--> Distribution of and Environmental Impacts on Microbial Mat Ecosystems of Little Ambergris Cay, Turks and Caicos

AAPG ACE 2018

Datapages, Inc.Print this page

Distribution of and Environmental Impacts on Microbial Mat Ecosystems of Little Ambergris Cay, Turks and Caicos

Abstract

Little Ambergris Cay (21.3° N, 71.7° W) is a ~6 km long, ~1.6 km wide island on the Caicos platform that was the focus of mapping campaigns during July 2016, August 2017, and after a direct hit from hurricane Irma in September 2017. Ongoing mapping of the cay provides context for interpreting the island’s geomorphic and stratigraphic evolution and insight into the genesis of ancient microbialite-grainstone reservoirs. Lithified upper shoreface and eolian ooid grainstones form a ~1-4 m high bedrock rim that is locally breached, allowing tides to inundate an interior basin lined with extensive microbial mats. Initial mapping in July of 2016 used unmanned aerial vehicle (UAV)- and satellite-based images, differential GPS (DGPS), and total station theodolite (TST) measurements to characterize environments across the cay. Nine facies were identified: oolitic grainstone bedrock, sand flats, cutbank and mat-filled channels, hardground-lined bays, mangroves, flat mats, polygonal mats, and mats with blistered surface texture. These facies were mapped onto a ~15 cm/pixel visible light orthomosaic of the cay made from more than 1500 nadir images taken by UAV. A corresponding stereogrammetric digital elevation model (DEM) was generated from the UAV images and validated with >1000 DGPS and TST measurements. In conjunction with the UAV-based facies map, the DEM confirms that all facies, including microbial mat types, vary strongly as a function of elevation; notably, mat morphology was observed to be sensitive to cm-scale changes in water depth. A UAV-based visible-light orthomosaic of the island was produced again in August of 2017. Some significant changes were noted relative to 2016, including the closure of several channels due to the constant influx of ooids transported along-shore by currents driven by easterly trade winds, and desiccation of mats in the interior basin due to channel closure. Additionally, a UAV-based VNIR (400-1000 nm) imaging spectrometer was used to map much of the cay at cm-scale resolution to more comprehensively evaluate the distribution of mats and sediment. On September 7th, 2017, the eye wall of hurricane Irma crossed Little Ambergris Cay with sustained wind speeds exceeding 170 MPH. Subsequent UAV remapping of the island revealed significant channel and bedrock erosion, scouring and removal of broad tracts of mats, and blanketing by ooid sediment of large portions of the interior basin, including smothering of microbial mats.