Abstract Pre-existing topography strongly influenced Sangamonian transgression in the Apalachicola area. Late Pleistocene neritic and estuarine Biloxi and Gulfport barrier-complex deposits indicate that interglacial sea level rose from at least -37 m, relative to present sea level, to above +3 m. The Biloxi Formation provides a Gulf of Mexico-wide stratigraphic marker. Subsequent regression first was accompanied by river-channel incision at a level slightly lower than interglacial- and much higher than full glacial-erosion levels. Large eolian dunes formed over Gulfport barrier surfaces, reflecting the regional extent of a Wisconsinan wind system. Antecedent Pleistocene topography also greatly influenced Holocene sedimentation and associated landforms. The late Holocene history of St. Vincent and “Little St. George” Islands and St. Joseph barrier spit is characterized by strandplain progradation. Unlike St. George Island, the St. Joseph barrier spit did not form through integration of emerging island cores. Multiple erosional episodes characterize the Quaternary barriers. Conclusive field evidence is lacking for Late Holocene sea-level fluctuations. Massive dilution by Apalachicola River runoff and resuspension/homogenization of the bay deposits by occasional hurricanes tend to diminish lateral and vertical salinity gradients in the record of the late Holocene sedimentary cycle. Unlike certain central Gulf Coast areas, contrasts in vertical salinity between Late Holocene lagoonal and neritic deposits in the Apalachicola are weak to nonexistent.

Abstract This chapter divides the Gulf Coastal Plain province into three major sectors: (1) the area west of the Mississippi embayment where sequences are fine-grained fluvial-deltaic and marginal marine facies; (2) the area east of the Mississippi embayment to peninsular Florida where sequences are chiefly fine-grained clastic fluvial, with relatively minor deltaic and marginal marine facies; and (3) peninsular Florida where sequences typically are brackish-water and nearshore-marine sand and calcareous facies. Also discussed are coastal plain tectonics and Quaternary vertebrate fossil localities. The Mississippi embayment is discussed elsewhere in this volume (Chapter 17, Autin and others). The Gulf of Mexico Coastal Plain of the United States extends from the Mexican border to the southern tip of Florida, a distance of about 2,350 km. The Pleistocene of the Gulf Coastal Plain is bounded inland by Neogene exposures, and seaward near the present coast by a narrow strip of Holocene sediments. Width of the Pleistocene plain varies from 60 to 140 km west of the Mississippi embayment, to 30 to 130 km east of the embayment. The entire Florida peninsula, with the possible exception of parts of the central highlands, is covered by a veneer of Pleistocene sediments. Surfaces of the Gulf Coastal Plain underlain by Pleistocene sediments are characterized by little relief and by seaward gradients of 0.2 to 3.0 m/km. Near the present coast of Texas and western Louisiana, thickness of the Pleistocene section ranges from about 240 to 460 m, whereas under the northeastern segment of the Gulf Coastal Plain