Phreatic and Vadose Diagenetic Modification of Pleistocene Limestone: Petrographic Observations from Subsurface of Barbados, West Indies

Abstract

Data from a shallow cored borehole through 105,000-year-old reef-tract sediments on the south coast of Barbados indicate that the carbonate sediments have been altered more rapidly and more extensively in freshwater phreatic diagenetic environments than in the vadose or marine phreatic diagenetic environments. The upper part of the borehole section has been exposed only to subaerial and shallow vadose processes since emergence from the marine depositional environment about 105,000 years ago. These sediments are recrystallized only partly. Total amounts of cement, mainly needle-fiber cement and dense micrite coatings, are high. Porosity is low. Dissolution, a minor feature in this part of the section, is associated with localized development of needle-fiber fabrics. Those parts of the borehole section which have been occupied by a freshwater phreatic lens have been cemented by calcite microspar. Aragonite grains have been dissolved on a massive scale, resulting in a very well-developed moldic porosity. Neomorphic grain alteration has been volumetrically important. Packstones and grainstones in the lowest part of the borehole are largely unaltered; metastable carbonate mineralogy dominates. This part of the sedimentary column has been subjected alternately to vadose conditions during low stands of sea level (> 70,000 years total) and to marine phreatic conditions during high stands of the sea (< 35,000 years total). Despite prolonged exposure to vadose conditions, diagenetic modification is slight. If the recrystallized lime muds contain greater than 25 percent cement (micrite), then mass-balance calculations suggest large amounts of calcium carbonate have been transported to that part of the sedimentary section exposed to freshwater phreatic diagenetic processes. The source of the additional carbonate may be updip in the aquifer, from mixing of underlying marine pore fluids with the freshwater pore fluids of the coastal water lens, from the overlying shallow vadose zone, or from marine water soon after deposition. Processes which affect early diagenetic modification of carbonate sediments on the south coast of Barbados operate more rapidly and more effectively in the freshwater phreatic environment than in vadose or marine phreatic environments.