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2. Underlying Geologic Template/Antecedent Topographic ControlDuring the pre-Holocene geologic history, a broad, topographically uneven surface was generated, which provided a fundamental first-order control on the Holocene coastal morphology. Pre-existing basins became estuaries and pre-existing highs became headlands (Evans et al., 1985; Duncan, 1993; Hine, 1997). The fact that the study area is situated in the middle of an ancient carbonate platform, which features extensive internal karst features and an external very low gradient, is of primary importance in shaping the Holocene, pre-transgressive surface. The age of the carbonate bedrock on top of which the modern coastal lithosomes are emplaced ranges from the Oligocene (?) in the northern part of the study area to Quaternary in the south. Extensive paleo-sinkhole activity is revealed in the seismic profiles, but there seems to be little active sinkhole activity today occurring on the inner shelf. Instead, the carbonate surface has been "planed off" by physical/chemical/biological erosion during repeated glacioeustatic, sea-level driven transgressions and regressions. This erosion has removed topography created by extensive deformation caused by internal and external karst dissolution thus forming a relatively even sequence boundary surface. This surface has minor, local relief featuring scarps, pits, and depressions, the latter of which become infilled by Holocene marsh deposits as sea-level rose since the Last Glacial Maximum. The Cenozoic deformation of the bedrock carbonates occurs at two scales: (1) warps and depression that are 100's of meters to perhaps 1 km in width and up to 10 meters in relief, and (2) large areas of regional depression forming paleo-shelf valleys 10's of kilometers in width and length and up to 30 m of subsurface relief. The latter controlled fluvial drainage patterns during sea-level lowstands and estuaries (Tampa Bay, Charlotte Harbor) during sea-level highstands (see Infilled Shelf Valleys and Their Significance section). The headlands at Indian Rocks and Venice are probably controlled by bedrock highs. Barrier-island location, orientation, and to some extent morphology are controlled by underlying rock relief formed by multiple sea-level highstand events (Evans et al., 1985). The island orientation wrapping around the Indian Rocks headland is an obvious example. However, many of the inner-shelf/barrier island cross-sections reveal that the bedrock surface rises faster directly beneath the barrier islands or provides a terrace, which supports the modern coastal system. Changes in bedrock gradient control the width of the back-barrier lagoon, which in turn affects the size of the tidal prism and the behavior of the inlets. This bedrock gradient change was probably created by the reoccupation of multiple, glacioeustatic highstand events during the Quaternary. |
