WERE SUPERSTORMS THE NORM? FENESTRAL POROSITY IN BAHAMIAN EOLIANITES AS EVIDENCE OF EXTREME STORMS DURING THE LAST INTERGLACIAL

Fenestrae (keystone vugs) are a common sight in the swash zone of beaches worldwide, and in carbonate settings like the Bahamas, are readily preserved by rapid cementation. Though typically associated with beach facies, fenestrae have been reported in last interglacial (MIS 5e) eolianites on islands throughout the Bahamas, often tens of meters above concomitant sea-level. Several mechanisms have been offered to explain fenestral porosity in MIS 5e eolianites, ranging from tsunamis to torrential rainstorms. However, detailed examination of exposures on Eleuthera, San Salvador, and Providenciales shows these fenestrae formed as large storm waves ran over, ran up, and ran out on coastal dunes at the end of the last interglacial.

Throughout the Bahamas, facies stacking patterns, emergent bio-eroded notches and fossil reefs record a series of rapid sea-level fluctuations toward the end of MIS 5e. As climate destabilized, global wind belts likely compressed and tropical storms intensified. In low-lying areas, storm waves regularly ran over coastal dunes, obliterating their eolian structures and reworking them into storm-beach ridges dominated by thick, tabular, fenestrae-rich beds. At moderate elevations and further from shore, storm waves ran up the dunes, forming zones of discrete, fenestrae-rich beds, often with evidence of scour. In the highest and most inland eolianites, where the effects of storm waves ran out, fenestrae formed in wispy, discontinuous beds, on the windward side of the dunes.

At several localities on multiple islands, fenestral beds in MIS 5e eolianites occur in separate horizons, signifying repeated inundation. Furthermore, with increasing elevation and distance from shore, the character of fenestrae-rich bedding changes as does the abundance and geometry of fenestral pores. This suggests a storm-wave origin to these structures, rather than rainstorms or a single event, and vividly illustrates the sobering potential for extreme storms at the end of an interglacial.