Abstract
Seven horizontal cores were taken from the reef-dominated margins of Salt River submarine canyon, St. Croix, U.S. Virgin Islands at water depths of 14-30 m. The pattern of east-to-west sediment transport in the area exerts a major control on present-day reef morphology, as well as accretionary styles recorded in the cores. Coral growth is inhibited by sedimentation on much of the eastern (heavily stressed) margin, and slopes are gentle. The regular pattern of alternating reef growth and sedimentary infill in the eastern cores reflects this highly variable environment. On the west wall, away from the source of incoming bedload sediment, coral cover is much greater, and a vertical reef wall forms the canyon margin. Slumping of the steep reef face has caused numerous repetitions of section and, in some instances, accretion rates higher than the growth rates of calcifying organisms occupying the present or past reef surface. This slumping process, along with the highly dissected character of the reef, results in larger sections of reef framework separated by open or sediment-filled vugs and caverns. 14 C dates indicate that this complex constructional history has resulted in at least 24 m of lateral accretion during Holocene time. Average lateral-accretion rates in the cores varied from 0.84 to 2.55 m/1,000 yr, with the highest rate occurring in the core from deepest water. Intervals of rapid accretion within the cores were not generally related to patterns of in situ coral growth, but rather with intervals of allochthonous material slumped from the shallower portions of the reef complex. The character of the cores illustrates the potential importance of detrital material and late-stage reworking in the accretion of shelf-edge reefs. The high rates of accretion at depth highlight potential problems with reef models based on net accretion paralleling the abilities of the present-day surface organisms to produce calcium carbonate.
