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A regional study of the Holocene sequence onlapping the west-central Florida Platform was undertaken to merge our understanding of the barrier-island system with that of the depositional history of the adjacent inner continental shelf. Key objectives were to better understand the sedimentary processes, sediment accumulation patterns, and the history of coastal evolution during the post-glacial sea-level rise. In the subsurface, deformed limestone bedrock is attributed to mid-Cenozoic karstic processes. This stratigraphic interval is truncated by an erosional surface, commonly exposed, that regionally forms the base of the Holocene section. The Holocene section is thin and discontinuous and, north or south of the Tampa Bay area, is dominated by low-relief sand-ridge morphologies. Depositional geometries tend to be more sheet-like nearshore, and mounded or ridge-like offshore. Sand ridges exhibit 0.5?4 m of relief, with ridge widths on the order of 1 km and ridge spacing of a few kilometers. The central portion of the study area is dominated nearshore by a contiguous sand sheet associated with the Tampa Bay ebb-tidal delta. Sedimentary facies in this system consist mostly of redistributed siliciclastics, local carbonate production, and residual sediments derived from erosion of older strata. Hardground exposures are common throughout the study area. Regional trends in Holocene sediment thickness patterns are strongly correlated to antecedent topographic control. Both the present barrier-island system and thicker sediment accumulations offshore correlate with steeper slope gradients of the basal Holocene transgressive surface. Proposed models for coastal evolution during the Holocene transgression suggest a spatial and temporal combination of back-stepping barrier-island systems combined with open-marine, low-energy coastal environments. The present distribution of sand resources reflects the reworking of these earlier deposits by the late Holocene inner-shelf hydraulic regime.

Hardbottoms are sequence boundaries and condensed sections that offer clues for the interpretation of the incomplete record of Tertiary continental shelf evolution. Seaward of 5 km, 50% of the inner west-central Florida shelf seafloor is flat hardbottom. These lithified surfaces are punctuated by shorefacing, scarped hardbottoms that trend shore-parallel (330??0?) and vary in relief (up to 4 m). Scarped hardbottoms are the only natural relief on the inner shelf and support a diverse benthic community, the activities of which erode the outcrops, producing undercuts in excess of 1 m. Outcropping hardbottom strata are comprised of distinct, phosphate-rich, mixed carbonate?siliciclastic lithofacies, that range in age from Miocene to Quaternary. Miocene units are dolomite-rich and mark the upper surface of the inner shelf bedrock (Hawthorn Group). Dolomite within these beds (silt-sized, cloudy centered rhombs) fall into two age groups, correlating with highstands at 15 and 5 Ma. This lithofacies is consistent with models that indicate an increased flux of organic matter ? resulting from topographically induced upwelling ? promoting dolomitization during early burial diagenesis in the sulfate-reduction zone. Quaternary units are calcite-rich and perched atop the shelf bedrock. Samples of these units record a complex diagenetic history and multiple sea-level fluctuations. Based on evidence of primary marine cementation, they are interpreted to be hardground (non-deposition) surfaces, forming as a function of sediment starvation and minimal sediment movement. Decreased highstand magnitude or duration may have resulted in the absence of a significant organic component to Quaternary hardbottoms, which, in turn, may prevent subsequent dolomitization. These outcrops are a potential source for sediments to the inner shelf, not only as habitat for biological sediment production, but also through their destruction. The undercut, shorefacing, scarped hardbottom morphology displayed by west-central Florida hardbottoms is indicative of bio-erosion. Preliminary studies indicate a potential mass of 0.04 kg m-2 yr-1 of siliciclastic sediment is released to the inner shelf.

This paper provides an overview for this special publication on the geologic framework of the inner shelf and coastal zone of west-central Florida. This is a significant geologic setting in that it lies at the center of an ancient carbonate platform facing an enormous ramp that has exerted large-scale control on coastal geomorphology, the availability of sediments, and the level of wave energy. In order to understand the Holocene geologic history of this depositional system, a regional study defined by natural boundaries (north end of a barrier island to the apex of a headland) was undertaken by a group of government and university coastal geologists using a wide variety of laboratory and field techniques. It is the purpose of this introductory paper to define the character of this coastal/inner shelf system, provide a historical geologic perspective and background of environmental information, define the overall database, present the collective objectives of this regional study, and very briefly present the main aspects of each contribution. Specific conclusions are presented at the end of each paper composing this volume.

The sedimentology, stratigraphic position, and benthic foraminiferal biostratigraphy of early- to mid-Holocene deposits from the west-central Florida shelf suggest that barrier islands developed along this coast as early as 8.3 ka, in an environment that was more arid than today. Predominant foraminifera of three paralic sedimentary facies deposited between 5.3 and 8.3 ka include miliolids, Elphidium spp., and Ammonia spp., all of which are common in back-barrier environments. Foraminiferal assemblages also suggest that early back-barrier sediments were deposited in a hypersaline environment, similar to that of the arid Laguna Madre of the western Gulf of Mexico. Modern back-barrier foraminifera in the Tampa Bay region are indicative of the humid subtropical climate of today. Thus, the climate of west-central Florida at approximately 8 ka was more arid than today, which is consistent with recent studies showing that climate in the Gulf of Mexico was dryer and cooler during this time period.

