BARRIER-ISLAND PROGRADATION AND HOLOCENE SEA-LEVEL HISTORY IN SOUTHWEST FLORIDA

Holocene barrier islands in Lee County, Florida, are composed of beach ridges organized into distinct, unconformable sets bounded by erosion surfaces. These beach-ridge sets are further differentiated on the basis of average elevation. Low sets are about 3-feet and high sets are about 6-feet above local mean sea-level. Beach ridges are composed exclusively of swash-zone deposits that consist of seaward-dipping, planar laminae. This marine origin is further documented by the uniform elevation and the regular, curvilinear geometry of individual beach-ridge crests. The oldest preserved beach-ridge sets in this region were deposited approximately 3000 BP. Elevationally distinct and geographically adjacent beach-ridge sets of apparently identical ages record a major fluctuation in sea-level (rise or fall) and/or wave-energy (increase or decrease). The date of this fluctuation is within the 200- to 400-year counting-error margin for the analyzed marine shells below which the radiocarbon technique cannot resolve differences. The geographic extent and lateral continuity of the low beach-ridge sets in this low-energy, micro-tidal region argue that the primary component of these fluctuations was a change in sea level rather than wave energy. Low beach-ridge sets are as geographically widespread and laterally continuous as are the high ones. Just the opposite situation would be predicted if the low sets were formed during decreased wave energy alone: wave refraction would be increased and result in an even more complex pattern of littoral drift. The more complex this pattern, the shorter the length of any given littoral-drift cell and, consequently, of beach-ridge sets. Five such fluctuations have been identified in these islands: (1) a rise of 4- to 6-feet at 2000 BP, (2) a fall of 3- to 5-feet at 1500 BP, (3) a rise of of 2- to 3-feet at 1100 BP, (4) a fall of about 2-feet at 500 BP, and (5) a rise over the past hundred or so years. The 500-year cyclicity is only apparent and reflects the precision of estimating differences in depositional ages of clastic deposits by radiocarbon-dating their constituent shell clasts. Each of these fluctuations resulted in barrier-island progradation or creation. Sand was supplied by erosion of the nearshore region and existing barrier islands. Each fluctuation had an initial depositional phase followed by an erosional phase as the supply rate fell below a critical threshold. A decrease in sand-supply rate reflects a source depletion and/or a redirection of the transport path.