Coastal morphology; Southwest Great Abaco Island, Bahamas

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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(

    7725

    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 E6 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...

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