FENESTRAL POROSITY IN BAHAMIAN EOLIANITES: EVIDENCE OF INTENSIFIED STORMS DURING THE LAST INTERGLACIAL

Though fenestrae (keystone vugs) are most commonly associated with beach environments in the Bahamas, fenestral porosity has been observed in last interglacial (MIS 5e) eolian facies on several islands. Many formation mechanisms have been invoked for eolian fenestrae (tsunamis, bank margin slumping, torrential rain) but detailed inspection of exposures on Eleuthera, San Salvador, and Providenciales indicates that such structures are formed as storm waves run over, run up, and run out on coastal dunes.

At the end of MIS 5e, several lowland dunes were run over and reworked by storm waves, forming chevron-shaped, storm-beach ridges with tabular, fenestrae-rich bedding. Eolian structures are typically obliterated in these chevron ridges, though rhizomorphs, root-balls, and eolian crossbeds are sometimes preserved. Within dune exposures at moderate elevations, fenestral porosity occurs in discrete wave run-up zones, often associated with scour. Fenestrae occur in different horizons within the run-up zones implying multiple events. In the highest and most landward dune ridges, fenestrae are preserved in thin, discontinuous beds in the seaward backsets where the effects of storm waves and windborne spray run out.

Facies stacking patterns, fossil reefs, and emergent bioeroded notches in the Bahamas together indicate that a series of rapid sea level fluctuations occurred toward the end of the last interglacial. In addition, the character of fenestral porosity within late MIS 5e eolianites suggests that storm activity intensified platform-wide as the last interglacial drew to a close. As climate deteriorated and glacial conditions set in, the Westerlies belt likely became compressed, which evidently gave rise to more frequent and powerful tropical storms than today.