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Geoforum, Vol. 6, pp. 237-246, 1975. Pergamon Press Ltd. Printed in Great Britain
Coastal Morphology; Southwest Great Abaco Island, Bahamas*
C. N. RAPHAEL, Ypsilantit
Summary: An examination of a portion of Great Abaco Island, Bahamas, reveals that the coastal zone may be subdivided
into two morphological units: barrier and lagoon complexes, and rock benches with boulder ramparts. The sand barriers,
colonized by multistoried vegetation, are stratigraphically thin and are characterized by narrowcoralline beaches and beach
rock. The imbricete boulder ramparts adjacent to deeper water, reveal that, on occasion, high wave energy conditions
occur. It is apparent that hurricanes and tropical storms are significant in modifying coastal southwest Great Abaco.
Introduction km’. The Banks are separated by the deep Northwest
Providence Channel into two distinct archipelagos; the
The Bahama Banks are the most extensive shelf seas in the Little Bahama Bank lies to the north and the Grand
tropical Atlantic. Separated from Florida Ily a deep channel, Bahama Bank to the south (Fig. 1). The isalnds occupy
the shallow platforms extend over an area of some 150,000 13,200 km* or about 7% of the area, are generally confined
1 I 79
I 77 75
-% %
Great Aboco z
c
0 26
Eleuthera c5
24
miles
C. Nicholas RAPHAEL, Professor of Geography, Eastern
Michigan University, Ypsilanti, Mich. 46197, USA.
The writer gratefully acknowledges the assistance of Dr. Eugene
JAWORSKI in the field.
237
Fig. 1
l Location of the Bahama
Banks and the study area.
238 GeoforumlVolume G/Number 31411975
to the margins of the Banks, and resemble very large
atolls. The seascape is one of the finest areas of contem-
porary limestone deposition in the world and in recent
years has attracted considerable interest.
Geologists probing for gas and oil, and investigating the
nature of carbonate deposition have concentrated their
studies on the intertidal and adjacent submarine zones
(e.g. MULTER, 1970; SHINN et a/., 1965).
Traditionally, the geologic history of the Bahama platform
has involved classic rigid earth ideas and conventional
concepts of carbonate deposition. One interpretation for
its origin suggests that the calcareous platform was
deposited on the eroded and dissected North American
craton and it therefore overlies a sialic basement (TALWANI.
et al., 1959). Others have inferred that the islands form a
mega-atoll and could conceivably represent carbonates
deposited on the periphery of volcanic islands which have
subsided beneath the sea (NEWELL, 1955). A third
possibility is that carbonates were deposited on a shallow
sea floor. As sedimentation continued, subsidence of the
ocean basin to great depths occurred. More recently
however, the concepts of sea-floor spreading and plate
tectonics have been used to explain the origin of the
banks. As postulated by Dietz and others, a small mediter-
North America seperated and rotated to the north away
from Africa (DIET& eta/. 1970). This rift was then filled
with elastic sediments derived from the adjacent continents
principally through the process of turbidity currents.
With continued separation of the North American and
African plates since Late-JurassicKretaceous time,
ecological conditions improved for incipient carbonate
deposition. Eventually, the Bahama platform was
separated from North America and Africa, and deposition
and subsidence continued.
Coastal geomorphic studies of the Bahama platform are few
and are restricted to the cays of the Great Bahama Bank. A
their origin.
geomorphological reconnaissance by Doran with its large
scale maps is confined to the extreme southeastern portion
of the archipelago, whereas Lind has limited his work to the
landforms of Cat Island (LIND, 1963, DORAN, 1955).
Both writers after identifying and mapping exposed
marine surfaces considered them as indicators of
a higher than present stillstand of the sea in the past 5000
years. Fieldwork undertaken on the southwest coast of
Great Abaco (Fig. 2) suggests that here many coastal
landforms are the product of hurricanes and tropical storms
and hence do not necessarily require higher stillstands for ranean was created during early to mid-Mesozoic time as
2(
77’25’
l
Fig. 2
Location of the investi-
gated sites and the offshore
topography (U.S. Naval
Oceanographic Office, H.O.
Charts 2499 and 5990).
