Reefs Of Barbados Research Articles

The Dry Valleys of Barbados: A Critical Review of Their Pattern and Origin

The major erosional feature on the emerged coral reefs of Barbados consists of a network of valleys. These valleys are generally dry, and the main problem discussed is that of their origin and subsequent desiccation. A secon- dary problem is provided by the courses of many valleys which have sections at right-angles to past and present shore- lines. Of the various hypotheses reviewed, that of surface run-off after heavy storms receives the most support. The possibility of climatic change is discussed and the contributory role of karstification considered. The valley pattern is judged to be a consequent one, its peculiarities resulting from primary depressions on the reefs. THE rock outcropping over six-sevenths of Barbados is a coralline limestone often referred to as the Coral Cap. In the north-east of the island this has been breached to reveal the underlying sandstones and clays of the Scotland district (Fig. i.) The Coral Cap has never been overlain by other strata and its surface descends west and south from elevations of over 350 m near the escarpment in a series of terraces. The origin of these terraces has been explained by K. J. Mesolella (i967), whose stratigraphical studies reveal an ordered sequence of fore reef, reef crest and back-reef deposits across each terrace. This indicates that the terraces are separate raised reefs, rather than wave-cut platforms on a single tilted reef or the product of a series of step faults. E. T. Price (1958) concluded that the declining silica/aluminium ratios with height on the limestone soils indicate a uniform rate of uplift. Mesolella et al. (i969) confirm this for the lower terraces by Thorium-Uranium dating, and they find an overall association of the height spacing of reefs with the time spacing of radiation maxima predicted by considering the changing tilt of the earth and the precession of the equinoxes. An almost uniform rate of uplift of 0-3 m per thousand years is indicated. This steady uplift has separated the reefs formed by the succession of high warm interglacial sea-levels in the Pleistocene. There has been some folding during the uplift (Fig. i) and the large-scale relief of the Coral Cap, with anticlinal domes and synclinal vales, faithfully reflects this. Two types of erosional feature are common on the limestone. The first consists of small dolines, usually discrete, predominantly circular in form and seldom greater than I5 m in depth, many containing sinkholes and many others ponds. The second and more striking features are the valleys which radiate from the high ground around the Scotland district and from the Golden Ridge dome. They are often incised to depths of 30 m and their sides slope com- monly at more than 30° and rarely less than I5°. The valley pattern is pinnate with some trellising. When analysed with respect to past and present shorelines, the trend of the former shown by the raised fringing reefs, 79 per cent of the total length makes angles greater than 6o0° to the shore, 7-3 per cent is between 30° and 6o0° and 13'7 per cent is less

Diagenesis and Porosity Modification in Acropora Palmata, Pleistocene Of Barbados, West Indies

ABSTRACT The uplifted coral reefs of Barbados have served as an excellent natural laboratory for the study of porosity changes and diagenesis in Pleistocene and Holocene carbonate material. The mineralogy and petrography of a single coral species, Acropora palmata, was examined in detail. Post-depositional modifications of the coral skeleton included entrapment of lime sand and mud, precipitation of void-filling aragonite needles in the marine environment, solution of the coral, alteration from aragonite to calcite skeletal mineralogy and the precipitation of calcite cement. The precipitation of calcite cement is volumetrically the most important porosity alteration in the history of these corals. The source of this void filling cement is shown to be the excess material remaining after the volume solution of aragonite skeleton and reprecipitation of calcite when the coral skeleton equilibrates mineralogically to the subaerial environment. It has become apparent from this study that a varied suite of rocks may result from the differential effects of the few processes mentioned above.

