Production and Cycling of Calcium Carbonate in a Shelf-Edge Reef System (St. Croix, U.S. Virgin Islands): Applications to the Nature of Reef Systems in the Fossil Record

ABSTRACT Geologic reefs are the end-products of not only constructive processes that produce calcium carbonate, but also of destructive processes (e.g., bioerosion and wave action) that reduce solid substrate to sediment, and of physical processes that rework the reef fabric and transport sediment. A study was conducted at Cane Bay on the island of St. Croix, U.S. Virgin Islands to quantify the suite of processes that have influenced Holocene reef development. Data on carbonate production, reef accretion, bioerosion, and sediment transport were used to construct a detailed "carbonate budget" and to apply the findings of this study to the character of reefs observed in the fossil record. Total carbonate production on the reef ranged from 0 to 5.78 kg/m2 per year, with a reef-wide average of 1.21 kg/m2 per year (1.13 by corals; 0.02 by coralline algae; 0.06 by primarily molluscs, forams and echinoderms). Based on 7 cores, only 0.91 kg/m2 per year of this have been retained within the reef interior. The remaining 0.24 kg/m2 per year of sediment, along with the 0.06 kg/m2 per year contributed directly by molluscs, etc., are deposited within reef channels and are probably flushed from the reef by major storms. Of the material recovered in the cores, 58% was either loose sediment and rubble or open void space. Much of the recognizable coral material was demonstrably reworked and out of place. Accretion rates across the shelf generally increased with water depth, as a result of active slumping along the steepening reef face over the past 2,000-3,000 years. The importance of detrital material in the reef fabric and the major role played by secondary processes that constantly rework the substrate have resulted in a reef whose interior is more of a "garbage pile" than an in-place assemblage of corals cemented together into a rigid "framework." The physical similarities between the detrital fabric of the reef at Cane Bay and those found in many ancient deposits imply that the general processes operating along the Cane Bay shelf have persisted during much of the evolution of ancient reefs. It is proposed that our modern models overemphasize the importance of in-place framework as a criterion for an "ecologic reef."