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The Upper Cretaceous sediments of the eastern Gulf region crop out in a crescentic band around the southwestern end of the plunging Appalachian Highlands in a belt 500 miles long and up to 75 miles wide. Their maximum thickness is estimated to be about 2,300 feet.
The Tuscaloosa formation is thickest in west-central Alabama, but thins northwestward to a feather edge in Tennessee, and thins toward the east to about 250 feet near Chattahoochee River. The Eutaw formation overlies the Tuscaloosa, from which it is separated by an important unconformity. The Eutaw thins both toward the northwest and toward the east, only the upper part being exposed in Tennessee and in Georgia. The Selma chalk overlies the Eutaw formation unconformably. It consists of nearly 900 feet of chalk in west-central Alabama, but is broken by a minor unconformity or diastem about 300 feet above its base, a few feet above a thin but persistent zone of hard, pure limestone layers interbedded with chalk, the Arcola limestone member. Northwestward in Mississippi the lower third o the Selma passes by merging and intertonguing into the Coffee sand; the upper third merges into the Ripley formation; and the middle third continues into Tennessee as an impure chalk unit. In the eastern part of Alabama the part of the chalk below the Arcola limestone member merges by intertonguing into the Blufftown formation, the middle third merges into and intertongues with the Cusseta sand, and the upper part merges into the Ripley formation. The Selma and Ripley are separated from the overlying Prairie Bluff chalk by an important unconformity. The Prairie Bluff chalk merges toward the north in Mississippi into the sandy Owl Creek formation and toward the east in Alabama into the Providence sand.
The distribution of the chalks indicates deposition in clear, only moderately deep water off the southwest end of the Appalachian Highlands during much of Upper Cretaceous time; drainage from the Highlands was mainly southeast and northwest while the chalk was accumulating.
The following zones have been traced through Mississippi and Alabama: the Exogyra ponderosa zone, which embraces the Tombigbee sand member of the Eutaw formation and the lower two-thirds of the Selma chalk and equivalent beds; the Diploschiza cretacea zone, a narrow zone in the Selma chalk, within and about 200 feet below the top of the E. ponderosa zone, traceable from Tupelo, Mississippi, to the longitude of Montgomery, Alabama; the Exogyra costata zone, embracing the upper third of the Selma chalk, the Prairie Bluff chalk, and equivalent beds; the Exogyra cancellata zone, traceable throughout the area in the lower 200 feet or less of the E. costata zone.
Correlations with the Upper Cretaceous series of the western Gulf region are indicated.
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