What Lies beneath: the Relationship between Antecedent Topography and Modern Geomorphology from Glovers Reef, Belize
Glovers Reef, off the coast of Belize, is a 260 km2 reef-rimmed platform that has over 800 patch reefs within its lagoon. The critical question is what are the foundations of these patch reefs. A single-channel seismic survey documents antecedent topography beneath the majority of the path reefs and a Holocene sediment wedge oriented with respect to the wind direction across the lagoon.
We used a single-channel seismic survey of approximately 111 km of grid lines with 0.5-2 km spacing, satellite imagery and SCUBA transects to determine relationships between antecedent topography, wind direction, and the modern geomorphology of Glovers Reef. Three seismic facies were identified: 1) high amplitude, continuous reflections over a transparent facies occur in the Holocene topographic lows, 2) chaotic to transparent facies occur on Holocene highs, and 3) an irregular set of high amplitude reflections above a transparent facies, which was interpreted as the Pleistocene horizon. Even when accounting for the pull-up effects caused by the Holocene reefs, Pleistocene topography exists beneath the majority of the patch reefs within the lagoon.
Maps of Holocene and Pleistocene seismic horizons document that the Holocene lagoon is deeper to the south than the north, while depth to the Pleistocene top is fairly constant at 18-19 meters across the lagoon. These differences in Holocene and Pleistocene bathymetry are primarily due to a 6-7 meter thick wedge of Holocene sediment that is thickest in the northeast, windward corner of the atoll and thins into the lagoon. An IKONOS-based benthic classification scheme constructed in ENVI software also illustrates the asymmetric nature of modern facies morphology with more and thicker facies belts located on the windward margin of the atoll. Only on the northern and eastern windward margins is the reef fully aggraded to a rubble rampart and a well-developed reef apron. Asymmetric facies belts are apparent at both kilometer (platform) and decameter (patch reef) scales. Additionally, 28 depth profiles across patch reefs within the lagoon show steeper windward versus leeward margins, suggesting that the energy gradient and subsequent sediment response continues well beyond the outer reef rim.
In summary, the modern geomorphology of Glovers Reef is a reflection of both Holocene sedimentation responding to wave and current exposure from the northeast and antecedent topography.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009