Virtual Landscapes of Texas

55

Gueydan Formation

ville sandstone is cemented by transparent crystalline calcite. The presence of clay pebbles, which rarely occur in the Fay ette except at the top of the formation, the coarser texture, the dominance of quartz over feldspar and the less argilla ceous character of the sandstone are features Which gener ally serve to distinguish the Oakville sandstone from that of the Fayette. Also the Fayette formation frequently carries beds of marine fossils that are indigenous to the formation, while the fossils indigenous to the Oakville are teeth and bones of land vertebrates, especially of Upper Miocene horses. However, small fragments and, occasionally, entire worn shells of Gryphea, Exogyra, and other marine inverte brates which have been washed into these Miocene beds from older sedimentary formations are of common occurrence. The presence of water- worn Cretaceous shells in the Oakville sandstone has been noted by Dumble. 29

A sample of lower Oakville sandstone was collected from the summit of a hill one and one-half miles southwest of Kittie on the Simmons-Kittie road in central Live Oak County. This is a friable, imperfectly cross-bedded to mas sive, light buffish-gray, medium-grained, calcareous sand stone which contains a few rounded pellets of brownish yellow clay up to 1 centimeter in diameter and numerous shell fragments visible to the naked eye. A petrographic description of this rock is given below as an example of the lithology of the Oakville sandstone.

PETROGRAPHIC DESCRIPTION OF LOWER OAKVILLE SANDSTONE FROM iy 2 MILES SOUTHWEST OF KITTIE IN LIVE OAK COUNTY

Sorting. — 2-1 mm. — trace; 1-% mm. — 3%; %-% mm. — 48%; Vi-Vs mm. — 33%; %-l/16 mm. — 4% clay and silt (mostly from clay pebbles) — 12%. The grains are largely subangular.

Composition of Washed Material. — Quartz (includes a few short double-ended pyramidal crystals) — 40%; orthoclase (mostly pinkish in color) — 6%; plagioclase (mainly oligoclase and andesine) — 7%; mi crocline — trace (frequent) ; chert (honey-yellow, pink, gray and black in color) — 20%; calcite (from cement) — 25%; water-worn pelecypod

29 Dumble, E. T., "The Cenozoic Deposits of Texas," Jour. Geol., Vol. 2, p. 557, 1894.