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

The Wills Point Formation, divided into basal Mexia Member, Kerens Member, and upper Solomon Creek Member, has been studied from shallow-well and outcrop sections in an attempt to interpret the paleoecology of the Foraminifera and to delineate the boundaries of the members. Paleoecologic interpretations are based on: (1) the number of species of Foraminifera 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 Foraminifera 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, sea-level increased gradually in depth with open-marine conditions prevailing. A maximum depth of no greater than 50 fm. is indicated for the section up to 150 ft. above the Tehuacana Member. Above this level, a gradual shallowing of water is indicated until, 330 ft. above the Tehuacana, the water was less than 10 fm. 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-fm.-deep facies existed from 330-460 ft. above the Tehuacana.

A further regression with fluctuating brackish to non-marine conditions caused the deposition of thin-bedded glauconitic sandstones between cross-bedded massive sandstones, and large flat-topped ferruginous concretionary layers 12-24 in. thick. At the top of the section studied, 687 ft. above the Tehuacana member, a thin section of gray clay containing an arenaceous fauna of five species was deposited in shallow brackish water.

The upper boundary of the Mexia Member of the Wills Point Formation is placed approximately 180 ft. above the top of the Tehuacana Member where the abundance of planktonic Foraminifera decreases to 6% and, with one exception at 360 ft., stays at or below that figure.

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

End_of_Article - Last_Page 1753------------