Sedimentary evidence of tsunamis impacting Anegada, British Virgin Islands

The relatively short global written record of tsunamis necessitates paleo-event analyses to assess event recurrence and size, both of which are essential to hazard vulnerability assessments. A major challenge in developing the geologic record of tsunamis is distinguishing between tsunami and storm deposits. We conducted field investigations on Anegada, British Virgin Islands, in 2009, 2011, 2017, and again in 2018 after Hurricane Irma to document and sample dozens of deposits (figure below shows 2009 trench and grab sample locations). Previous research has shown that forams are different in Anegada’s storm and tsunami deposits. We focus here on testing the utility of fine-scale (1 cm or less) vertical variations in grain size and CT scans of cores for discriminating between tsunami and storm formation. Tsunami deposits and turbidites often exhibit suspension grading, a type of normal grading that is a signature of sediment falling out of suspension. Suspension grading is identified by the entire grain size distribution fining upward, which is caused by particles with higher settling velocities (larger particles for the same particle shape and density) raining out of the water column and depositing on the bed before particles with lower settling velocities (smaller particles for the same particle shape and density) as a fast flow slows. Deposits on Anegada with suspension grading likely formed by a tsunami. In contrast, other deposits on Anegada sampled within several hundred meters inland of the shoreline appear massive or inversely graded in the field and may appear laminated in CT scans. Lack of grading or inverse grading at the decimeter scale has been observed in modern storm deposits and laminations are apparent in CT scans of Hurricane Irma deposits on Anegada. Deposits on Anegada with these features likely formed during storms (e.g., hurricanes). Our studies of Anegada deposits indicate that fine-scale vertical variations in grain size and CT scans are a useful tool for discriminating between tsunami and storm deposits. The increased ability to identify tsunami deposits will aid in the development of the geologic record of tsunamis and lead to improved hazard assessment.