GeoforumIVolume G/Number 31411975 239
Physical Setting
Tectonically, the Bahama Banks are considered stable; no
major displacements have yet been reported. The absence
of volcanic activity and submarine trenches would seem
to confirm crustal stability. However, as in most active
carbonate areas, subsidence is active. A bore hole on
Andros revealed that the carbonates have a thickness of at
least 5000 m and that these shallow water deposits date
from early Cretaceous time (GOODELL & GARMEN, 1969).
This and other borings clearly imply downwarping since
the upper Mesozoic. Although the dominant geological
theme has been shallow water carbonate accumulation and
subsidence, other slight changes in elevation have been
detected. Doran suggests positive displacements of 8m on
Little lnagua (DORAN, 1955). This conclusion is based on
the presence of coral in growth position at that level.
In southwest Great Abaco a rock bench paralleling the
coast varies vertically as much as o.6m. At Rocky Point
the bedrock is inundated at normal high tide whereas
Fig. 3
A rock surface exposed during
low tide near Rocky Point
Fig. 4
The bench, terminated by a boulder rampart, dips north- westerly towards Rocky Point where it is at about mean sea level. The wave-cut notch is exposed during low tide.
240
southeastward this level calcarenite surface is above high
spring tides (Figs. 3 and 4). Also tension cracks, some of
which have been welded, tend to substantiate some slight
regional unrest. Local uplifts of similarly ‘stable’ areas
of the Pacific have also been reported by NEWELL and
BLOOM (1970). Thus, slight eustatic sea level changes on
this coast may be difficult to determine because of
slight tectonic modifications.
Southwest Great Abaco was selected for this study
because of contrasting submarine and subaerial conditions
Banks in the vicinity of Sandy Point are shallow; many
portions are intertidal exposing elongated sand flats
twice per day. The mean depths here are less than 3m and
thus a favorable harvesting area for conch fishermen who
use 4m poles to harvest these bottom dwellers. From
Rocky Point southeastward the coast is adjacent to the
Northwest Providence Channel where depths of 10m are
rapidly encountered in the rocky offshore area. Swell
waves are normally high (0.6-lm) and break directly on
the rocky scarp which is less than 1 m in elevation. Due
to the character of the submarine bottom, low waves
are normally generated and are associated with
depositional landforms such as at Sandy Point.
Exposed rock coasts adjacent to deeper water such as
Rocky Point, are being attacked by higher waves.
Great Abaco is frequently struck by tropical storms and
hurricanes. Accurate data indicate that between 1871
and 1973 forty-nine tropical cyclones of hurricane
intensity passed within 150 km of its southwest coast
(CRY, 1965). Within this period of about a century, 6
hurricanes and numerous tropical cyclones passed over
southern Great Abaco. Tannehill reports 7 hurricanes
within 150 km of Sandy Point between 1804 and 1871
(TANNEHILL’, 1956). Although pre-nineteenth century
data are sketchy, between 1500 and 1796, 7 storms
of at least tropical cyclone intensity have been
documented for the Bahamas (MILLAS, 1968). The
combined data of Cry and Tannehill and others reveal
that between 1804 and the present, 58 hurricanes passed
near the study area. More than one-half of these
tropical depressions were located between Florida and
Great Abaco for at least 24h.
Although rainfall is characterized by distinct seasonality
(Aw, Koppen), the coastal vegetation is luxuriant and
l The upper story includes sea grape (Coccoloba uvifera), coconut
palm ~CCJCOS sp.l, palmetto LSabalpalmettol and occasionally Australian pine (Casuarina equisetifolia). Near ground level the thickets are dominated by sea oxeye (Borricbia arborescents),
low bushy senna (Cada sp.), the shrub cosmopolitan espancil (Soriana maritima) and dense vines (Smilax sp.).
