ABSTRACT: A Modern Example of Reef Backstepping from the Eastern St. Croix Shelf, U.S. Virgin Islands

HUBBARD, DENNIS K., West Indies Laboratory, St. Croix, USVI

Descriptions of rapidly accreting Acropora palamata reefs have raised questions about sea level rise as a limiting factor in Holocene (and by extension, more ancient) reef development. Some workers have argued that a reduction in light and/or calcification by increased sediment or nutrient levels is a more likely mechanism.

Core data from eastern St. Croix shed light on this argument. Shelf-edge reefs (present depth ~20 m), dominated by A. palmata, rimmed the eastern St. Croix shelf between 6000 and 9000 y.b.p. Despite their occurrence in depths ideal for rapid A. palmata growth, they apparently stopped accreting between 3800 and 5500 y.b.p. while shallower reefs closer to shore continued to track rising sea level until the present time.

Seismic data reveal that the deeper reefs formed on a narrow Pleistocene karst(?) ridge. The reefs tracked rising sea level until their lower margins built to the edges of the underlying ridge. At that point, bioeroded sediment and rubble, previously incorporated into the accreting reef, were now lost into deeper water. This geometric constraint slowed accretion despite a likely maintenance of high growth rates by individual corals. As water depth increased, A. palmata was replaced by slower growing head corals, and the reef was gradually left behind. Neither sudden sea-level rise nor a change in oceanographic regime was required. The likelihood that ancient reefs localized on similar topographic highs infers that this mechanism has been important in many ancient systems "paradoxicall " left behind by rising sea level.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)