Cenomanian-Turonian rocks in Trans-Pecos Texas and adjacent Mexico consist of thin shelf limestone and corresponding thick basin argillite. The lithic differences between the two gross rock bodies are due to deposition, respectively, upon the Diablo and Coahuila platforms and within the Chihuahua trough southwest of the Diablo platform. These rock bodies are correlated first lithologically. Paleontologic correlation is based on separate zonations of the limestone shelf facies and the argillaceous basin facies. A rough real delineation of the different facies followed by locally precise correlation between the areas aids in locating the northeastern edge of the Chihuahua trough during the Late Cretaceous Epoch.