Origin and Development of Holocene Inner Shelf Sequences, Southwest Florida

Richard A. Davis, Jr., Jonathan M. Klay

The Holocene stratigraphic record on the inner shelf off the coast of southwest Florida between Cape Romano and Cape Sable is quite thin. The gradient of this coast is and has been, extremely low: about 1:3,000. The combination of coastal configuration, available sediments, and processes have, however, resulted in a stratigraphy that reflects the diverse depositional environments that have occupied this region for the past several thousand years. Overall energy levels have been low through most of the period when this record accumulated. A combination of relatively high-energy tidal processes and low-energy wave-dominated processes have produced this stratigraphy.

The pre-Holocene stratigraphy includes a karstic Pleistocene (?) limestone surface overlain by a muddy, low-energy coastal sequence of terrigenous sands. These lower Holocene strata represent the transgressing coastal marsh or mangrove and estuarine environments from about 7,000 to 3,000 years ago when sea level was rising slowly over this broad shelf. Sea level has been near its present level for the last 3,000 years, during which this inner shelf environment has changed dramatically.

During the past few thousand years or less, the sequence that has accumulated is largely restricted to the northern portion of the area where tide-dominated conditions prevail. The outer part of this area is characterized by tidal sand ridges strongly resembling those in the present North Sea but on a smaller scale. Landward of these ridges, in Gullivan Bay, a sequence of muddy sands represents a sediment sink supplied primarily by tidal currents through the sand ridge system in combination with runoff from the adjacent mangrove islands. The southern two-thirds of this inner shelf system lacks significant terrigenous Holocene sediments due primarily to a lack of supply.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.