Great Inagua Research Articles

Quaternary Eustatic Sedimentary Accretion of Southern Bahamas Archpelago: ABSTRACT

Surficial geologic mapping indicates that the southern half of the Bahamas Archipelago is forming by the accretion of discrete depositional sequences resulting from successive eustatic sea level changes: (1) multiple beach and dune ridges, (2) estuarine, (3) lacustrine, (4) shallow subtidal, (5) reef and reef rubble, and (6) megadune complexes. The lithologies are accreted along unconformable erosional-solutional contacts--marine terraces and subaerial caliche crusts. During periods of significant transgression, sequences 1-5 are accreted. Sediments are predominantly skeletal and peloid. During periods of significant regression, megadune complexes are accreted. Ooids are the dominant sediment. Erosional-solutional features reflect areas of subaerial exposure and/or coastline erosion. Terraces at 10, 20, and 40 ft elevations are preserved along arid eastern Great Inagua Island. The calichification of Bahamian Quaternary carbonates has concentrated insoluble residues (quartz, feldspar, heavy minerals, crandallite, micrometeorites). Insoluble residue analysis provides a basis for the correlation of accreted eustatic sedimentary sequences. End_of_Article - Last_Page 289------------

Morphological channeling by structural constraint: convergence in styles of dwarfing and gigantism in Cerion, with a description of two new fossil species and a report on the discovery of the largest Cerion

A survey of all Cerion taxa and geographic variants reveals that a distinctive shape—a narrowly spired “smokestack” shell more than three times higher than wide—occurs only in dwarfs and giants and never in the much more common populations of normal size. The smokestack shape evolved once in giants (in the newly described species C. excelsior from Great Inagua and Mayaguana—the largest of all Cerion), but at least seven and probably eight times independently in dwarfs. A study of complex allometric patterns in Cerion's ontogeny, and of covariance sets in growth, indicates that smokestacks can easily evolve in dwarfs and giants by the common route of relative increase in whorl number during an allometric phase that adds height but no width to the shell. (Giants simply add more whorls to a shell with whorls of normal size; dwarfs grow a normal number of whorls in a shell with small whorls and reduced maximum width.) This pathway is not open to Cerion of normal size, and the restriction of smokestacks to both extremes of the size range records a channel set by Cerion's invariant allometries and the geometry of spiral growth in general, not an immediate adaptation conferring special advantages via the elevated spire itself.

Nutrition and grazing behavior of the green turtle Chelonia mydas

The apparent digestibility coefficients for 4 size classes of the green turtle Chelonia mydas feeding on the seagrass Thalassia testudinum were measured in Union Creek, Great Inagua, Bahamas, from September 1975 to August 1976. The values ranged from 32.6 to 73.9% for organic matter; from 21.5 to 70.7% for energy; from 71.5 to 93.7% for cellulose; from 40.3 to 90.8% for hemicellulose; and from 14.4 to 56.6% for protein. Digestive efficiency increased with increases in water temperature and body size. There was no seasonal variation in the nutrient composition of T. testudinum blades. Grazing on T. testudinum may be limited by its low quality as a forage, a result of its high fiber content and possible low protein availability. Turtles did not graze at random over the extensive beds of T. testudinum, but maintained “grazing plots” of young leaves by consistent recropping. They thus consumed a more digestible forage-higher in protein and lower in lignin-than the ungrazed, older leaves of T. testudinum. The selectivity of green turtles for either a seagrass or algal diet may reflect the specificity of their intestinal microflora.

Mixed terrigenous—Carbonate sedimentation in the Hispaniola—Caicos turbidite basin

Sedimentation in the 9500 km 2, 4100 m deep Hispaniòla—Caicos Basin is dominated by turbidity currents. Carbonate turbidites originate from the Bahama Islands, Great Inagua and Caicos at the north end of the basin. Mixed carbonate—non-carbonate flows come from Hispaniola and perhaps Cuba. Most flows originate on insular slopes rather than in shallow water. The relatively low CaCO 3 content of hemipelagic sequences throughout the entire basin reveals that the influence of non-carbonate Hispaniola—Cuba sources is widespread. The basin was sampled with closely spaced piston cores. Sand-layer isopach and frequency maps reveal four or five major basin entry points for turbidity currents. Flow size is proportional to the size of source areas. Average volumes of flows originating from Hispaniola—Cuba, the largest source, are 10 9 m 3. This compares to an average flow volume of 10 6 m 3 for flows derived from the smallest source area, the Southeastern Caicos Bank. Measures of turbidity-current activity, such as thickness and frequency, change in a regular fashion away from each entry point. Average lutite thickness (combining hemipelagic and turbiditic lutite) are greatest near the basin entry points. On the abyssal plain occupying the south half of the basin, Bouma turbidite sedimentary structure sequences tend to be complete. However, on the Caicos Fan, the sedimentary structure sequences in turbidites are characterized by missing or repeated units. Six radiocarbon dates of two widespread presumed pelagic units in the basin yielded younger dates in stratigraphically older positions. The reversed dates are assumed to reflect storm erosion of older sediment on adjacent insular shelves. Consideration of a north—south reflection seismic profile over the basin indicates that the present sediment regimen has pertained through much of the Neogene. The coherence, convergence and termination of reflections in the seismic section are consistent with and tend to confirm conclusions based on the core study regarding the greater extent and volume of sediment deposits derived from the Hispaniola source area.

