ABSTRACT

The outcrop of the Pecan Gap Chalk in northeastern Texas can be divided into two parts at the change in strike from westbound to southbound. The eastern part, about 60 kilometers in length, is composed of chalk with partings and thin beds of marl, which overlie the Wolfe City Sandstone unconformably. The upper contact with the Marlbrook marl varies from disconformable to questionably conformable. The southern part, about 30 kilometers in length, is composed of a silty marl unit, hitherto unnamed, and an overlying chalk and calcarenite with silty quartz. These two units are conformable to slightly unconformable with one another and apparently conformable with the Wolfe City and Marlbrook. At the junction of the two parts of outcrop, there is an area of about 2 square kilometers with exposures of a quartzose arenite and calcarenite that is distinguished by abundant glauconite-phosphorite, abundant foraminifers, coccoliths, and pelecypod prisms, thin, even bedding, and prominent mottles of burrows. These three units are described as new members of the Pecan Gap, and the formation is emended accordingly.

The relations of the three units to one another and to the chalk of the eastern belt is unclear, particularly in the environs of Farmersville, Texas, where the sequence seems discordant with that elsewhere. The appearances of the planktonic foraminifers, Globotruncana ventricosa (White) and G. calcarata Cushman, seem to bear out the stratigraphic complexity of the Farmersville area; elsewhere they suggest absence of the silty marl along the eastern outcrop and approximate equivalence of the chalk and calcarenite of the southern outcrop and the Pecan Gap of the northern part. They indicate that much of the section of the southern part of the outcrop is missing farther south and along the eastern outcrop.