AAPG ACE 2018

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Diverse and Spatially Extensive Microbial Mat and Ooid Sand Depositional System, Little Ambergris Cay, Turks and Caicos Islands

Abstract

Development of microbially-influenced reservoirs requires understanding of modern processes and analogs. Little Ambergris Cay (21.3 N°, 71.7° W; ~6 km long x ~1.6 km wide) is developed on a broad bank marked by converging ooid shoals, influenced by strong easterly trade winds (avg. 7.5 m/s), and culminating in a linear ooid shoal that extends almost 25 km from the western tip of the cay. Lithified upper shoreface to eolian ooid grainstones form a ~2 m high bedrock rim that protects an extensive interior tidal marsh with well-developed (0.1-30 cm thick) microbial mats. The rim is Holocene in age and shows outward accretion from 1185 to 450 years before present. It is locally breached to allow tidal flows to inundate interior bays floored by microbial mats. Three mat types are observed based on texture: dark toned “blister mat”, which flanks the bays where they intersect with the bedrock rim; light-toned “polygonal mat” which covers broad tracts of the bay and is exposed at low tide; and lighter-toned “eps mat” which is generally submerged even at low tide. Over 30 different mat locations were studied and sampled for groundwater salinity, pH, DNA content, photosynthetic efficiency, C and S isotope composition, lipid biomarkers, and taphonomic state. The island was mapped using multispectral Landsat images (m-scale resolution), Quickbird Earth images (50 cm-scale resolution), and photogrammetry from three UAVs. In July of 2016, the UAVs captured more than 1500 nadir images from a ~350 m standoff distance and were processed to generate a 3-band visible light mosaic map of the island with ~15 cm/pixel resolution. To detect and maps changes, this survey was repeated again in August of 2017, and again after Hurricane Irma passed directly over the cay in September of 2017, this time with 150 m standoff. The drones also captured images with a 5-20 meter standoff to quantify sub-cm-scale bed textures, including those expressed by the different microbial mats. Topography and nine sedimentologic facies were mapped at cm-scale resolution based on 910 differential GPS data points, and the thickness of the Holocene sediment fill (0 to >2m) was estimated using a depth probe and laser theodolite. Eight vibracores sampled facies extending to depths of 3 meters. These cores suggest the cay initiated as an ooid shoal, developed cemented margins marked by beach and eolian facies, which created a relatively recent interior tidal marsh.