ABSTRACT

Pleistocene glacioeustatic sea-level oscillation on the stable Bermuda Platform is expressed in a succession of shallow-water carbonates interrupted by lowstand unconformities. In Bermuda, the maximum highstands 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 pore waters during lowstands and highstands following on deposition.

Two representative Pleistocene shoreline sections were studied to see whether early diagenesis has recorded these pore-water changes. The sediments of both sections show multiple generations of cement. Optical and scanning electron microscopy, cathodoluminescence microscopy, X-ray diffraction, microprobe studies and stable-isotope analyses were used to determine the diagenetic environments involved. Diagenetic features of multiple pore-water changes (herein termed early-diagenetic oscillation) 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. Isopachous high-Mg calcite cement whose Mg content now varies with the degree of meteoric altera ion eventually lithified the sediment. There are several types of meteoric low-Mg calcite cement. These sediments have undergone several cycles of deposition, cementation, erosion, and mechanical abrasion of interparticle cements. (2) Firmly cemented stable substrates (cliffs in Watch Hill Park) reveal complicated early diagenetic sequences. Cements with marine mineralogy comprise high-Mg calcite (isopachous blades, peloidal aggregates, fibrous microstalactites) and acicular aragonite. Freshwater influence is indicated here by leaching, neomorphism, and low-Mg calcite cement.

Regardless of the degree of substrate cementation, freshwater alteration was mainly vadose whereas marine cementation was either phreatic or vadose or both. Early diagenetic oscillation is easier recorded in coastal successions than in lagoonal sediments, mainly because marine cementation is more active nearshore. Became the coastal environment is prone to wave destruction, the potential for preserving these diagenetic features is usually low. Data published on tectonically unstable areas (Enewetak Atoll; Barbados) suggest that early diagenetic oscillation may characterize stable coastlines.