ABSTRACT

The Catahoula Formation is unique in that it records the last significant influx of volcanic detritus supplied to Gulf Coast sediments. The study area is situated on the northeast flank of the San Marcos arch in Lavaca, Gonzales, and Fayette Counties, Texas. The Catahoula Formation was mapped and subdivided into a basal Chita Sandstone Member (fluvial channel facies) and an overlying and/or laterally contiguous Onalaska Clay Member (floodplain and levee facies). The basal unit of the Chita Sandstone is typically a channel deposit of light gray, conglomeratic sandstone resting unconformably on limonitic claystones and very fine grained sandstones of the late Eocene Whitsett Formation. The basal Chita Sandstone unit consists of graded to poorly laminated beds containing lag deposits of silicified wood and/or mudstone clasts. Chita Sandstone units grade laterally and vertically into tuffaceous, very fine grained sandstone, siltstone, and claystone units of the Onalaska Clay Member which is conformably overlain by channel deposits of early Miocene Oakville Sandstone. The Oakville Sandstone is comprised of lenses of coarse-grained calclithite beds (sandstone containing an abundance of carbonate rock fragments). The Oakville calclithite beds record an episode of early Miocene uplift of the Edwards Plateau along the ancestral Balcones fault zone.

Chita units consist of well sorted to poorly sorted, silty fine-grained to pebbly coarse-grained, lenticular sandstone exhibiting fining upward cycles of festoon cross-beds, plane beds, and ripples. A volcanic provenance contribution is suggested by: 1) abundant volcanic quartz (22% of total quartz); 2) lithic 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 units consist of tuffaceous mudstone and clay-ball litharenite beds.

Differing heavy mineral suites and quartz-feldspar-lithic modes in the Catahoula Formation and coeval Gueydan Formation support the hypothesis that the San Marcos arch was a drainage divide in middle Tertiary time. In the study area north of the San Marcos arch Catahoula sandstones contain a mean Q:F:L ratio of 80Q:5F:15L and a zircon-tourmaline-rutile index of 90%. South of the San Marcos arch (data from Lindemann and McBride) Gueydan sandstones contain a mean Q:F:L ratio of 31Q:28F:41L and a zircon-tourmaline-rutile index of 33%.

Abundant Ca-montmorillonite, reworked biomicrite clasts, montmorillonite rim cement, caliche beds, and wood replaced by length-slow chalcedony in the Catahoula Formation all support an alkaline diagenetic environment. Pervasive opal and chalcedony pore-filling cements are restricted to surficial outcrops and reflect Pleistocene and/or Holocene leaching of volcanic ash under alkaline conditions. It is likely that leaching of volcanic ash in the Catahoula Formation could result in uranium mineralization in Catahoula channel and overbank facies.