Bedrock controls on barrier island development: West-central Florida coast

Abstract

Fifty vibracores and 130 km of high-resolution seismic profiles were taken in the lagoons and nearshore area of northern Pinellas County, Florida, to determine the role of antecedent topography upon barrier island location, origin and development. East—west seismic profiles show a terrace with 1–1.5 m of relief located along the seaward trend of the barrier islands between −4 and −6 m MSL. This antecedent topography is composed of a relatively erosion resistant Miocene limestone and the terrace is usually present as an increase in slope. A contour map of the pre-Holocene surface shows that this subsurface feature is continuous along the entire study area and is mimicked by the opposing orientations of Honeymoon and Caladesi Islands. The correspondence of the modern barrier system to the antecedent topography indicates that: (1) the islands formed on the terrace; or that (2) landward migration of the barriers was stopped by grounding on this feature. Stratigraphic analyses of lagoonal and nearshore environments were conducted in order to establish the overall response of the northern Pinellas coastal system to the late Holocene sea-level rise. This analysis indicates a basically transgressive Holocene sequence with lagoonal sediments occurring seaward of the islands. Underlying the lagoonal deposits in all areas is a 1 m thick relict Pleistocene unit that has been flooded, reworked and vegetated by intertidal to subtidal organisms. A model of development for these barriers is proposed which incorporates an early transgressive history followed by a recent progradational phase. The progradational phase was probably initiated by the late Holocene slowdown in sea-level rise and an increased sediment supply as the barrier islands migrated over a relict sediment source. The antecedent topography thus served as a stabilization point for landward barrier migration and seaward progradation. The antecedent topographic control of barrier island location described by this model is dependent upon the coincident decline in the rate of sea-level rise and an increased sediment supply.