CONTRASTS WITHIN AN OUTLIER-REEF SYSTEM - EVIDENCE FOR DIFFERENTIAL QUATERNARY EVOLUTION, SOUTH FLORIDA WINDWARD MARGIN, USA

Closely spaced, high-resolution, seismic-reflection profiles acquired off the upper Florida Keys (i.e., north) reveal a platform-margin reef -and-trough system grossly similar to, yet quite different from, that previously described off the lower Keys (i.e., south). Profiles and ma ps generated for both areas show that development was controlled by an tecedent Pleistocene topography (presence or absence of an upper-slope bedrock terrace), sediment availability, fluctuating sea level, and c oral growth rate and distribution. The north terrace is sediment-cover ed and exhibits linear, buried, low-relief, seismic features of unknow n character and origin. The south terrace is essentially sediment-free and supports multiple, massive, high-relief outlier reefs. Uranium di sequilibrium series dates on outlier-reef corals indicate a Pleistocen e age (similar to 83-80 ka). A massive Pleistocene reef with both aggr adational (north) and progradational (south) aspects forms the modern margin escarpment landward of the terrace. Depending upon interpretati on (the north margin-escarpment reef may or may not be an outlier reef ), the north margin is either more advanced or less advanced than the south margin. During Holocene sea-level rise, Pleistocene bedrock was inundated earlier and faster first to the north (deeper offbank terrac e), then to the south (deeper platform surface). Holocene overgrowth i s thick (8 m) on the north outer-bank reefs but thin (0.3 m) on the so uth outlier reefs. Differential evolution resulted from interplay betw een fluctuating sea level and energy regime established by prevailing east-southeasterly winds and waves along an arcuate (ENE-WSW) platform margin.