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Structure Contour Map on the top of the Miocene Beneath the Barrier/Inlet System of Pinellas, Manatee and Sarasota Counties, Florida

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Chief Scientist
D.F. Ufnar, Department of Geology, University of South Florida, Tampa, FL
R.A. Davis Jr., Department of Geology, University of South Florida, Tampa, FL

structure contour map

Figure 1: Structure contour map of the Miocene surface beneath the west Florida barrier islands.

Defining the antecedent topography beneath the west Florida barrier islands is paramount to understanding the development and subsequent morphodynamics of the system. Data from numerous vibracores, seismic reflections, and probe surveys, have been mapped to illustrate the configuration of the bedrock surface beneath the islands (Figure 1). These data suggest that pre-Holocene deposits have had a significant impact upon island genesis and subsequent morphologic modifications (Davis and Kuhn, 1985; Evans et al., 1985).

The post-Miocene surface (unconformity) between Anclote Key and Venice Inlet is an erosional surface excavated into deposits of the Miocene Hawthorne Group. Hawthorne sediments consist of a complex assemblage of variably phosphatic (<1 to 60%) sands, clays, limestones and dolostones. The Hawthorne in south Florida is subdivided into the lower, carbonate dominated Arcadia Formation, and the upper, siliciclastic dominated Peace River Formation (Scott, 1988). The Hawthorne Group records erosion and reworking of older strata, and deposition in nearshore and inner shelf environments during several Miocene sea-level and phosphogenic events (Scott, 1988).

The quartzitic limestone of the Arcadia Formation outcrops in several areas of the mapped region. In particular, the Tampa Limestone Member of the Arcadia (defined by its lack of phosphate) is exposed in the intertidal zone near the mouth of the Anclote River, and on the landward side of the intracoastal waterway at Indian Rocks beach (Figure 1). Carbonate rocks of the Arcadia Formation are also exposed in the intertidal zone of Sarasota Bay, however it has yet to be determined if they are part of the Tampa Member. Siliclastic and phosphatic material (including phospahatized bone fragments and sharks teeth) derived from Hawthorne sediments are found in the Midnight Pass area, Venice Inlet and the proximal surf zone areas.

In general, the mapped region can be subdivided into three zones. The northern portion which extends from Anclote Key to Indian Shores (Sand Key). The central Portion which extends from Indian Shores to Longboat Pass, and the southern portion extending from Longboat Key to Venice Inlet. The northern and southern portions of the study area are characterized by a relatively thin post-Miocene sediment package. Mapping in these areas reveals an abrupt north-south trending break in the slope of the bedrock surface. In cross-section, the break resembles a subtle escarpment, and has relief on the order of 2-3 meters. The break in slope is generally found directly beneath, or just seaward of the barrier islands 2-5 m below mean sea-level. In some areas, small-scale (relief <10 meters) paleovalleys are coincident with the present location of inlets (e.g. Clearwater Pass, New Pass, and Big Sarasota Pass)(Figure 1). The central portion of the study area (the mouth of Tampa Bay) is characterized by a large paleovalley incised into the bedrock surface. The deepest portion of this trough is 52 meters below MSL, and is located beneath the northern tip of Egmont Key. Perhaps the location of the deepest portion of the trough has influenced the location of the present Egmont Channel. Furthermore, the post-Miocene valley fill beneath Tampa Bay is of great interest, for it may contain pre-Holocene deposits not present elsewhere along this coast. Some karst features are also discernible in the mapped region. The most noteworthy are on the mainland coast in northern Pinellas and southern Pasco Counties. Solution processes have generated relief on the order of 20 meters in this area.


  • Davis, R.A. and B.J. Kuhn, 1985, Origin and development of Anclote Key, West-Peninsular, Florida, Marine Geology, 63, 153-171.
  • Evans, M.W., A.C. Hine, D.F. Belknap, and R.A. Davis Jr., 1985, Bedrock controls on. barrier island development: west-central Florida coast, Marine Geology, 63, 263- 283.
  • Scott, T.M., 1988, Lithostratigraphy of the Hawthorne Group (Miocene) of Florida, Florida Geologic Survey Bulletin 59, 148 p.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Morphodynamics > Structure Contour Map on the top of the Miocene Beneath the Barrier/Inlet System of Pinellas, Manatee and Sarasota Counties, Florida U. S. Department of the Interior | U.S. Geological Survey
Center for Coastal & Regional Marine Studies

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Updated May 06, 2013 @ 09:24 AM  (JSS)