Stratigraphy of Cotton Valley Beds of Northern Gulf Coastal Plain

Abstract

The Upper Jurassic Cotton Valley beds comprise an entirely subsurface sequence of sandstone, shale, and limestone in southern Arkansas, northern Louisiana, northeastern Texas, central Mississippi, and western Alabama. It is herein demonstrated that this sequence, originally defined as a formation, contains two units of formational rank which comprise the Cotton Valley group. The lower of the two formations is herein named the Bossier formation. It consists of dark gray shale and sandstone in north-central Louisiana and southern Arkansas, where it attains a thickness of 1,900 feet, and passes eastward into redbeds in the Monroe uplift of northeastern Louisiana. It extends slightly into southern Arkansas where it is overlapped by the upper formation of the Cotton Valley gro p, herein named the Schuler formation. This formation comprises two members differentiated by lithology and color, and exceeds 2,300 feet in maximum thickness. The lower member, herein named the Shongaloo member, consists of nearshore red shale, sandstone, and conglomerate and equivalent offshore fossiliferous gray shale, limestone, sandstone, and basal conglomerate. The upper member, herein named the Dorcheat member, comprises nearshore pastel, varicolored shale and sandstone, and equivalent offshore fossiliferous gray shale, sandstone, and limestone. Paleontological information contributed by R. W. Imlay indicates that the Bossier formation is middle and upper Kimmeridgian, and that the Schuler formation may be Portlandian and Tithonian.