Barbados Mixing Zone Dolomite: Paleohydrologic Control on Dolomite Distribution

John D. Humphrey

Petrographic, stable isotopic, and chemical data suggest that late Pleistocene dolomites from southeastern Barbados, West Indies, precipitated in the zone of mixing between the coastal meteoric phreatic lens and normal seawater. A model combining late Pleistocene glacio-eustasy, regional tectonic emergence, and diagenesis constrains both the timing and the rate of dolomitization. Dolomitization occurred during the glacio-eustatic high stand and initial stages of sea level lowering, 195,000 years ago. Dolomitization was locally very rapid, with up to 100% dolomitization occurring within about 5,000 years.

Outcrop sampling reveals that the surface extent of the dolomite is primarily confined to a portion of a coral terrace estimated to be about 195,000 years old. This areal distribution of dolomite relates directly to the paleohydrologic setting. Underground streamwater flow and sheetwater development on Barbados is controlled primarily by the paleotopography of the Tertiary Oceanic Formation aquiclude which underlies the late Pleistocene coral cap. The dolomite occurs within a well-defined paleotopographic low. At time of dolomitization 195,000 years ago, this low accommodated a high-flow meteoric phreatic lens (sheetwater). Persistence of the phreatic lens in the paleotopographic low during initial stages of a glacio-eustatic sea level lowering contributed to sustaining conditions nec ssary for dolomitization. A combination of high meteoric phreatic discharge with high-energy conditions along this windward coastline provided vigorous mixing between the fresh and marine waters, resulting in local dolomitization. This paleohydrologic and diagenetic model affords the inference that substantial dolomite probably occurs in the subsurface of southeastern Barbados.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.