SEA-LEVEL AND DIAGENESIS - A CASE-STUDY ON PLEISTOCENE BEACHES, WHALEBONE BAY, BERMUDA

Pleistocene fluctuations of sea level have left marine and aeolian lim estones intercalated with glacial red soils on the Bermuda Carbonate P latform (Atlantic, 64-degrees-50'W, 32-degrees-20'N). Successive eusta tic highstands of similar amplitude drowned the tectonically stable pl atform and piled up similar sets of sediments. Up to three Pleistocene beaches are stacked in shorelines sections. Post-depositional diagene tic histories of these beaches can be linked to repeated changes in se a level and pore waters. This paper presents field evidence and petrog raphic results (microscope, X-ray, cathodoluminescence, SEM-EDAX) for the diagenetic histories of two superimposed Pleistocene beaches in Wh alebone Bay, Bermuda North Shore. The younger beach was deposited duri ng isotopic stage 5e, about 120 ka ago. The age of the older beach may be isotopic stage 9 or older. Diagenesis drastically altered the olde r beach before the stage 5e transgression. Primary high-Mg calcite (HM C) and aragonite were no longer present. Marine skeletal grains were i nstead leached or recrystallized to low-Mg calcite (LMC). Primary and secondary pore space were largely reduced by LMC cement. Lines of need le relics reminiscent of marine aragonite cement occur as inclusions w ithin syntaxial rim cements around echinoderm grains, indicating that a marine influence had at least once interrupted this period of freshw ater alteration. Finally, before the rocks became buried by the sedime nts of the younger beach, a crust of marine, bladed HMC cement was pre cipitated throughout the pore space. The younger beach consists of ske letal grains that are, apart from the effects of non-selective dissolu tion, essentially unaltered. The sediments are only weakly lithified b y cryptocrystalline LMC showing an alveolar texture, tangential fibres and other features characteristic of calichification. A younger post- depositional marine influence is not recorded. These results suggest t hat, under favourable conditions, diagenetic processes can document se a-level fluctuations. The recorded fluctuations, however, are difficul t to assess because even major sea-level highstands may not produce a diagenetic imprint.