Facies Geometries Within Pleistocene Reefs of Barbados, West Indies

Abstract

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.