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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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