RUN OVER, RUN UP, AND RUN OUT: A STORM WAVE ORIGIN FOR FENESTRAL POROSITY IN LAST INTERGLACIAL EOLIANITES OF THE BAHAMAS

Fenestral pores form in the swash zone of beaches worldwide, as waves wash over dry sand, trapping air bubbles in the underlying sediment. In the carbonate environments of the Bahamas, where cementation rates are accelerated, beach fenestrae (keystone vugs) are readily preserved and lithified. Though most commonly associated with beach facies, fenestral porosity has been observed in last interglacial (MIS 5e) eolian deposits on islands throughout the Bahamas, in some cases up to 43 meters above sea-level.

To explain the presence of fenestrae in the MIS 5e eolianites, numerous formation mechanisms have been proposed; from tsunamis and bank margin slumps, to torrential rainstorms. However, detailed descriptions of ten MIS 5e eolianite outcrops on the islands of Eleuthera, San Salvador, and Providenciales, demonstrate rather conclusively that these fenestrae formed as intense storm waves ran over, ran up, and ran out on coastal dunes.

There are several lines of evidence that support a storm wave origin for the eolian fenestrae of MIS 5e. Eleuthera, San Salvador, and Providenciales are all situated on the Atlantic margin of the Bahamas Bank, where they are exposed to the full impact of tropical storms. Furthermore, at each locality fenestrae-rich beds occur in separate horizons, signifying repeated inundation by multiple events. Finally, with increasing elevation and distance from shore, the character of fenestral bedding changes, as does the abundance and geometry of individual fenestral pores.

During the last interglacial, lowland dunes in the Bahamas were repeatedly run over by storm waves and reworked into storm-beach ridges with abundant tabular, fenestrae beds, and only minor remnants of eolian cross-beds and root structures. At moderate elevations and further inland, wave run up formed discrete packages of fenestral beds within the dunes, often associated with scour and rip-ups. Within the highest and most distal dune ridges, storm waves ran out, leaving thin, discontinuous, fenestral beds in the seaward-dipping backsets. With the current warming trend and acceleration of sea-level rise, the frequency and intensity of tropical storms is expected to increase. Thus, the powerful superstorms of the last interglacial may serve as a solemn harbinger of things to come.