Southeastern Section - 68th Annual Meeting - 2019

Paper No. 24-5
Presentation Time: 3:35 PM


TORMEY, Blair R., Program for the Study of Developed Shorelines, Western Carolina University, Cullowhee, NC 28723 and HEARTY, Paul J., Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712

Fenestral porosity (beach bubbles) forms continually on sandy beaches worldwide, as waves wash over dry sand, trapping the interstitial air below. In carbonate settings, rapid cementation often preserves these features. Though typically associated with intertidal beach facies, fenestrae can be found at higher elevations, in last interglacial (MIS 5e, ca. 125 ka) chevron storm-beach and eolian ridges throughout the Bahamas.

Wanless and Dravis (1989) first implicated storm waves to explain fenestral porosity in last interglacial eolian ridges on Providenciales (+30 m). Two more localities (Eleuthera, +43 m; San Salvador, +12 m), led Bain and Kindler (1994) to alternatively propose that eolian fenestrae formed by sheet flow of rain down steep lee slopes. Subsequently, the rainfall model has been invoked repeatedly to explain supratidal MIS 5e fenestrae (e.g., Mylroie, 2018), despite an abundance of detailed, outcrop-based, evidence to the contrary.

We have described in detail fenestral porosity in MIS 5e chevron and eolian ridges from dozens of localities across the Bahamas archipelago. In lowland chevron ridges, fenestrae are abundant in thick, continuous beds, with repeated low-angle, aggradational sequences in outcrop and thin section. In built-up dune ridges, fenestrae occur in discrete zones, concentrated in seaward-dipping, low-angle beds (not foresets). This pattern is expected with wave attenuation along exposed Atlantic coastlines. Multiple localities exhibit wave run-up deposits, including onlapping fenestrae beds, current scour structures, and rip-up clasts.

In the Bahamas, high-elevation supratidal fenestral porosity is unique to MIS 5e strata. To the point, if seasonal rainfall creates fenestral porosity, it should be observed in elevated deposits of all ages. With a lack of supporting outcrop evidence, or a documented modern analog, the rainfall model for fenestral porosity should be diligently tested or abandoned. In contrast, the documented outcrop evidence in support of a storm-wave origin for MIS 5e fenestrae is widespread and overwhelming.

The fenestral porosity in MIS 5e chevron and eolian ridges indicates intense superstorms during the last interglacial. These observations have ominous implications for our warming world, where extreme storm events are becoming the norm.