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.