Paleozoic rocks beneath the Mesozoic Gulf coastal plain beds in Mississippi are in the transition area from the Appalachian Mountain system in Alabama to the Ouachita Mountain system in Arkansas. Strata ranging in age from Cambrian to Pennsylvanian aggregate more than 15,000 ft in thickness in the Black Warrior basin of northern Mississippi and in the Central Mississippi deformed belt. The oldest rock penetrated is cherty dolostone of the Knox Group. The age and composition of the basement are unknown.

The Ordovician and Silurian of northeastern Mississippi are mainly carbonate rocks along the southwestern edge of a widespread lower Paleozoic carbonate facies that extends from the Alabama Appalachians northwestward beyond the Nashville dome. A tongue of sandstone and mudstone in the Upper Ordovician of northeastern Mississippi is evidently the distal part of the regional Sequatchie clastic wedge which was derived from the Appalachian area of southern Tennessee. Toward the west across central Mississippi, eastward-pinching tongues of black shale in the Ordovician and Silurian suggest gradation to the Ouachita black shale facies. The Arkansas Novaculite of Devonian-Early Mississippian age extends from the Ouachita Mountains across northern Mississippi.

In northeastern Mississippi, the Mississippian-Pennsylvanian sequence is dominated by a thick northeastwardly regressive wedge of shallow-water clastic rocks. The lower part of the clastic sequence grades northeastward to the Mississippian limestone facies in Alabama. The basal Pottsville quartz-pebble conglomerate descends stratigraphically westward across Mississippi, and the higher Pottsville in eastern Mississippi is cyclical carbonaceous shale, sandstone, and coal. The probable Mississippian-Pennsylvanian section of western Mississippi includes dark-colored chert and siliceous shale which are comparable to rocks of the Ouachita facies.

Evidently the lower Paleozoic carbonate facies of eastern Mississippi grades westward to the Ouachita dark-colored shale facies and the upper Paleozoic shallow-water regressive clastic facies grades westward to the deep-water clastic facies of the Ouachitas. Both facies changes apparently trend north-northwest across central Mississippi; possibly the facies boundaries formed along a steep shelf edge at the western margin of a relatively shallow platform in eastern Mississippi.