Onalaska Clay Member (flood-plain and levee facies). The basal unit of the Chita Sandstone is typically a channel deposit of ash-gray, conglomeratic, medium to coarse-grained sandstone in graded to poorly laminated beds containing silicified wood chips. Fining-upward sequences of moderately sorted sandstone units consist of (McBride classification): sublitharenite (42%), quartz-arenite (29%), subarkose (16%),
and litharenite (13%). Modal analyses of 41 thin sections show that framework grains consist of quartz (80%), rock fragments (15%), feldspar (5%), and trace amounts of micas and heavy minerals. Porosity of Chita Sandstone units ranges from 0 to 35%; mean percent of cement (mostly chalcedony, cristobalite, and opal) is 12.8%.
The Catahoula Formation is unique in that it records the last significant influx of volcanic detritus supplied to Gulf Coast sediments. Volcanic contributions include (1) abundant volcanic quartz (22% of total quartz); (2) rock fragments consisting mostly of silicic shards, felsite clasts, and tuffaceous clay clasts; (3) fresh sanidine (sanidine/orthoclase ratio = 1.2); and (4) a heavy mineral suite dominated by euhedral, elongated zircons. Onalaska Clay consists of mudstone and clay-ball lithoarenite beds composed mostly of montmorillonite, volcanic ash, and quartz silt. Volcanic ash in the Onalaska Clay is a likely source of most of the uranium mineralization in contiguous sandstone units. Silica leached from volcanic ash in the Onalaska Clay has been redeposited as pore-filling seq ences of chalcedony (length slow and length fast), cristobalite, and opal cement in Chita Sandstone units.
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