Geoforum/Volume G/Number 3/4/l 975
diverse. Sand barriers, separating shallow lagoons from
the sea, are colonized by grasses, especially sea oats
(Uniola paniculatal and saltmarsh grass (Distichlis
spicata), and prostrate pioneer plants such as beach
morning glory (lpomeapes-caprae) and sea-purslane
(Sesuvium portulacastrum). Landward, where slash-
burn or plantation activity has occurred, the vegetation is
two-storied. * Distinct vegetation communities similar to
those noted on Great Abaco colonize other tropical coasts
(SAUER, 1962). Even with the passage of hurricanes the
low vegetation adjacent to the coast has a low mortality
rate and little difficulty reestablishing itself in spite
of heavy seas and washover deposits. This is most evident
in the coastal areas where the three fathom contour is
displaced seaward such as between Blackwood and George
Points, and Sandy and Rocky Points. Here a shallow shelf normally protects the beach to within a few meters of
the high water mark.
Coastal Types
The coasts, which reflect the submarine environment, may
be divided into two morphological units: barrier and
lagoon complexes, and rock benches with boulder
ramparts.
The barriers of southwest Abaco are for the most part
composed of biogenetic sand and have maximum elevations
of 3 m (Fig. 5a). Test pits indicated that sedimentary structures were absent, although buried soil profiles were
common. A representative pit on the Sandy Point barrier
revealed 25 cm of medium to coarse sand overlying 40 cm
of fine organic sand. At a depth of approximately 0.6 m eolianite was encountered which became more consolida-
ted with depth. The surficial stands are composed of coral
and shell fragments. However, entire conch shells
(Strombussp. ) do occur suggesting that waves as well as
wind are agents responsible for deposition and modifica-
tion of the barriers.
Where severe coastal erosion has occurred such as at
Sandy Point, exposed eolianite 0.6 m thick outcrops above
the level of spring tides. Coastal distribution of eolianite
is patchy. However, more prominent exposures occur
where coastal erosion of the barrier is evident. Outcrops
which dip 7-9 degrees seaward are bored by terrestrial
organisms, and commonly contain root fragments and
shells, especially Callista eucymata (Dall). Petrographic
and hand lens inspection revealed that the eolianite
fragments are finer, more rounded, and more tightly
packed than beach rock sediments. Cementation of eolianite
is possibly caused by the seepage of rain water charged
GeoforumlVolume G/Number 3/4/1975 241
a.Sandy Point Fresh Sand
b. George Point
I
1
0 60 120 270 330
12 c. Hole in the Wall
g E’6 z
0 1 0 15 30
d. Rocky Point
b 15 30 I5 60 75
e. Blackwood Point
Boulder Rampart
Carbonate Bedrock
Fig. 5
l Selected coastal orofiles
with calcium carbonate through porous dunes (RUSSELL,
1962). Its development is assisted by occasional wetting
of the deposit and is associated with seasonal rainfall. On
Great Abaco water tables fluctuate about 0.3 m with the
oscillating tides.t Such a fluctuation will also cause
periodic wetting of the barrier and probably accounts for
t Dr. Daniel TURNER (Eastern Michigan University) has carefully
recorded the oscillation of water levels in four open wells in
the settlement of Sandy Point. He reports that water level
fluctuations in wells, due to the rise and fall of the tide, average
around 0.3 m. During a heavy rain in February, 1969, the
water levels rose to the surface of the ground causing some
flooding of the village.
242 GeoforumlVolume G/Number 3/4/1975
Fig. 6
. Parallel bands of beach rock exposed during low spring tide at Sandy Point.
the more indurated eolianite with depth. Thus, the alter-
nate wetting and drying may be caused by seasonal
precipitation from above or fluctuation of the water table
below; quite likely both are significant as they are inter-
related.
Beach material which has been indurated by calcareous
cement is referred to as beach rock. Its occurrence is
thought to be controlled by location and temperature of
ground water (RUSSELL & McINTIRE, 1965). Beach rock
observed on Great Abaco is regionally associated with
eolianite and therefore is localized. The outcrops are
composed of coarse coral and shell fragments giving the
appearance of a cemented shell hash. Beach rock is
intertidal ranging from low spring tide to approximately
high spring tide (Fig. 6). The individual bands dip 10-l 1
degrees seaward and look cuesta-like. Between the parallel
bands, intertidal pools are colonized by coral (Parities sp. ),
urchins (Echinoiderms sp. ), rock-boring marine worms
and other organisms.
