24 Reconstructing Paleo-Environmental Changes II

Thursday, 28 March 2019
2:00 PM - 5:05 PM
Gold Ballroom

SUPERSTORM WAVES, NOT SEASONAL RAIN: THE ORIGIN OF "BEACH BUBBLES" AT HIGH ELEVATIONS IN THE BAHAMAS DURING THE LAST INTERGLACIAL

Paper number: 24-6
Program: Theme Sessions
Time: 3:35 PM – 3:55 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 3

doi: 10.1130/abs/2019SE-326947

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.