3. Infilled Shelf Valleys and Their Significance
Tampa Bay marks the northern extent of the shelf-valley depressions. Infilled valleys are not found further north, although sinkholes and warped strata are common. From Tampa Bay south to the Venice headland numerous shelf-valley depressions are present, which contain progradational infilling sequences. These structures probably extend as far south as the Sanibel Island area. The extent and orientation of these systems is not well known, and many valley systems are limited to within 20 km of the present coastline with no particular orientation as yet determined. The majority of these features no longer have topographic expression. Tampa Bay, an exception, owes its origin to a large shore-normal valley system that has not been completely infilled and extends considerably further seaward beneath the inner continental shelf. Still, the Tampa Bay shelf-valley system appears to disappear between 20-30 m water depth west of the present bay mouth.
A dense grid of high-resolution, single-channel seismic data was collected at the mouth of Tampa Bay in an attempt to define the stratigraphy, determine the controls on deposition, and define the underlying structure of a shelf-valley system that underlies this area. The seismic data were tied into nearby wells and boreholes for lithologic and age control. Sequence stratigraphic methods were incorporated in order to develop an integrated chronostratigraphy for the depositional infilling of the shelf-valley system.
Five seismic sequences were identified infilling the Tampa Bay incised-valley system (Duncan, 1993). The lowest sequence (A) is characterized by gently prograding sigmoidal clinoforms and is interpreted as a product of highstand shedding from south of the study area. More carbonate-rich sediments are indicated based on well-log correlations. Sequences B and C indicate an east/northeast source of sediments and are characterized by steeply prograding reflectors in the north of the study area (possibly of deltaic origin) and flat-lying or chaotic units in the central and south, which are interpreted as a combination of reworked siliciclastic and nearshore marine deposits. These deposits may be the west Florida analog to the thick siliciclastic unit underlying southern Florida and the Florida Keys (Warzeski et al., 1996; Cunningham et al., 1998). Finally the upper sequences (D and E) are represented by predominately flat-lying reflectors with common progradational character near Egmont Channel. These sequences have been interpreted as shallow marine deposits and reworked sediments (siliciclastic) controlled by nearshore wave and tidal processes similar to today.
Based on well-log correlations (Ferguson, 1997), seismic reflection characteristics, and comparisons to regional lithostratigraphic and biostratigraphic work, the chronostratigraphy of the infill of the Tampa Bay shelf-valley system is as follows:
The Arcadia Formation (underlying sequence boundary 1--SB1) within the Tampa Bay area was deposited during the early to middle Miocene highstand of sea level. SB1 and was formed during a sea-level lowstand during the middle Miocene, sometime after ~15 Ma. Sequence A was deposited during the middle Miocene infilling the shelf-valley system from the south. A major sea level lowstand occurred during the late Serravalian/early Tortonian (about 10 ma) forming SB2 and shifting the focus of deposition further northward in the shelf-valley system. Sequences B and C are deposited during the late Miocene/early Pliocene representing the transition from low-energy, more carbonate deposition (sequence A) to the higher energy, fluvial deltaic deposits interpreted as Upper Peace River (sequence C) within the shelf valley. SB4 caps these fluvial deltaic deposits sometime during the Pliocene and the onset of shallow-marine deposition of the Quaternary (sequences D and E) begins. SB5 marks the Pleistocene-Holocene boundary.
The modern mixed carbonate-siliciclastic shelf sediments are a product of continuing carbonate marine deposition interspersed with reworked siliciclastics, the majority of which have been sequestered in the coastal zone since the late Miocene/Pliocene. Climatic and oceanographic conditions responsible for this siliciclastic pulse are unique during the Neogene of Florida and provide a stark contrast to deposition before and after the late Miocene/Pliocene.