Geology Reference
In-Depth Information
High-energy storms generate waves and currents that
affect sea bottoms, stir up sediment and create charac-
teristic sedimentary structures (e.g. hummocky cross-
stratification) and textures (e.g. tempestites). Severe
storms erode coasts, destroy tropical islands and reefs,
take part in onshore and offshore sediment transport
and may contribute to extensive supratidal sedimenta-
tion. Shallow-marine waters respond to wind energy
by
•
Box 12.2.
Selected references discussing the impacts of
hurricanes on organisms and sediments in modern shal-
low-marine carbonate environments. Frequent hurricanes
occur today in belts 7° to 25 °N and S of the equator.
Major reef areas suffering regular hurricanes occur in
the northwestern and southwestern Pacific, the northern
and southern Indian Ocean, and the northwestern Atlan-
tic Oceran including the Caribbean Sea.
Reefs
:
Many papers compare pre- and post-hurri-
cane situations of specific reefs. A recent overview was
given by Scoffin (1993). The list here includes only pa-
pers not cited by Scoffin: Blanchon and Jones 1997;
Braithwaite et al. 2000; Davis and Hughes 1983; Gagan
et al. 1987, 1988, 1990; Guozhong 1998; Heap et al.
1999; Hubbard 1992; Kaufman 1983; Knowlton et al.
1988; Kobluk and Lysenko 1987; Lugo-Fernandez et al.
1994; Moran and Reaka-Kudla 1991; Rogers et al. 1982;
Russ and McCook 1992; Schönberger 1989; Woodley
1981; Yamano et al. 2001; Young et al. 1999.
Carbonate platforms and ramps:
Aigner 1985; Boss
and Neumann 1993; Bourrouilh-Le Jan 1979, 1980,
1998; Davaud and Strasser 1984; Galli 1989; Hine 1982;
Seguret et al. 2001; Vermeer 1963; Wanless et al. 1988.
Beaches
:
Ball et al. 1967; Coch 1994; Collins et al.
1999; Davis et al. 1982; Knowles and Davies 1991; Lee
et al. 1994; Siringan and Anderson 1994.
coastal swell (storm floods) resulting from upland
winds that raise the sea level,
•
surface waves producing breakers at the coast, and
•
ground waves that become transformed into onshore
and offshore bottom currents.
Severe storms are represented by cyclones and hur-
ricanes, occurring in tropical latitudes, and by intense
winter storms (blizzards) in middle and high latitudes
forming at fronts between cold and warm air masses.
The development of strong storms is controlled by
changes in atmospheric pressure and wind intensity.
The importance of storm deposits in the geological
record has attracted the attention of many workers. The
papers listed in Box 12.2 describe the effects of mod-
ern storms known as cyclones, hurricanes and typhoons.
A
cyclone
is an intense cell of low atmospheric pres-
sure with a central eye surrounded by a circular wind
system that rotates clockwise in the Southern Hemi-
sphere and anti-clockwise in the Northern Hemisphere.
The system follows tracks that generally curve away
from the equator.
Tropical cyclones
form over the tropi-
cal oceans and range from 100 to 1000 km in diameter.
Wind velocity may increase to up 230 mph and more.
Tropical cyclones transport fine sediments and nutri-
ents from shore to reef zones. The term
typhoon
desig-
nates a tropical cyclone occurring in the Pacific region.
A
hurricane
is a tropical cyclone in which the wind
speed equals or exceeds 118 km/h and may reach more
than 300 km/h as in the example of Hurricane Gilbert
described below. Locations crossed by the central eye
experience winds from different directions that gener-
ate storm surges causing catastrophic damages within
a zone of approximately 30 km on either side of the
eye.
sediment redistribution than the quick passage of a
weakened hurricane. The major outcomes of severe
storms are (a) transport and deposition of storm sedi-
ments on shelves and export to slope and basinal envi-
ronments, and (b) the destruction of reef structures.
These processes are discussed in the following two sec-
tions.
12.1.2.1 Storm Deposits (Tempestites) on
Shelves, Ramps and Platforms
Storm beds are most common on shelves and ramps in
windward settings, because of the generation of sedi-
ment-laden storm surges that transport sediment from
onshore to deeper offshore outer ramp settings via uni-
directional return flows. After the storm is over, the
sediment is redistributed across the ramp and finally
deposited in the form of sheets of limited extension in
low-energy environments below storm-wave base (Fig.
12.3). These storm deposits exhibit distinct proximality-
distality trends as shown, for example, in the Arabian
Gulf. Storm deposits of attached platforms (e.g. Florida)
display strong variations in internal composition. The
effects of storms on isolated platforms (e.g. Bahamas)
are concentrated at platform margins (Hine et al. 1981).
Sediment transport and redeposition produce milli-
meter- to meter-thick storm layers, contribute to the
The varying intensity, size and strength as well as
the forward speed and duration of strong storms are
critical constraints on the storm's capacity to modify
shallow-marine environments (Boss and Neumann
1993). Impacts from hurricanes are proportional to the
area and the period over which hurricane conditions
persist. Slower-moving hurricanes will cause more ex-
tensive and geologically relevant surface damage and