The shelf-valley system underlying Tampa Bay, Florida?s largest estuary, is situated in the middle of the Neogene carbonate Florida Platform. Compared to well-studied fluvially incised coastal plain valley systems, this shelf-valley system is unique in its karstic origin and its alternating carbonate-siliciclastic infill. A complex record of sea-level changes, paleo-fluvial variability and marine processes have controlled the timing and mechanisms of this `compound? shelf-valley infill. A dense grid of high-resolution, single-channel seismic data were collected at the mouth of Tampa Bay, in an attempt to define this stratigraphy, determine the controls on deposition, and define the underlying structure of this shelf-valley system. The seismic data were correlated with 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. Sequence boundaries generally show erosional truncation and karstification, with downlap of overlying sequences. Structure contour and isopach maps indicate that the Tampa Bay shelf-valley system has remained in essentially the same location since its formation in the early Miocene, although the provenance of sedimentary infill has changed. This change is due to increasing amounts of siliciclastic material during the Neogene. Seismic facies interpretations indicate lower-energy, northward prograding deposition dominated by predominantly carbonate sediments within the lowest Sequence A. Higher energy, siliciclastic fluvio-deltaic deposition within sequences B and C originates to the east and northeast of the shelf-valley system related to a Pliocene pulse of sedimentation onto the Florida Platform. Finally, marine processes (longshore transport, ebb-tidal delta formation) dominate the upper two sequences (D and E), reworking these siliciclastic sediments into a spatially mixed carbonate-siliciclastic depositional setting.

The west-central Florida inner shelf represents a transition between the quartz-dominated barrier-island system and the carbonate-dominated mid-outer shelf. Surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large-scale sediment redistribution. The west Florida shelf is the submerged extension of the Florida carbonate platform, consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover. A total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition. Results show that sediment consists of a combination of fine quartz sand and coarse, biogenic carbonate sand and gravel, with variable but subordinate amounts of black, phosphorite-rich sand. The carbonate component consists primarily of molluskan fragments. The distribution is patchy and discontinuous with no discernible pattern, and the transition between sediment types is generally abrupt. Quartz-rich sediment dominates the inner 15 km north of the entrance into Tampa Bay, but south of the Bay is common only along the inner 3 km. Elsewhere, carbonate-rich sediment is the predominate sediment type, except where there is little sediment cover, in which cases black, phosphorite-rich sand dominates. Sediment sources are likely within, or around the periphery of the basin. Fine quartz sand is likely reworked from coastal units deposited during Pleistocene sea-level high stands. Carbonate sand and gravel is produced by marine organisms within the depositional basin. The black, phosphorite-rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area. The distribution pattern contains elements of both storm- and tide-dominated siliciclastic shelves, but it is dictated primarily by the sediment source, similar to some carbonate systems. Other systems with similar sediment attributes include cool-water carbonate, sediment-starved, and mixed carbonate/siliciclastic systems. This study suggests a possible genetic link among the three systems.

Sediment vibracores and surface samples were collected from the mixed carbonate/siliciclastic inner shelf of west?central Florida in an effort to determine the three-dimensional facies architecture and Holocene geologic development of the coastal barrier-island and adjacent shallow marine environments. The unconsolidated sediment veneer is thin (generally <3 m), with a patchy distribution. Nine facies are identified representing Miocene platform deposits (limestone gravel and blue?green clay facies), Pleistocene restricted marine deposits (lime mud facies), and Holocene back-barrier (organic muddy sand, olive-gray mud, and muddy sand facies) and open marine (well-sorted quartz sand, shelly sand, and black sand facies) deposits. Holocene back-barrier facies are separated from overlying open marine facies by a ravinement surface formed during the late Holocene rise in sea level. Facies associations are naturally divided into four discrete types. The pattern of distribution and ages of facies suggest that barrier islands developed approximately 8200 yr BP and in excess of 20 km seaward of the present coastline in the north, and more recently and nearer to their present position in the south. No barrier-island development prior to approximately 8200 yr BP is indicated. Initiation of barrier-island development is most likely due to a slowing in the Holocene sea-level rise ca. 8000 yr BP, coupled with the intersection of the coast with quartz sand deposits formed during Pleistocene sea-level highstands. This study is an example of a mixed carbonate/siliciclastic shallow marine depositional system that is tightly constrained in both time and sea-level position. It provides a useful analog for the study of other, similar depositional systems in both the modern and ancient rock record.

A series of migrating shore-normal sandbars with wavelengths of 75?120 m and heights up to 2 m have been identified off the northern tip of Anna Maria Island, a barrier island on the west-central Florida coast. Similar features have been described elsewhere since the 1930s and termed `transverse bars?. The transverse bars identified off Anna Maria Island are found for about 3 km along the coast and extend 4 km offshore, well outside the normal surf-zone width. No cusps or any other associated beach expression is evident despite the fact that the bars come to within about 75 m of the beach. Sediments on the crests of the bars are a well-sorted fine quartz sand, whereas sediments in the troughs are a poorly sorted coarse carbonate shell hash. Historical aerial photographs and repeated high-resolution bathymetric surveys provide a means of quantifying the migration of the transverse bars. Analyses of orthorectified aerial photographs from the early 1940s through the mid 1990s clearly show movement or migration taking place in the bar field. In the 40-yr period from 1951 to 1991, the southern edge of the bar field moved 200?350 m to the south, with an average long-term migration rate of 8 m/yr. Repeated bathymetric surveys over an 8-month period give an average short-term migration rate of 21 m/yr to the south. Wave and current measurements suggest that southerly winds associated with the passage of cold fronts drive near-bed currents to the south that are strong enough to initiate sediment transport and cause the southerly migration of the transverse bars.