Geology Reference
In-Depth Information
Bahama Bank, Bahamas, within one month in
September 2004, provided a unique opportun-
ity to examine the impact of strong tropical
storms on active Holocene carbonate systems.
If storm-generated morphological changes were
ever expected to occur, they should be readily
apparent after the passage of two storms over the
same area within a 1 month period.
The purpose of this paper is to describe the
impacts of these storms, explicitly exploring
the hypothesis that storms are not a major infl uence
on the geomorphology of the shoals in this region.
To further explore the effects of storms, a wave
model studies the depths of infl uence of storm
waves on more simplifi ed carbonate bathyme-
tries such as a gentle ramp or a fl at, unrimmed
carbonate platform. The results of the modelling
suggest that larger storms are not necessarily more
infl uential on the geomorphology of platform-top
carbonate shoals since the larger waves are more
likely to break over the platform edge. The results
are important in understanding the fundamental
processes involved in shaping the geomorpho-
logy of carbonate systems. Additionally, they raise
questions regarding the realism of interpretations
of widespread storm deposits in analogous
systems in the ancient geological record.
1500 m
Break in Reef
Atlantic Ocean
Reef
Linear Bars and
Blowouts
Stranger's Cay
ADCP2
ADCP1
Joe's Cay
Jack's Cay
Little Bahama Bank
Carter's Cay
N
Fig. 1. QuickBird image of the study region in the Northern
Abacos, Bahamas. QuickBird satellite image courtesy:
DigitalGlobe. The yellow dots indicate the locations of the
bottom mounted ADCPs ('ADCP1' in August and 'ADCP2'
in September). The green, red, dark blue and light blue
dots indicate the locations of the Dobie wave gauges.
There was one at the dark blue dot during the entire
sampling period for calibration purposes, and the other
rotated through the other three locations to indicate
differences in the wave fi eld during this time period.
and it consists mainly of a skeletal sandy bottom.
Between the reef apron and the island system
(a distance of ~4.5-5 km), the sea-bottom consists
mostly of peloid-skeletal packstone to grainstone
largely stabilized by seagrass, and abundant patch
reefs. (In this paper, although these sediments
are not rocks, for ease, the Dunham classifi cation
based on grain support and mud content is used to
describe the modern depositional texture.)
Rocky islands formed by Pleistocene aeolianite
are separated by inlets that focus the tidal fl ow.
Flood and ebb tidal deltas in the area consist of gen-
erally lobate forms, convex away from the islands
with endpoints near the inlets (Fig. 1). The ebb
deltas extend up to 2 km from the islands towards
the reef, but the fl ood deltas mostly remain within
1.3 km of the islands. Although the inner portions
of these lobes are mainly seagrass-stabilized fi ne-
sand-sized peloid-skeletal packstone with a few
small local patch reefs, the main channels are gen-
erally hard bottomed (assumed to be Pleistocene
bedrock) with growth of hard and soft corals and
sponges. Outwards from the inlet centre, this
hard bottom passes into coarse lag and eventually
into a seagrass-stabilized area before reaching the
active oolitic shoals, which form the crest of
BACKGROUND
Study area
The study focuses on observations from the
northwestern portion of the Abaco Island chain,
on the northern edge of Little Bahama Bank.
Little Bahama Bank is an isolated carbonate plat-
form bound by the Atlantic Ocean to the north
and east, the Straits of Florida to the west, and
Northwest Providence Channel to the south. A
discontinuous barrier reef is located at the north-
ern shelf break (Fig. 1). The western portion of the
northern boundary of the study area contains a
5-km-wide breach in the reef complex, through
which many open-ocean waves can pass without
being affected by the bottom. This area is deeper
(~10+ m deep) than the adjacent parts of the shelf
located behind the reef (~6-7 m deep), and the
sandy bottom includes many areas with symmet-
rical skeletal sand ripples (~0.15 m high, spaced
about 0.30 m apart), although large (up to 5 m
relief) patch reefs are common. East of this break,
the barrier reef is continuous for 10.5 km, and is
fl anked by a reef apron that extends ~1 km onto
the platform. The apron is generally ~4-5 m deep,
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