Seismic Stratigraphic Investigation of West Florida Slope, Gulf of Mexico: 3. Regional Stratigraphy and Structure

The post-Early Cretaceous geologic development of the West Florida Slope was studied by analysis of 2,500 n.m. (4,630 km) of seismic sparker lines and seven core holes. Slope deposition was not a simple, continuous process, but was interrupted by significant depositional breaks that appear in the faunal record and as reflective events on seismic profiles. Sediments between these breaks are interpreted as genetically related sequences that can be mapped and used to interpret slope stratigraphy. At the end of the Early Cretaceous the present slope area was a broad shallow-water shelf which probably extended westward to the present West Florida Escarpment. This escarpment is interpreted as mostly a constructional shelf margin forming a gulfward extension of the Washita-Fredericksburg trend of Louisiana and Texas. The role of major faulting as a controlling factor in producing the escarpment was not investigated in this study, although we failed to observe major faults on our sparker data. Late Cretaceous subsidence of the shelf produced a rapid transition from shallow- to deep-water carbonates, with water depths in the order of 3,000 to 6,000 ft (900 to 1,800 m) on the outer slope by the end of the Cretaceous. Deep-water conditions prevailed throughout the Cenozoic as the area gradually filled to its present bathymetric configuration. Lowermost Tertiary beds along the outer slope commonly are crumpled and distorted. Post-Eocene topographic lows tend to be filled with Oligocene and early Miocene beds. During the middle Miocene large clinoforms of argillaceous carbonate built as progradational features from shallow-water areas on the north. Thinner upper Miocene beds were deposited as lenses on the clinoform slopes. Pliocene and Pleistocene deposition continued this trend to its present configuration. The DeSoto Canyon area has a long Cenozoic history of nondeposition and erosion. Lithified shallow-water dolomite and limestone are present in Lower Cretaceous and lowermost Upper Cretaceous beds. Younger sediments consist mostly of unlithified deep-water carbonates with planktonic foraminifers, coccoliths, and radiolarians associated with varying amounts of terrigenous clay. Mineralogically, the carbonate is mostly low-magnesium calcite, but Pleistocene sediment also contains some high-magnesium calcite and aragonite. The mineral dolomite is present in Pleistocene, Pliocene, and uppermost Miocene unconsolidated sediments as well as in Lower Cretaceous lithified beds. Possibly the younger dolomite formed in deep-water unlithified sediments at or near water depths in which they now are. The most common sedimentary structures are banding and burrowing. The banding consists of alternating light (coarse) and dark (fine) layers of carbonate that form couplets varying in thickness from less than 0.5 in. (13 mm) to more than 6 in. (15 cm). The banding is very regular, and grain size is a function of the relative abundance of foraminifers, coccoliths, and clay matrix. The couplets may be formed by small, frequent density currents, but the mechanism is not understood. Burrow End_Page 193------------------------ mottling commonly is superimposed upon the banding structures and may destroy them partly or completely. Questionable turbidites, more than 6 ft (180 cm) thick, are present in some calcareous clay sequences in the northern part of the slope.