The geomorphological significance of beach rock is that it
is exposed only on eroding coastlines and the Sandy Point
barrier is no exception (RUSSELL, 1962, p.205; RUSSELL &
MCINTIRE. 1965, p.20, 23). The beaches are narrow and
with high tide and moderately strong onshore winds the
low barrier is undercut. According to local informants at
Sandy Point, 25 m of shoreline has been lost since 1965.
This is substantiated by the erosion of roads leading to the
beaches and dead Casuarina trunks which litter the shore.
Stratigraphically the barrier-lagoon complexes are thin.
Sections in several auger holes and pits indicate that the
barrier is about 3 m thick south of Sandy Point and
overlies a carbonate bedrock (Fig. 5a). The lagoon margins
are fringed with red mangrove (Rhizophora mangle) which
attain heights up to about 6 m. Landward of the red
mangrove to approximately high tide, the lagoon shore is
colonized by black mangrove (Avicennia nitida) 5-6 m
high. The open waters of the lagoon, with the exception
of localized sand, are free from sediment. Drowned sinks
or blue holes and subaerial sinkholes suggest that the hard
carbonate beneath the lagoon and barrier is pre-Recent in
age. Considering the dense vegetation canopy fringing the
lagoon, peat deposits are relatively thin and pinch out at
the mean high tide level. The peat forms a dense reddish
organic mat with occasional living Rhizophora roots.
Cores, 1 m long, revealed 0.6 m of alternating peat and
shelly sand above the bedrock. The barrier and lagoon
occupied by low mangrove (R. mangle) is veneered with
40 cm of dense peat. Beneath the peat the flat carbonate
surface was again encountered. One other estuarine
environment visited northeast of Sandy Point was cored
to a depth of 1 m before the hard carbonate surface was
encountered.
The auger holes and cores indicate that the carbonate
surface located at approximately low tide is the base upon
GeoforumlVolume G/Number 3/4/1975 243
Fig. 7
l An imbricate boulder rampart between Blackwood and Rocky Points. The bench exhibits a whitish-gray zone and is not inundated during high tide.
which the barrier and shallow lagoons are resting. The
elevation, widespread occurrence, and lateral seaward
extent of the carbonate surface suggest that the modern
patch reefs are growing on this surface. The barrier and
the relatively thin peat deposits overlying carbonate rock
suggest these coastal features are probably Recent in age.
On southwest Abaco, a rock bench extends from Rocky
Point to the vicinity of Blackwood Point and has an
average elevation of 0.6 m above mean sea level (Fig. 5d).
At Hole in the Wall another bench surface is encountered
at an elevation of 10 m (Fig. 5~). Inland, both benchesare
terminated by a spectacular boulder rampart composed of
limestone blocks up to 1.5 m across (Fig. 7)*. The bench in
the vicinity of Rocky Point averages 30 m in width and
displays color zonations noted on other benches in the
Bahamas and Florida (MULTER. 1970). An intertidal yellow
zone is characterized by rock pinnacles and intensive
weathering; a black zone inundated by spring tides
illustrates some effects of intertidal erosion; and a smooth
grayish-white zone covered during storms extends to the
base of the boulder rampart. The only unconsolidated
materials are the large boulders. Smaller gravels are welded
onto the flat surface particularly on the intermediate black
zone. A prominent feature are tension cracks which have
been filled by such fragments and cemented. Planation and
solution weathering have left these tension cracks 12-15 cm
above the bench surface.
The erosion occurring on tropical limestone coasts is
complex and involves chemical, biological, and mechanical
processes. The general consensus is that solution weather-
* In the literature it is also referred to as a storm ridge, boulder
ridge, or typhoon rampart.
ing in some form is responsible for notch development (NEWELL, 1964). Notches in the bench observed near Rocky
Point are sporadically distributed and are best developed
in sheltered embayments. At Hole in the Wall, a well-
defined, continuous notch and visor parallels the lee of
that narrow calcareous peninsula suggesting that chemical
and biological agents may indeed be more significant than.
mechanical activity. The level at which active notches occur
also varies. Fairbridge states that the maximum undercut
below the visor occurs at mean sea level whereas Russell
believes notches are cut “between sea level and an
elevation of 1.5 m or more, with visors to about 3 m
higher” (RUSSELL, 1963; FAIRBRIDGE, 1947). In the
investigated areas the notch floor is just below spring tide,
and the visor is partly inundated during high tide (Fig. 4).
