Accretionary Styles in Shelf-Edge Reefs, St. Croix, Virgin Islands: ABSTRACT

Abstract

Fourteen reef cores were taken in water depths of 8-30 m (25-100 ft) along two contrasting shelf margins on St. Croix, Virgin Islands. Seven of the cores were drilled vertically into the deep reef rimming the narrow (150-300 m, 500-1,000 ft) shelf of Cane Bay. Seven horizontal cores were drilled into the steeply sloping reef walls of Salt River submarine canyon. The Cane Bay cores indicate initial Acropora colonization on a rubble slope of 5,000-6,000 years B.P. (at a water depth of 5 m or 16 ft). A subsequent 3,000-year hiatus in reef accretion was ended by recolonization of head corals (at a water depth of 8 m or 25 ft). The Salt River cores show alternating Montastrea annularis framework and open or sediment-filled voids up to 2 m (6 ft) across. This pattern is related to the complex system of channels, caverns, and overhangs common in this steep reef face. At Cane Bay, vertical accretion rates averaged 1.16 m/1,000 years (45.7 in./1,000 years), with a range of 0.17-1.69 m/1,000 years (6.7-66.5 in./1,000 years). Lateral accretion rates at Salt River averaged 1.03 m/1,000 years (40.6 in./1,000 years), with a range of 0.84-1.38 m/1,000 years (33.1-54.3 in./1,000 years). The surprisingly high lateral accretion rates at Salt River are largely the result of down-faulted reef blocks causing repetitions of section. Comparison of reef-accretion rates based on the cores with calcification potential for the reef (primarily as coralgal growth) indicates that more than 60% of the carbonate incorporated in the reef is ultimately reduced to sediment. Unlike that on Pacific reefs, this material is moved seaward and off the shelf edge, primarily during major storms. Such early degradation of the reef relates to the under representation of reef framework in the ancient record. End_of_Article - Last_Page 488------------