ABSTRACT

The Midway group of Texas, a predominantly shale, sandy shale, and sandy limestone section of Paleocene age, was deposited in an open sea whose depth fluctuated between very shallow and deep. This concept is based on a study of samples from Tehuacana Creek, Limestone County, in east-central Texas. The interval sampled extends upward for 278 feet from the Cretaceous-Paleocene contact. Lithologic examination revealed a complete cycle of deposition, with a basal glauconitic sand overlain successively by sandy shale, sandy limestone or limy sandstone, glauconitic sand, and shale.

The foraminiferal content indicates that the very lowest beds of the Midway (Littig member of Kincaid formation) are marine, not littoral, probably mid- to outer neritic. Seemingly the unconformity between this section and the Cretaceous developed without the latter being exposed subaerially. Biostratigraphic and paleoecologic interpretation demonstrates that the shales and sandy shales of the lower part of the succeeding Pisgah member were deposited in water deeper than 200 meters. As the water became more shallow, sands and calcareous muds interbedded with sands of the upper Pisgah and Tehuacana member were deposited. This shallow water environment favored an abundant growth of a few species of Foraminifera. The Mexia member (lower unit of Wills Point formation) represents a gradual return to a deeper water environment characterized initially by a glauconitic sand followed by glauconitic muds and a rich fauna of both deep and shallow water species. The dark pyritic muds of the upper part of the Mexia member were deposited in deep water (below 200 meters). The fauna is rich in variety of genera and species of Foraminifera but poor in number of individuals.

The Midway group of Texas (=Danian) has been included in the Globigerina compressa-daubjergensis zone by Loeblich and Tappan. An analysis of the distribution of Foraminifera showed that 4 zonules can be established at this locality. They are, numbered from the base upward:

4. Marginulina tuberculata (Plummer) zonule

3. Polymorphina cushmani Plummer zonule

2. Discorbis newmanae Plummer zonule

1. Alabamina midwayensis Brotzen limbata (Plummer) zonule.

Figure 1. Areal extent of Midway outcrops in Gulf Coastal Plain. Arrow points to Tehuacana Creek locality.

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