INFLUENCE OF PLEISTOCENE BEDROCK TOPOGRAPHY ON HOLOCENE PATCH REEF DEVELOPMENT IN BISCAYNE NATIONAL PARK, FLORIDA

The topographical features of pre-Holocene limestone bedrock underlying the Biscayne National Park (BNP) reef tract were mapped in 1998 an 1999 using a Geoacoustic boomer source and a sub-bottom profiler (Edgetech model SB-0512). The sharp impedance contrast between unconsolidated sediments and Pleistocene bedrock was clearly visible but could rarely be seen beneath well-cemented patch reefs and bank-margin reefs. Surface topography of the Pleistocene surface was dominated by small-scale depressions of various sizes with very few positive relief features. Depressions were concentrated on the shelf between -8 and -14 m below present mean sea level and ranged from small V-shaped depressions less than 10 m across and 1 m deep, to large U-shaped incised channels more than 200 m across and 10 m deep. Nearly all depressions are now in-filled with sediments to the level of the seafloor. Extensive patch reef growth is estimated to have started soon after the platform was flooded by rising sea-level approximately 8 kyr ago. A GIS platform was used to overlay the position of modern reefs with underlying bedrock topographic maps. Depending on their relationship to underlying bedrock features, modern patch reefs were divided into four types: (1) located on irregular bedrock topographic features (e.g. channel margins); (2) located on flat bedrock (e.g. no apparent bedrock control); (3) located on margins of semi-consolidated Holocene sand banks associated with irregular bedrock; (4) located on flat unconsolidated sediments overlying flat bedrock control. Seismic sections clearly show how patch reef growth around channels is limited to the margins of the channel and not in the sediment-filled center. This is in agreement with previous studies that suggest that reefs cannot form on preexisting topographic depressions, but are favored to grow on topographic highs or irregular surfaces. The complex shelf topography created by incised channels and numerous depressions may also play a role in explaining the high abundance of patch reefs in this part of the Florida reef tract.