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.

A subsurface investigation of the coastal areas in the Apalachicola delta region on the northwest Florida coast indicates that the Pleistocene sediments, which thicken to the southwest, have been deposited on an uneven Miocene surface of variable age. The thickest Pleistocene section, west of the mouth of the Apalachicola River, consists of two superimposed sequences of terrigenous clastic sediments. Each sequence grades upward from coarse to fine, and represents two major Late Pleistocene sea-level fluctuations. Radiocarbon dates and stratigraphic relationships indicate that the upper of these sequences probably represents a mid-Wisconsin transgression of the sea, and the lower represents a Sangamon transgression. The thickest section of Recent sediments lies in the old incised valley of the Apalachicola River which was cut during the last lowering of sea level. These deltaic, prodeltaic, and bay sediments represent the gradual filling of the drowned river valley, which was inundated by the Recent rise in sea level. The bays, barrier islands, and spits are the only other areas of any significant Recent sedimentation with the exception of one offshore basin to the southwest of the present river mouth. Radiocarbon dates, stratigraphic relationships, and environmental interpretations suggest that there was a relatively high stand of the sea that was very near present sea level sometime between 24,000 and 40,000 years B.P. Geomorphologic evidence in the area indicates that this high stand may have been slightly higher than present sea level and probably corresponds to the Silver Bluff shoreline of Florida and Georgia. No evidence has been found for a Recent higher sea-level stand, and radiocarbon dates and sediments indicate that sea level was approximately 10 to 15 feet below its present level sometime between 4000 and 4500 years ago. A rather rapid rise probably occurred after this time to a position slightly below present sea level, from which it has gradually risen to its present position.

ABSTRACT The distribution and composition of the coastal and inner-shelf clastic sediments of northwest Florida have been strongly influenced by the activity of the Apalachicola River. The largest river in the northeastern Gulf of Mexico, the Apalachicola has dominated sedimentation in the region since Neogene time. High-resolution shallow seismic investigations of the inner shelf, combined with coastal borehole records, indicate that the bulk of the clastic sediment cover of the coast and inner margin is late Quaternary in age, and probably Sangamonian or younger. Late Quaternary sea level fluctuations have exerted a major influence on the timing and character of sedimentation in the Apalachicola Embayment.