Geology Reference
In-Depth Information
stones are (a) mixing of grains derived from source ar-
eas that may be different with regard to facies and age,
and (b) vertical sequence of sedimentary structures and
textural types indicating changes in energy levels dur-
ing deposition.
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the growth form, tendency of attachment, strength
and community composition of reef-building organ-
isms,
•
the length of time after a hurricane before a subse-
quent storm disturbs reef recovery. Recurring storms
may selectively destroy fragile corals. This can re-
sult in gaps in otherwise distinct ecological zona-
tions (Geister 1992).
12.1.2.2 Impact of Tropical Storms on Reefs
Geologists like to relate the origin of allochthonous reef
material, e.g. fore-reef breccias (Pl. 27/2) or isolated
reef boulders found in slope position to catastrophic
storms devastating the reefs. This is a pleasant inter-
pretation, but how do severe storms actually affect reefs
and how can we trace their activity in the geological
record?
The
recovery period
of modern reefs (the time taken
for the reef to return to pre-hurricane conditions) lasts
between a few years and some tens of years. Scoffin
(1993) reports numbers between 5 and 40 years. Re-
settlement is selective and depends on the available
substrates. On soft muddy substrates resettlement of-
ten starts with sponges and soft corals, on hard sub-
strates with cyanobacteria and sponges.
Modern tropical storms and hurricanes cause sig-
nificant perturbations that affect reef communities and
destroy reef architecture (Hughes 1993; Scoffin 1993).
Damage by hurricanes is common in shallow reefs. The
effect of hurricanes and cyclones may be significant
over tens to hundreds of square kilometers, but reef
destruction is often patchy. Hurricane waves and storm
surges erode reef crest, corals, reef rocks and sediments
down to more than 20 m depths and produce mud- to
boulder-sized fragmented reef material. This material
accumulates
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Changes in diversity over time influenced by severe
storms have been explained in the context of the
'inter-
mediate disturbance hypothesis'
(Connell 1978), sug-
gesting that the highest number of corals species will
be reached at intermediate levels of the frequency and
size of natural disturbances. The
effects of hurricanes
on reef diversity
vary with depths, reef zones, the time
since the last disturbance of the ecosystem, and the mor-
phology of dominant species (Rogers 1993). Applica-
tion of the intermediate disturbance hypothesis to mod-
ern ancient reefs struck by severe storms requires data
on pre- and post-hurricane species number and cover.
as talus on and at the foot of the forereef slope, in
grooves on the reef front and on submarine terraces,
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as storm ridges consisting of shingle, boulders or
single blocks deposited on intertidal reef flats,
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as beach ridges formed by leeward migrating shingle,
The Hurricane Gilbert - Cozumel Case Study
In September 1988, Hurricane Gilbert, on its devas-
tating path through the western Caribbean, passed di-
rectly across the Mexican Island of Cozumel, off the
eastern shore of Yucatan Peninsula. The eye of the hur-
ricane was situated directly over the northernmost tip
of the island. The storm reached an extraordinary
strength (about 320 km/h) and a minimum surface pres-
sure (885 hPa), damaged coral reefs on the shelf and at
the shelf margin, and also destroyed the tropical forest
on the island (Sanchez et al. 1999).
The shelf of Cozumel Island had been intensively
studied only a few months before with regard to plat-
form and reef morphology and distribution patterns of
benthic organisms. Priority was given to the carbon-
ate-producing fauna and flora, and the quantitative as-
sessment of the shallow-water biotopes on the insular
shelf (Muckelbauer 1990). With these detailed obser-
vations of the pre-hurricane situation at hand, a new
survey was started (Muckelbauer 1991, unpublished
research report, Institute of Paleontology Erlangen) in
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as scattered debris in back-reef lagoons, and
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as drapes of carbonate sand and mud in forereef and
in lagoonal areas.
Common and in the geological record recognizable
results of strong storms
are the disintegration of reef
framework, coral breakage, dislodgment of heads of
massive corals, transport of variously-sized reef blocks
and of carbonate sand, and scarring on standing corals
caused by waterborne debris.
The extent of damage that will in turn influence the
chance of recognizing storm effects in ancient reefs de-
pends on many factors including
•
the morphology and size of the reefs,
•
the violence of strong storms relative to fair-weather
effects and the state of tides,
•
the angle under which the cyclone affects the reef
slope,
•
the shelter afforded by differences in water depths,
topography or by the structure of the reefs,