Characteristics of three turbidites, Hispaniola-Caicos Basin

The absence of analyses of complete sedimentary units resulting from single turbidity currents has been a major shortcoming of turbidite studies. Previous studies have extrapolated single-flow characteristics by averaging attributes of groups of flows. Distinctive source areas surrounding the 9,500-km 2 , 4,100-m-deep Hispaniola-Caicos Basin allow long-distance correlation of beds and afford an unusual opportunity to study turbidites resulting from single-flow events. On the basis of data from closely spaced piston cores, three individual turbidites have been mapped in their entirety. Two of the flows contain abundant terrigenous material; they originated from Hispaniola or Cuba. The third flow is almost exclusively bioclastic; it originated from Great Inagua Carbonate Bank. All of the flows traveled about 100 km across the flat basin floor. They range in total volume from 8.9 × 10 8 m 3 to 30.8 ×10 8 m 3 . The flows cover areas ranging from 3,500 to 5,200 km 2 , indicating the importance of lateral spreading on the basin floor. Maximum thicknesses, including upper pelitic intervals, are well over 200 cm. The flow deposits generally thin away from their basin entry points but thicken by ponding in basin depressions. Marked horizontal sorting occurs on the basis of constituent particle density and size. In two flow deposits, Bouma interval structure units A through E are present everywhere. In the third turbidite, the complete Bouma sequence is present only near the basin entry point, and the upper flow regime units (A and B) disappear within 30 km of the basin edge.

The Typhlops biminiensis Group of Antillean Blind Snakes

pattern, Typhlops biminiensis Richmond and Typhlops caymanensis Sackett are considered to comprise a single Antillean group of the genus. New distributional records are given for T. biminiensis (Cayman Brac, localities in Cuba and the Bahamas), and two new subspecies are described. T. biminiensis paradoxus is described on the basis of two specimens from Great Inagua and is characterized by a narrow rostral scale and extensive pigmentation. T. biminiensis epactia is described on the basis of five specimens from Cayman Brac and is characterized by a narrow rostral, small size and slender build. It is postulated that the biminiensis group entered the West Indies along the Cayman Ridge rather than through western Cuba. Observations on the habitat of the members of this group indicate that they are primarily xerophiles in contrast to the ecologically more tolerant, partly sympatric congener, Typhlops lumbricalis Linnaeus.

Recent Dolomitization of Pleistocene Limestones by Hypersaline Brines, Great Inagua Island, Bahamas: ABSTRACT

Pleistocene carbonate rocks were sampled beneath a shallow saline lake on Great Inagua Island, Bahamas, to test the hypothesis of dolomitization by seepage refluxion. Commercial salt has been produced in part of the lake by solar evaporation of sea water for more than 100 years; hence, the writers reasoned that brines formed might have affected the Pleistocene carbonate bedrock. The upper few inches of rock beneath salt ponds consists of fine-grained, moldic dolomite; no dolomite was found in the Pleistocene limestone marginal to the lake or in rock underlying the lake in areas away from the salt ponds. Relict structures in the moldic dolomite show that it formed by replacement of partly cemented, aragonitic, oolitic limestone identical with the underlying and adjacent Pl istocene limestone. The dolomite crystals are cryptocrystalline (< 5 µ) in size and have a calcium-rich composition (about Ca60Mg40 to Ca56Mg44). The dolomite is enriched in O18; measured apparent fractionation values for ^dgrO18 between the dolomite and the aragonite and calcite of the parent oolitic limestone range from 3.8 to 4.6^pmil. This O18 enrichment should be expected from dolomitization resulting from interaction of carbonate material with hypersaline brines formed by solar evaporation. Radiocarbon dates of the dolomite range from 2,930 to 3,420 yr B.P. These dates, older than anticipated if the dolomitization is controlled by brines formed since salt production began, may be in error because o incorporation of old carbon atoms from the replaced Pleistocene limestone. End_of_Article - Last_Page 522------------