--> Timing of Mixing-Zone Cementation in a Late Pleistocene Coral Reef, Barbados, W.I., by J. L. Banner, L. R. Edwards, T. N. Kimbell, and J. D. Humphrey; #90986 (1994).

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Abstract: Timing of Mixing-Zone Cementation in a Late Pleistocene Coral Reef, Barbados, W.I.

Jay L. Banner, Lawrence R. Edwards, Tod N. Kimbell, John D. Humphrey

The timing of cement precipitation relative to sea level oscillations is an important but poorly understood factor in the evolution of porosity in carbonate platforms. Uranium-series isotopes, which can provide a precise geochronometer for carbonate minerals, were measured on aragonite cements, host rocks, and groundwaters from Late Pleistocene coral reef terraces on Barbados. Previous studies of these tectonically-uplifted terraces have delineated a high-resolution eustatic record and an active meteoric diagenetic system. The U-series results provide unique information on the timing of diagenesis and evolution of diagenetic fluids.

The cements occur in large dissolution vugs (>5 cm) in a core recovered from forereef grainstones along the southeast coast of the island. Aragonitic corals from this facies have higher initial ^dgr234U values than seawater, typical of open-system behavior, and 230Th model ages of 300 +/- 50 ky. Trace element and C and O isotopic variations indicate that the cements formed from mixed seawater and freshwater. Such a mixing zone may have migrated through the host limestones during any of several Late Pleistocene sea-level highstands. The cements have 230Th ages that range from 80 to 86 ky, 5.3 to 10.2 ppm U, and initial ^dgr234U values (79 to 86 per mil) intermediate between modern meteoric groundwaters on Barbados (0 to 36 per mil) and sea ater (150 per mil). In general, the cements' ages, U contents, and ^dgr234U values increase downcore. The results indicate that the cements formed in a coastal mixing zone from progressively fresher waters during a sea-level highstand that peaked at 80 ky (i.e., stage 5a). Thus, the cements may post date deposition of their host limestone by as much as 220 ky. These results are in contrast to generalized models of diagenesis, in which mixing zone diagenesis is an early phase of a platform's evolution.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994