ABSTRACT

The Simsboro Formation of the Wilcox Group between the Trinity and Brazos Rivers, Texas, is characterized by white, coarse- to fine-grained, cherty, feldspathic, muscovitic sand in a white clay matrix, and minor beds and lenses of white to light-gray, silty clay. The clays and sands are of continental origin. The clay mineral suite consists of kaolinite, illite, and a very small amount of mixed layered illite-montmorillonite, whereas calcium montmorillonite is the most common clay in the overlying Calvert Bluff Formation and underlying Hooper Formation.

The Simsboro is distinct from the light- to dark-gray, carbonaceous, waxy, plastic clays of the Hooper Formation, which also contains lenses of buff, fine-grained sand, ironstone slabs, and ferruginous concretions.

It is also distinct from the Calvert Bluff Formation which is composed of interbedded, thinly laminated, buff to light-gray silt and light-gray clay; fine-grained, cherty sand, light-gray, plastic clay; lignite; and ironstone slabs and ferruginous concretions 0.5 to 5 feet in diameter.

Weathering characteristics are used to distinguish the montmorillonitic Hooper and Calvert Bluff Formations from the predominantly kaolinitic Simsboro Formation.