Sedimentology Of Hurricane Irma Deposits On A Modern Carbonate Platform

Little Ambergris Cay is a 6 km long uninhabited island on the Caicos platform in the Turks and Caicos Islands. Hurricane Irma passed directly over Little Ambergris Cay on Sept 7, 2017, making it an ideal location to study the deposits formed during the hurricane and the continued effects of fair weather currents and wind on their sedimentology. The primary goal of this project is determining the differences in patterns of hurricane-related vs. fair weather sediment deposition in a modern carbonate system. This also enables us to observe how the new deposits evolve in the context of the island's continued exposure to fair-weather conditions. Samples and drone images were collected in March and July 2018 (6 and 10 months after Hurricane Irma, respectively), to observe the extent of the re-working over time. Samples were collected from multiple hurricane deposits, including washover fans in the interior of the island and lobes at the mouths of tidal channels. We collected 71 sediment samples in March 2018 and 33 in July 2018. All samples were rinsed, dried, and analyzed using a Camsizer to examine variations in grain size, roundness, and sorting in relation to depth and location on the island. Aerial imaging of the island through the use of a drone was also conducted in order to establish the changes in position of the sediments associated with current and wind re-working, and shows a westward migration due to longshore transport. Several hundred overlapping visible light drone images of the sediment deposits were taken in September 2017 and March and July 2018 using a Phantom 4 Pro quadcopter. The images were processed with Agisoft Photoscan photogrammetry software to generate orthomosaics of the lobes and washover fans at 5cm and 7mm resolution, respectively, and corresponding digital elevation models. Re-working of the hurricane deposits was also tracked at sub-week timescales using 3m resolution approximate true color images from the PlanetScope satellite network.The grain size of washover fan sediment decreases westward. We observed that the channel mouth lobes had migrated 10 m west between March and July 2018, demonstrating substantial reworking by longshore currents and used Camsizer data to examine the evolution of grain size, shape, and sorting associated with this reworking.