ABSTRACT

The last large-scale transgression of the Tertiary seas in northeast Texas began with the deposition of marine clays of the Wills Point Formation (upper Midway Group) over the carbonate sediments of the Tehuacana Member of the Kincaid (basal Midway Group).

The Wills Point Formation, divided into basal Mexia Member, Kerens Member, and topmost Solomon Creek Member, has been studied from shallow well and outcrop sections in an attempt to interpret the paleoecology of the Foraminiferida and to delineate the boundaries of the members. Paleoecologic interpretations are based on: (1) the number of species of Foraminiferida per sample; (2) the number of specimens per 1 gram sample; and (3) the characteristic dominance of the fauna, whether benthonic or planktonic.

By employing the criteria set forth by Walton (1964) that a decrease in number of species of Foraminiferida indicates an approach toward marginal-marine conditions, and an increase in numbers of species indicates a marine transgression, the writer postulates that at the beginning of deposition of Mexia sediments, the sea fluctuated gently and increased in depth with open marine conditions prevailing. Maximum depth no greater than 50 fathoms is indicated for the section up to 165 feet above the base of the Mexia Member. Upward from this level, a gradual shallowing of water is indicated until 270 feet above the base of the Mexia, where the water is less than 10 fathoms deep.

The decrease in number of species and the increase in dominance of the Haplophragmoides-Ammobaculites fauna suggests that marginal-marine conditions of an intertidal to 2-fathoms facies existed from 270 to 445 feet above the base of the Mexia.

A further regression with fluctuating brackish to non-marine conditions caused the deposition of thinly-bedded glauconitic sandstones between cross-bedded massive sandstones, and large flat-topped ferruginous concretionary layers 12 to 24 inches thick. At the top of the section studied, 635 feet above the base of the Mexia Member, a thin section of grey clay containing an arenaccous fauna of 5 species was deposited in shallow brackish water.

The top boundary of the lowermost Mexia Member of the Wills Point Formation is placed approximately 165 feet above the base of the Mexia Member where the abundance of planktonic Foraminiferida drops to 6% and, with one exception at 360 feet, stays at or below that figure.

The top boundary of the Kerens Member is placed at the top of the continuous shallow brackish-water, wholly arenaceous fauna 445 feet above base of Mexia Member. The brackish to non-marine, non-foraminiferidal section above is included in the Solomon Creek Member, whose upper boundary cannot be determined. The massive sandstone and overlying bed of Ostrea duvali which form the Caldwell Knob, lowermost Member of the Wilcox Group, are not present along the Trinity River in the Navarro-Henderson Counties area.