Geoscience Reference
In-Depth Information
Fig. 9.17
Storm-generated coral rubble ridges. A series of ridges, up to 3 m high, occur at these sites, recording evidence of successive
storm events. Triangulos, Campeche Bank, Gulf of Mexico.
et al. 1996). The scale and extent of disturbance
are often highly variable even over short spatial
scales, but in high-impact areas reduced live
coral or mangrove cover may be an immedi-
ate consequence of disturbance. Disturbance
may be exacerbated in reef environments by the
subsequent effects of cyclone-related rainfall,
which may induce localized coral bleaching (Van
Woesik et al. 1995), and in mangrove settings,
increased rainfall may reduce soil salinity levels
or cause erosion due to surface runoff.
From a sedimentological perspective, cyclones
exert a direct influence on sediment transport,
and, over geological time-scales, should be
regarded as an important control in tropical
sedimentary environments. A detailed review
of the geological effects of cyclones within reef
environments is provided by Scoffin (1993), who
identified a range of erosional and depositional
processes. These include both on- and offshore
transport of sediment and rubble, much of
which is derived from physical breakdown of
the coral communities. Onshore transport can
result in the development of rubble cays or
ridges (Fig. 9.17; Hayne & Chappell 2001) and
sediment deposition in the lee of these features
(Fig. 9.18), whereas off-reef transport produces
local talus deposits. In shallow water, storm-
generated rubble can be a major constituent of
reef facies (Blanchon et al. 1997). Storm surges
may also result in significant sediment mobiliza-
tion in shallow water, leading to beach, coral
cay and subtidal erosion.
Studies in St Croix provide an insight into the
volumes of sediment that may be mobilized during
hurricanes. At Salt River Canyon, an estimated
2
10
6
kg of sediment were flushed from the reefs
during the passage of Hurricane Hugo in 1989,
and at nearby Cane Bay an estimated 336,000 kg
of sediment were flushed offshore from a single
reef channel (Hubbard 1992). Recent work on
the Great Barrier Reef (GBR) shelf demonstrates
the extent to which cyclones also influence sedi-
mentation across much wider carbonate shelf
systems (Larcombe & Carter 2004; see also
Chapter 10). Cyclones, which typically approach
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