DIAGENESIS IN COASTAL CARBONATES RELATED TO PLEISTOCENE SEA-LEVEL, BERMUDA PLATFORM

Pleistocene glacioeustatic sea-level oscillation on the stable Bermuda Platform is expressed in a succession of shallow-water carbonates int errupted by lowstand unconformities. In Bermuda, the maximum highstand s of the last 400,000 yr ranged within 10 m around the present level, Coastal carbonates of various highstands are exposed along the present shoreline. These carbonates were penetrated by meteoric and marine po re waters during lowstands and highstands following on deposition. Two representative Pleistocene shoreline sections were studied to see whe ther early diagenesis has recorded these pore-water changes. The sedim ents of both sections show multiple generations of cement. Optical and scanning electron microscopy, cathodoluminescence microscopy, X-ray d iffraction, microprobe studies and stable-isotope analyses were used t o determine the diagenetic environments involved. Diagenetic features of multiple pore-water changes (herein termed early-diagenetic oscilla tion) depend on whether substrates were loosely or firmly cemented. (1 ) In loosely cemented beach sands (Belmont Group, Grape Bay) truncated layers of Mg-bearing low-Mg calcite covered many grain surfaces. Isop achous high-Mg calcite cement whose Mg content now varies with the deg ree of meteoric alteration eventually lithified the sediment. There ar e several types of meteoric low-Mg calcite cement. These sediments hav e undergone several cycles of deposition, cementation, erosion, and me chanical abrasion of interparticle cements. (2) Firmly cemented stable substrates (cliffs in Watch Hill Park) reveal complicated early diage netic sequences. Cements with marine mineralogy comprise high-Mg calci te (isopachous blades, peloidal aggregates, fibrous microstalactites) and acicular aragonite. Freshwater influence is indicated here by leac hing, neomorphism, and low-Mg calcite cement. Regardless of the degree of substrate cementation, freshwater alteration was mainly vadose whe reas marine cementation was either phreatic or vadose or both. Early d iagenetic oscillation is easier recorded in coastal successions than i n lagoonal sediments, mainly because marine cementation is more active nearshore. Because the coastal environment is prone to wave destructi on, the potential for preserving these diagenetic features is usually low. Data published on tectonically unstable areas (Enewetak Atoll; Ba rbados) suggest that early diagenetic oscillation may characterize sta ble coastlines.