The conspicuous absence of well-developed notches in
many localities and the presence of tension cracks indicate
that the visors collapse rapidly. Immediately seaward, in
5-6 m of water, several large tabular boulders were
observed suggesting that this is the case.
Landward of the cliff face, a rampart approximately 2.5 m
high is normally encountered (Fig. 5d). Although occasion-
ally composed of sand or a mixture of sand and blocks,
the principal constituents are blocks of varying size. The
larger blocks are tabular and exhibit an imbricate structure
leaning inland. The average length along the long axis is
aboutl.2 m, however blocks with lengths of 2.5-4 m are
frequently encountered. Boulders composing the higher
elevations of the barrier generally are more spherical and
have diameters of about 0.6 m. The barriers composed of
blocks have contrasting vegetative covers (Fig. 8). Generally
the boulders are covered with a dense coppice of woody
vegetation, especially sea grape (C. uvifera). Others are
244 Geoforum/Volume G/Number 3/4/1975
Fig. 8
The leeward side of a boulder rampart which has washed over and exhibits a tongue- like morphology. To the right and left .of center the ramparts have been vegetated.
Fig. 9
A gravel ridge at Blackwood Point consisting of coral fragments. The coral is derived from patch reefs 3 km offshore.
virtually devoid of vegetation and the blocks are loose and
not compacted. Morphologically, the boulder ramparts
resemble washover fans commonly noted on sand barriers
which have been breached by storm waves. One such
washover was lobate in shape with the tongue of the ram-
part extending 60 m back onto a sand ridge. The rampart
was so fresh that no pitting, staining, or any other type of
weathering was evident. Isolated blocks from this and the
other ramparts were noted 70 m leeward of the barrier
crest. Occasionally large coral heads are incorporated in
the ramparts indicating that patch reefs as well as the cliff
face is a source area for the ramparts.
A barrier profiled at Blackwood Point has characteristics
of the Sandy Point area as well as the Rocky Point area
(Fig. 5e). The offshore zone slopes seaward and is
colonized by living coral approximately 1-2 km out to sea.
A low sand barrier separates a shallow mangrove lagoon
from the platform. Mangrove (R. mangle) is‘attempting to
gain a foothold on the rocky offshore zone as evidenced
by a few scattered prop roots. Sporadic peat deposits occur
suggesting that the mangrove may have been more
extensive in the past. However, occasional high energy
waves discourage extensive development. Between the
high tide strandline and the established sand barrier a
GeoforumlVoturne G/Number 3/4/l 975
rubble ridge composed of coral averaging 25 cm in
diameter and unharvested conch (Strombussp. ) shells
has been deposited (Fig. 9).
Sea Level, Tropical Storms, and the Coast
With the retreat of Pleistocene glaciers at higher latitudes
meltwaters returned to the ocean basins and sea levels
rose. The eustatic rise, which began about 20,000 years
ago, continued until 4000 years ago when a standstill was
reached (BLOOM, 1970, REDFIELD, 1967, COLEMAN &
SMITH 1964). Therefore the present level of the sea is at
its highest since the last interglacial. Some writers, however,
suggest that in the past 4000 years sea level eustatically
deviated 2-3 m from the present level (FAIRBRIDGE, 1961,
BLOCH, 1965). Lind concluded that many Holocene
coastal landforms on Cat Island, including boulder
ramparts and sand barriers are a response to late-
Recent higher than present stand of the sea (LIND,
1969). A seaward decrease in spacing and width of
accretionary ridges in the southeast Bahamas has been cited
as probable evidence of a eustatic sea level drop by Doran
(WRAN, 1955, p.32). The only investigated coastal area
clearly suggesting higher stillstands of the sea is at Hole in
the Wall (Fig. 5~). Rather than uplift or Recent eustatic
change, this bench probably represents a pre-Recent level.
