Modern Coastal Tempestite Deposition by a Non-Local Storm: Swell-Generated Transport of Sand and Boulders on Eleuthera, The Bahamas

Kat Wilson and David Mohrig

Sedimentology (2021)

This project presents an analysis of the transport conditions of a storm deposit (i.e., tempestite) produced by a non-local cyclone. Observations and analysis of sand-to-boulder transport and washover deposition in March 2018 at Gaulding Cay Quarry, Eleuthera, The Bahamas, confirm that swell waves can cause coastal change and effect the depositional record > 1000 km from the storm center. Drone video, news reporting, deposit stratigraphy, grain size measurements, and wave data were all used  to define three phases of washover fan construction: an aggradational phase associated with base-level rise and quarry filling, a progradational phase associated with quasi-constant base-level, and fan incision tied to base level fall as discharge through an outlet channel exceeded input discharge by overtopping waves.

Washover fan location was controlled by antecedent topography and represented only a fraction of the swell-impacted coastline. Sand and boulders were transported simultaneously, forming a compositionally and texturally complex poorly sorteddeposit. Drone video, sediment-transport estimates, and bedrock erosion all indicated that overwash exceeded sediment availability. As a result, the measured washover fan was estimated to be an order-of-magnitude smaller than its potential volume if conditions had been transport-limited. This study highlights the importance of pre-event topography and independent measures of event duration on accurately reconstructing storm properties from the sedimentary record, as well as the challenges in reconstructing storm location, and therefore storm intensity and frequency, from sedimentary deposits alone.

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