Southeastern Section - 67th Annual Meeting - 2018

Paper No. 32-7
Presentation Time: 3:50 PM


TORMEY, Blair R., Program for the Study of Developed Shorelines, Western Carolina University, 108A Old Student Union, Cullowhee, NC 28723, HEARTY, Paul J., Consultant, Wilmington, NC 28412 and DONOVAN, Bailey G., Department of Geological Sciences, East Carolina University, 101 Graham Building, Greenville, NC 27858

A trilogy of sedimentary deposits in the Bahamas reveals a dramatic destabilization of climate and intensification of storms at the end of MIS 5e (~120-118 ka). Waves from intense storms repeatedly inundated islands across the platform from Abaco to Caicos, leaving a range of sedimentary deposits that vary according to coastal geomorphologic, topographic, and bathymetric conditions. These intense events deposited massive boulders on the cliffed coastline of north Eleuthera, kilometer-scale, storm-beach ridges in lowland areas, and distinct wave runup beds on high and distal eolian ridges.

The geology of Eleuthera (180 km long) is remarkable; the trilogy of MIS 5e storm wave deposits can be observed along the entire island. Geologic evidence of superstorms is documented in stratigraphic sections, photographs, and petrography from fourteen known and new MIS 5e sites on Eleuthera.

On north Eleuthera, seven megaboulders (some greater than 900 tons) were emplaced atop the steep, cliffed coastline. Published stratigraphy links the megaboulders to in situ middle Pleistocene rocks at the base of the cliff; they were not sourced from the cliff top as has been recently postulated. A basal-cliff source of the megaboulders requires far greater wave energy than observed in modern storms, confirmed by smaller adjacent storm-tossed Holocene boulders, and the Glass Window Bridge – dislodged during recent events.

The megaboulders at Glass Window are impressive, but they are a local geomorphic and bathymetric phenomenon. Far more convincing are the MIS 5e storm-beach and runup deposits that record the repeated inundation of storm waves across the archipelago. In the lowlands of Eleuthera, including Thomas Street, Winding Bay, Half Sound, Tarpum Head, St. Anne’s, and Rock Sound, new observations show complex, wave-deposited, storm-beach ridges, dominated by tabular, fenestrae-rich beds, with scour and rip-ups. In higher and inland eolian ridges, new sites include Ocean Hill and Bannerman Town, where wave energy was diminished, leaving multiple, onlapping sequences of thin, fenestral beds.

In addition to Eleuthera, similar deposits have been documented at numerous sites across the Bahamas and beyond, offering a stormy and ominous lesson from the last interglacial; a harbinger of the consequences of our warming climate.