Geology Reference
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due to the current-transport of sediment from the shelf
to the shelf margin during and after the hurricane. Shelf
reefs show no notable changes in the frequency of soft
sediments.
• Carbonate-producing organisms are distinctly re-
duced in frequency (Fig. 12.6B).
• Corals and hydrozoans are reduced to almost one
half their pre-hurricane level (Fig. 12.6A).
• Growth forms of corals: Branched corals exhibit a
decisive reduction in all reef sites (Fig. 12.7B). The
coverage of massive and platy corals has not been re-
duced except platy corals in shelf-edge reefs.
• Considering all corals in regard to the relative cov-
erage of all corals dendroid corals have lost out to mas-
sive corals.
• Total coral diversity (species richness) has been
maintained, but the ranking of species according to their
frequency in the reefs has changed considerably.
• Differences in coral diversity between reefs located
in different water depths have become more pronounced
due to great reductions in species dominating in pre-
hurricane time.
• Reefs in deeper water are more greatly destroyed
than those in shallow water, due to the higher vulner-
ability of dendroid corals dominating in deeper water.
• Different algal groups reacted differently: The cov-
erage of brown algae has increased significantly, that
of green algae decreased. Red algae show no signifi-
cant change.
• Calcareous algae have not declined quite as drasti-
cally as corals but still significantly.
• The frequency of calcareous green algae has been
variously reduced. Halimeda is reduced by one half.
Other calcareous green algae show a slight increase.
• Surprisingly, the coverage of calcareous red algae has
not changed.
• The coverage of cyanobacteria growing on trans-
ported coral fragments and dead coral heads has in-
creased significantly.
• The destruction of the non-calcareous sessile reef
organisms (mainly sponges, octocorals, hydroids, soft
algae) has been hit by the hurricane similar to that of
the scleractinian coral fauna.
• High-growing finger-, bowl-, tube- and vase-shaped
sponges have clearly lost ground. Low-growing encrust-
ing and boring sponges have gained in distribution both
in near-coast reefs and in shelf-edge reefs, because
many of these sponges live in protected cryptic habi-
tats.
rather than the reestablishment of the reef community.
The shifts as well as the creation of large no longer
organically covered areas show that the impact of hur-
ricanes on reefs is not solely destructive, but that the
ecosystem also benefits from severe storms. The ini-
tial recovery stage is indicated by the strong increase
in the frequency and distribution of cyanobacteria.
How to recognize storm effects in ancient reefs ?
Box 12.4 reviews criteria that indicate disruption of
reef growth by severe storms. Most criteria must be
evaluated in the outcrop, but some should also be evi-
dent on a microfacies scale.
12.1.2.3 Significance of Carbonate Storm
Deposits
Severe storms acting in shallow-marine environ-
ments change sedimentation patterns, influence the dis-
tribution of organisms, and modify the course of di-
agenesis. Ancient storm sediments provide information
on the following topics:
Depositional processes: Tempestites and reefs offer
the possibility of understanding the physical controls
on debris production.
Stratigraphy: The recognition of the imprint of hur-
ricanes and other strong storms on the sediment record
provides a useful stratigraphic marker for the study of
ancient reef systems.
Paleoclimate: Because the frequency and the pre-
dominant tracks of intense hurricanes are expected to
have varied in time as a function of global climatic
changes (Emanuel 1987; Barron, 1989; Gray 1990), hur-
ricane-related tempestites are used as paleoclimatic
proxies.
Paleogeography: Storm-dominated, windward plat-
form margins are characterized by sand bodies trend-
ing parallel to the platform margin. Windward-leeward
orientations of ancient reef complexes can be deduced
from distributional patterns of fore-reef sediments.
These criteria as well as modeling of windward-lee-
ward effects based on paleowind directions allow large-
scale and medium-scale paleogeographic maps to be
reconstructed.
Diagenesis: Hurricanes are frequently accompanied
by heavy falls of rain. Strong rain may trigger vadose
dissolution and karstification processes of carbonate
platforms (Bourrouilh-Le Jan 1998) and influence do-
lomitization in tidal flats (Bourrouilh-Le Jan 1979).
Reservoir rocks: Onshore directed storms can accu-
mulate shoals and sheets of porous carbonate sands of
reservoir quality along coastlines. Draping of storm-
The Cozumel study describes the situation just one
year after the hurricane. Most of the changes observed
reflect the biotic loss and shifts in the dominant taxa
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