The Pliocene and Pleistocene, embracing only 5.5 m.y., were times of very rapid sedimentation on the continental shelf and slope of the northern Gulf of Mexico. During this period the center of maximum deposition shifted over 200 mi (320 km) southwestward from just west of the present mouth of the Mississippi River to 100 mi (160 km) south of the present shoreline at the Louisiana-Texas border. This shifting of the center of maximum deposition was accompanied by 50 mi (80 km) of southward progradation of the continental shelf edge to its present position near the 600-ft (200 m) isobath. Hydrocarbon productive trends follow the shifting path of the depocenter.

Rapid sedimentation took place upon substrata which included several thousand feet of mobile salt, plus a comparable thickness of mobile prodelta clay. The weight of the accumulating sediments has caused movement of the underlying mobile material, with the result that the structural configuration of the strata in the Plio-Pleistocene depocenters is complicated by large piercement salt and diapiric shale massifs, which are estimated to occupy about 20 percent of the total area at a depth of 12,000 ft (3,657 m) in the Pleistocene depocenter.