Revealing Pleistocene Topography below Contrasting Holocene Shoals around Ocean Cay,
Bahamas, with High-Resolution Sub-Bottom Profiles
F. Eduardo Cruz
Comparative Sedimentology Laboratory, University of Miami, Miami, FL
Abstract
High-resolution sub-bottom profiling reveals an unexpected shallow Pleistocene
topography and the internal geometry of Holocene shoals around Ocean Cay on the western
margin of Great Bahama Bank. The continuous sand belt north of ocean Cay is anchored on a
Pleistocene high, while south of Ocean Cay discontinuous tidal deltas are situated on shallower
Pleistocene bedrock.
The modern 1-3 km wide and 35 km long sand belt south of Bimini is the only example
of a shoal complex on the leeward side of the Great Bahama Bank. The complex consists of
shoals north of Ocean Cay, commonly referred to as Cat Cay Shoals, and flood tidal deltas south
of Ocean Cay. Over the past two years, our observations from the modern shoals at these bankmargin documented considerable variability in grain type, age, and shoal geometry between the
north and south areas linked to the bathymetry, physiographic setting and tidal flow regime. Here
we report on the investigation of the antecedent topography in these two areas by interpreting
approximately 120 km of shallow, high-resolution seismic data acquired using a full spectrum
digital sub-bottom profiler that emits a FM pulse with frequency range of 0.5 – 12 kHz. The
cross-sectional subsurface images are correlated to a series of closely-spaced cores collected
around Ocean Cay. The topography of the subsurface Pleistocene bedrock and the thickness of
Holocene deposits are extracted by mapping the sea bottom and the first major Pleistocene
horizon in the 2D grid of 18 lines.
Preliminary results from this interpretation indicate that the laterally continuous sand belt
north of Ocean Cay developed behind a subtidal to exposed rock ridge that is located seaward of
the active sand belt. This ridge acts as anchor for the Cat Cay Shoals and probably prevented
vigorous sediment transport. Unexpectedly, the top of Pleistocene along the sand belt is deeper
north of Ocean Cay compared to the southern area. This variable depth of the Pleistocene results
in contrasting thicknesses of the Holocene deposits; it is thicker than 5 meters in the north but
usually less than 5 meters in the south.