Facies Geometries Within Pleistocene Reefs of Barbados, West Indies

Gradual tectonic uplift of the island of Barbados during the Pleistocene was accompanied by the periodic development of reef tracts around the island in association with high stands of sea level. Investigation of geometries within these elevated reef-carbonate bodies was on two scales. In a broad-scale approach, the individual reef tracts were treated as single carbonate bodies. We found that the areal distribution of reef-tract trends conforms to the outlines of topographically high areas of the island; that the reef tracts thin over the topographically high areas; and that individual reef crests are upwarped over the topographic highs. These observations indicate that tectonic uplift of Barbados has been differential throughout the developmental history of the coral cap the highest rates of uplift occurring in the topographically high area. A smaller scale approach was taken by determining facies relations, and the geometry of facies composing the individual reef tracts. Examination of vertical sections through the reefs tracts indicates that the Barbados Pleistocene reefs are closely analogous to modern West Indian reefs. Geometries of individual reef-tract facies record interaction of rates of (1) reef growth, (2) glacio-eustatic sea-level fluctuations, and (3) tectonism. The possible rates at which each of these three major variables took place during the formation of the Pleistocene Barbados reefs were estimated. We conclude that tectonic uplift of Barbados at 1 ft/1,000 years was considerably less important than rates of glacio-eustatic sea-level fluctuation and reef growth. Furthermore, it appears that rates of glacio-eustatic sea-level fluctuation could easily exceed rates of reef growth. Reef-tract development probably did not begin until rates of sea-level rise at the beginning of a high sea stand had slowed to a pace that could be equalled by rates of reef growth. During early phases of the hi h sea stand, as sea level continued to rise, reef-tract development was largely upward. During later phases of the high sea stand, sea level reached a peak and then began to fall. Upward reef-tract development gave way to seaward progradation of the reef tract. Eventually, rates of lowering glacio-eustatic sea level exceeded rates at which the reef could prograde seaward, and reef-tract development was terminated. The rhythmic nature of Barbados reef-tract deposition may well be an expression of a mechanism for climatic change and associated glacio-eustatic sea-level fluctuations which has operated to varying degrees throughout geologic time. Therefore, an analysis of tectonic, eustatic, and reef-growth rates not only provides a better understanding of the complex stratigraphy of the Barbados coral cap, but also provides the basis for a general model of reef sedimentation for other parts of the geologic record.

The Leeward Reefs of St. Vincent, West Indies

The distribution, species composition, and zonation of reefs on the island of St. Vincent are described and compared with corresponding characteristics of several other Caribbean and Bahamian reefs. Reef-building organisms are restricted to the leeward coast adjacent to the older volcanic mountains where broad, shallow shelves are present and where the influx of inorganic sediment is at a minimum. Well-developed reef structures are few in number and are somewhat restricted in size. However, those present show distinct zonation and a diverse coral, sponge, and algae assemblage. St. Vincent reefs are generally-smaller than the reefs of Barbados, Jamaica, or the Bahamas. The assemblage of organisms is much less diverse, but the over-all morphology is similar in most cases.

Carbonate Diagenesis: Equilibration of Sedimentary Mineralogy to the Subaerial Environment; Coral Cap of Barbados, West Indies

ABSTRACT It has long been recognized that ancient carbonate rocks are composed of the stable minerals calcite and dolomite, whereas their Recent sediment analogs are predominantly composed of the unstable minerals aragonite and high-magnesium calcite. Further, it has been noted that many subaerially exposed Pleistocene carbonates exhibit mineralogical stability and lithification, thus suggesting that the stabilization process proceeds rather rapidly in fresh water diagenetic environments. The subaerially exposed Pleistocene coral reefs of Barbados provide an excellent opportunity to investigate the stabilization process in greater detail. Owing to its unique history of Pleistocene tectonic uplift, Barbados records a more detailed history of Pleistocene high sea level stands than any other Caribbean shoreline yet documented by radiometric dates. The youngest terrace on the island (82,000 years B.P.) is composed of generally unstable mineralogy throughout its extent. Terraces older than 500,000 years are of stable mineralogy. Terraces of intermediate age exhibit varying proportions of stable and unstable mineralogy. Further, different portions of Barbados have decidedly different climates. Thus reef tracts of equivalent age can be studied in a high rainfaIl-low evaporation terrain and in a low rainfall-high evaporation terrain. Mineralogical stabilization proceeds more slowly in the drier climate. A model is proposed for the stabilization process at or near the outcropping surface of an aragonite-calcite sediment. Kinetics of calcite nucleation, kinetics of calcite crystal growth, and availability and transport of water can each be expected to be the rate step in various geologically realistic situations. Finally the model is extended to include mineralogical stabilization in meteoric water in the subsurface.

THE DIGESTIVE TRACT AND ITS RELATION TO FEEDING HABITS IN THE STENOGLOSSAN PROSOBRANCH CORALLIOPHILA ABBREVIATA (LAMARK).

Coralliophila abbreviata is found on partly eroded colonies of Montastrea annularis in the coastal reefs of Barbados and contributes to the destruction of this coral. The habitat and feeding habits have been examined and a histological study has been made of the digestive system. Stomach and faecal material indicate that both the animal tissue and zooxanthellae are ingested. Enzyme tests demonstrate the presence of protein and carbohydrate digesting enzymes. Since this animal has no radula or jaws, it is thought that the muscular proboscis is used as a pump during the ingestion of food. The single salivary duct, formed from the ducts of the paired salivary glands, opens into the oral tube on the dorsal side of the mouth. It is suggested that its secretion may aid in penetration of the coral epidermis before ingestion begins.