Anomalous pleistocene palaeo-sea-levels at Bermuda and their control on littoral depositional cycles which culminate in the formation of landward-advancing dunes (eolianites)

Abstract

The Bermuda islands are constructed predominantly of aeolian dunes, termed eolianites, whose episodic accumulation has been correlated with Pleistocene sea-level oscillations. Despite a long-standing, now disputed, notion of Bermuda as a “tide-gauge” for Pleistocene glacio-eustacy there has never been a consensus on Bermuda’s palaeo-sea-level history. Most recently there has been disagreement over the interpretation of littoral deposits of the Belmont Formation. Based on U-series ages from coral fragments and analyses of sedimentary lithofacies, it is contended here that a sea level of ≥4.5m above present mean sea level at the penultimate interglacial is represented by these Belmont deposits. It is inferred from the anomalously high elevation of this sea-level imprint relative to the estimated global eustatic sea level of the time that glacio-hydro isostatic and possibly, to a lesser extent, tectonic influences have contributed to a composite RSL (relative sea level) signal at Bermuda. This and other interglacial highstands at Bermuda left their imprint in the form of exceptionally well exposed emergent coastal facies assemblages. The most complete assemblages are shown to have developed in two stages, S1 and S2, respectively during a rising RSL and a falling RSL. S1 records beach progradation, barrier construction and back-barrier inundation. S2 begins with emergence of an ultimately wooded backshore, and ends with its burial by advancing dunes sourced on expanding beaches at a highstand termination. Past hypotheses that Bermuda’s dunes were static aggradational structures which accumulated rapidly in storms are tested by analyses of eolianite stratification, wind data and drift potential. It is demonstrated that the eolianites are the remnants of mobile landward-advancing bedforms constructed predominantly when winds above the threshold velocity were directed onshore across source beaches. The model developed for the evolution of beach-dune systems on Pleistocene Bermuda is applicable to present-day clastic coasts which are vulnerable to RSL rise