For reasons of regional relationships, the base of the Wilcox group is best placed at the base of the Seguin formation as redefined by Beckman and Turner, 1943. This formation is a transgressive shallow-marine sequence of glauconitic sands topped by the Caldwell Knob oyster bed. The latter is a biostrome, 0–23 feet thick. The Seguin formation is 92 feet thick in central Texas. The Seguin formation is overlain by the Hooper formation (newly proposed), about 325 feet thick in central Texas. The Hooper is a regressive partly marine, partly non-marine sequence of glauconitic sands and lignitic clays. An erosional disconformity separates the Hooper from the overlying Simsboro sand, a non-marine kaolinitic very coarse sand, about 75 feet thick in central Texas. The Simsboro sand is succeeded by a great mass of non-marine sediment composed of shales, clays, silts, sands, old soils, lignites, channel sandstones, and clay-ironstone lenses. The Butler clay, previously regarded as occupying the stratigraphic position of the Hooper, rests on the Simsboro sand at the Butler clay type locality. Hence the Butler clay is merely the basal part of the Calvert Bluff formation from which it can not be separated on lithologic grounds.
The top of the Wilcox group is placed at the base of the Carrizo sand, a fluviatile sand, which rests with a regional erosional disconformity on the Wilcox. A hilly buried topography exists at this break and is recognizable in the Tyler basin, central Texas, and possibly other marginal updip areas.
The data presented are the work of Ralph Giannone, William W. Sharp, Jr., and H. B. Stenzel.