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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Vol. 67 (1997)No. 1. (January), Pages 1-16

Anatomy of a Fringing Reef Around Grand Cayman: Storm Rubble, Not Coral Framework

Paul Blanchon (*), Brian Jones, William Kalbfleisch

ABSTRACT

Our fair-weather perception of modern reefs has led to the implicit assumption that their development is controlled by processes that govern the siting of in-place coral growth. Yet more ephemeral processes, such as storms and hurricanes, assume much greater importance over longer time scales because few reefs escape their influence. To discover the importance of storms on reef development, we analyze the zonation, anatomy, and architecture of a fringing-reef complex around Grand Cayman. We find that the surface zonation of in-place corals is merely a facade and the reef core is in fact composed of meter-thick layers of coral-cobble rudstone capped by crusts of coralline algae. The large size and abraded condition of the rudstone clasts shows that these layers are not the product of f ir-weather processes but the result of destruction and deposition during hurricanes. As hurricane waves cross coral-mantled zones of the inner shelf, they destroy live coral stands and deposit the clasts as a rubble layer covering the entire reef complex. Between storms, this rubble foundation is stabilized by coralline-algal crusts and recolonized by rapidly growing corals, leading eventually to full reef regeneration before the next hurricane. This cyclic pattern of destruction and regeneration consequently produces a fringing-reef complex with a core composed of hurricane-generated rubble-not coral framework as previously assumed.

In addition to explaining reef anatomy, hurricane control also explains the variation in reef architecture along shelf, uniform reef location across shelf, and reef absence along certain shelf sections. As hurricane waves cross a mid-shelf scarp, they start to break and destroy coral growth over most of the inner shelf. Coral rubble generated by these waves is deposited 350 (± 50) m from the mid-shelf scarp on margins exposed to the largest waves, but only 250 (± 50) m on semi-protected margins that experience smaller, fetch-limited waves. In areas where the width of the inner shelf is < 250 m, hurricane waves throw rubble ashore and a fringing reef does not develop. During sea-level rise, this influence of shelf width on rubble deposition controls the timing of reef nitiation, and that in turn controls reef architecture. Reefs initiate first on low-gradient coasts with wide shelves, and gradually extend around higher-gradient coasts as sea level rises and shelf width increases. Thus, older reefs are located farther offshore, front deeper lagoons, and have thicker and narrower profiles than younger reefs.

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