The investigated areas of Great Abaco suggest that the
accretionary landforms are products of present environ-
mental conditions. When the eye of Hurricane Betsy
passed 65 km south of Sandy Point in 1965 the settlement,
which stands 2-3 m above sea level, was inundated with
1 m of water according to local informants. Thus, it is
obvious that the low barrier can be overtopped. Although
not topographically well developed, washover fans do
occur on the Sandy Point barrier. As noted, this barrier
has shells incorporated in it testifying to the fact that high
wave energies do occasionally occur. Washover deposits are
thin and are generally distributed over a short horizontal
distance (Fig. 5a). Test pits and auger holes reveal that the
maximum accumulation occurs on the crest of the ridge
adjacent to the sea. A comparison with post-hurricane
profiles on Mauritius generally reveals similar deposition
(McINTIRE & WALKER, 1964). In fact, beach-ridge
development in Tabasco, Mexico, is attributed to storm
activity (PSUTY, 1967). The surface morphology of the
barriers is related to overtopping by storm waves.
Variation in height between the two beach ridges south of
Sandy Point and contrasting sediment size at Blackwood
Point are possibly due to storm and do not necessarily
indicate sea level changes (Fig. 5a, b). On the average, the
width of washover deposits is about 10 m. However,
245
stringers of fresh sand have penetrated 3-12 m inland and
on occasion reached the lagoon. The interfingering peat
and sand deposits on the lee of a barrier such as at Sandy
Point probably indicate occasional periods of intensive
storm activity followed by calm conditions and normal
mangrove development.
The amount of sedimentation associated with hurricanes
is not always predictable. A comparison of two hurricanes
of comparable size and intensity passing over the
Florida-Bahama area illustrated that supratidal sedimenta-
tion can not be associated with every hurricane (PERKINS
& ENOS, 1968). The washover fan examined at Sandy
Point may have accumulated during one tropical
storm and may not necessarily represent a deposit
increasing in thickness with every storm. Similarly the
thick upper peat on the fringe of the lagoon suggests that
washover sands seldom reach the lagoon. Because no sand
was introduced into the lagoon fringe, does not mean no
storms occurred during the accumulation of the dense peat.
Although the shallow platforms at Blackwood Point are
normally indicative of low energy coastal environment, the
very coarse sediment of the low barrier clearly reveals that
on occasion storms are capable of breaking up the reef
offshore, traversing the platform, and depositing the
rubble l-1.5 m above the high tide line and onto the
backslope of the barrier. These storm-accumulated gravels,
except for their uncemented nature, are similar to those
noted on Pacific atolls where they have been deposited by tropical storms (cuRR~Yeta/. 1970; NEWELL & BLOOM,
1970, PP. 1874-1877; MARAGOS etal., 1973).
Boulder ramparts also favor a storm hypothesis. The
boulders are stacked and display an imbricate structure
indicating that they were shoved rather than tumbled onto
the platform. They are dispersed randomly over the entire
bench and therefore not entirely concentrated at the barrier
front. If Recent sea levels were significantly higher, it
would seem likely that a pre-modern shoreline would have
survived and consisted of reworked or wave-washed sedi-
ments. No evidence of pebbles or small cobbles similar to
the size fraction noted at Blackwood Point were observed.
Tabular boulders noted at the base of the boulder rampart
at different coastal localities exhibited varying degrees of
weathering suggesting that they were deposited at different
times. Perhaps the most striking evidence are boulder
ramparts which have the configuration of washover fans.
The possibility of moving such large boulders can only be
associated with storm conditions.
Although there may have been eustatic sea level changes
in the past few thousand years, many of the landforms of southwest Great Abaco appear to be closely related to
tropical storm activity. Sands in the Sandy and George
246 Geoforum/Volume G/Number 31411975
Point lagoons are either confined to the lee of the barrier
or are absent. The gently seaward slope of the bench and
widespread occurrence of the offshore bedrock strongly
suggest that this feature is a nip-coastal form. The lack of
broad accretionary topography and the presence of thin
washoever fans, narrow beaches, and beach rock at the
investigated sites reveal that coastal erosion is now
occurring. Tropical storms and hurricanes are not normal
events, however, they are a significant phenomena contri-
buting to the coastal morphology of southwest Great
Abaco.
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