The Eagle Ford Group of the Upper Cretaceous Gulfian Series is one of the most stratigraphically complex terrigenous units in the East Texas basin. At the type locality in Dallas County, Texas, the Eagle Ford consists of bluish-black, carbonaceous sediments exceeding 400 ft in thickness. In this area, the Eagle Ford includes the Tarrant, 15-20 ft of brownish-gray calcareous sandstone, the Britton, 250-300 ft of interbedded brown calcareous mudstone, and the Arcadia Park, 100-200 ft of dark gray calcareous mudstone.

Eastward into the basin, the Eagle Ford thickens to 500 ft as the upper Eagle Ford acquires 100 ft of Lake Crockett terrigenous clastics on top of the Arcadia Park. Westward out of the basin, the Eagle Ford thins by truncation and changes lithologic character; consequently the previously named subdivisions are no longer recognizable. Near Waco, the lower Eagle Ford consists of mostly montmorillonitic clays with disseminated calcium carbonate, called the Lake Waco Formation, and the upper Eagle Ford consists of dark gray, blocky shales, named the South Bosque Formation. The upper 30-50 ft of the South Bosque is completely noncalcareous. In the deepest portions of the basin near the Cretaceous continental margin, 150-200 ft of Eagle Ford and Woodbine are underlain by the Buda Limestone nd overlain by the Austin Chalk.

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