Abstract

The Pleistocene of Barbados should be of interest to students of cyclic sedimentation for the following reasons: (1) Radiometric dating of the various elevated reef tracts provides the most convincing evidence to date that Pleistocene sea level fluctuations are in part ascribable to climatic variation resulting from the systematic perturbations (disturbances) of the earth’s orbit. If it is sufficiently demonstrated that these perturbations have significantly affected Pleistocene climates, then it is reasonable to propose that a rhythm of similar origin is superimposed on the whole geologic record. (2) The Pleistocene record on Barbados provides some insight as to how coral reefs and reef associated facies react to cyclic eustatic sea level fluctuations. The models emphasize the importance of sediment production rather than coral ecology and may therefore be of general applicability. (3) Diagenetic modification of the reefs and associated sediments is systematic and closely related to variations in post-depositional environment attendant to cyclic fluctuations in sea level and the more gradual uplift of the island.

Radiometric dating by the Th230 growth method undertaken in collaboration with Broecker et. al., has established an absolute chronology for the lower six of the numerous uplifted reef tracts of Barbados. The chronology of reef tracts strongly suggests growth of individual reef tracts in response to sea level rises associated with periods of warm climate induced by solar radiation maximums. This has been superimposed on the relatively slow and continuous tectonic uplift of the island through out the Pleistocene. With lowering of sea level after each high stand, each reef tract was left high and dry. Continuing tectonic uplift has in most cases moved each older reef to a position slightly above the sea level attained with the next younger high stand.

It has long been recognized that reefs may prograde seaward over their fore-reef talus if they grow during extended periods of sea level still-stand. Barbados data suggested that in certain geometrically favorable situations reefs may similarly prograde during periods of gradual regression. The local geometry is critical in determining the amount of fore-slope detritus which must accumulate in order that progradation may occur. Progradation results in the development of a sheet structure which shows normal reef zonation in vertical profile. The sheet structure dips seaward at a slight angle which is governed by the interplay of progradation and rate of sea level lowering. With rising sea level, a thriving reef community may build upward to keep pace with the rising sea-air interface. When this occurs, a barrier reef develops with a lagoon separating the reef from the island. If rising lea level is not accompanied by upward growth of existing reefs, new reef developments will form on bathymetric prominences or as fringing reefs along the new shore line.

Cyclicity of sea level fluctuations imposes changing diagenetic environments onto the sediments. With lowering sea level, sediments of marine origin are commonly invaded by fresh water (phreatic zone). As regression continues, the water table falls and the sediments are left in the zone of fresh water undersaturation (vadose zone). Significant digenetic modifications accompany this transition.