The lithologic unit previously considered to be of Sabine (Wilcox) age in the Sabine uplift area of Louisiana is divisible on the outcrop into the conformable beds carrying Wilcox fossils and upper Midway fossils. The Midway fauna occurs throughout several hundred feet of section above the typical Midway shale reflection on electrical logs. Isopach maps and cross sections developed from electrical logs and paleontological reports of deep test wells from east Texas to southern Alabama show that during the time of deposition of the marine Midway and Wilcox of Texas and Alabama, a great deltaic mass, reaching a thickness in excess of 3,000 feet, was accumulating in the Mississippi Embayment of eastern Louisiana and central Mississippi. The presence of interpretable fossil as emblages appearing in beds which interfinger with the deltaic mass along its northwestern margin is important in determining the age of the mass and in field mapping in the Sabine uplift area.

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