ABSTRACT

The several distinct lithofacies of the Late Jurassic Cotton Valley Group in Louisiana and Arkansas are presently masked by accepted stratigraphic terminology. Although the existing, subdivisions have been adequate, recent dense well control necessitates new stratigraphic nomenclature to emphasize lateral and vertical lithologic relationships.

New nomenclature, to emphasize lithologic variation, is proposed here for the Upper Jurassic in Louisiana and Arkansas. In Louisiana the Cotton Valley Group is subdivided here into four formations. The Knowles Limestone (new) at the top is a tabular unit of limestone and shale. Forty miles south of the Arkansas-Louisiana boundary the massive, white Terryville Sandstone (new) underlies the Knowles. Toward the north the Terryville Sandstone descends stratigraphically and the northward-thickening Hico Shale (new) intervenes between the Terryville and Knowles formations. Five distinct tongues of sandstone extend northward from the massive Terryville Sandstone into the gray Hico Shale. The Terryville Sandstone is underlain by the Bossier Formation (restricted) that pinches out northward beneath the progressively older sandstone. The entire gray shale--white sandstone sequence of Louisiana grades northward in Arkansas into a maroon shale and sandstone succession for which the name Schuler Formation (restricted) is retained.