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Carbonate Shelf Margin Morphometrics: Insights From Multibeam and Airborne LIdar Bathymetry of the Caicos Platform


Steep-walled carbonate margins are of high geologic interest and provide critical insight into depositional facies transitions and mark regions of significant syndepositional fractures, faults and high-permeability features develop that have significant impact on hydrocarbon recovery. Shelf margins are dynamic environments especially in icehouse climate conditions when high-amplitude, high-frequency sea-level changes can create significant heterogeneity along the margin. Two different datasets—airborne LIDAR and echo sounder multibeam—are used in this study to map the shape and character of the sea floor bathymetry and island topography along the leeward margin of the Caicos Platform. The airborne lidar dataset was acquired with two different laser wavelengths to enable high-resolution mapping of the topography (0.35-m spacing) and underwater bathymetry (1-m spacing) to a water depth of -30 m and covered 92 sq kms. Two different frequency of echo sounder multibeam (200 kHz and 400 kHz) were used to image depths of -30 to -350 m below sea level. The resolution of the mulitbeam was 1 to 2-m spacing for the 200 kHz and 0.5 to 1-m for the 400 kHz. In addition, the multibeam head was mounted using a 15-degree wedge to improve resolution on the steep-walled (vertical) margin. Over 80 km of the leeward margin of the Caicos Platform was mapped by one of the three methods. Using the combined dataset, 3D morphometrics (i.e., the quantitative analysis of form, size and shape) of the margin were measured in order: (1) provide insight to the complexity of shape; (2) create a large database of critical dimensions not easily obtained from seismic data on steep-walled margins; and (3) enable discussion on controls of erosive and deposition variability along a reservoir-scale carbonate margin. Sampling of the dataset occurred in two ways. The first is 400, 2D profiles spaced every 200 m were developed and the geometric parameters were recorded for comparison. Significant variability was measured in upper 150 m of water depth including depth of platform top, shape and dip of platform top to upper slope, and upper slope angle. 3D mapping of the shelf margin dataset revealed morphologic changes including: (1) erosive notches in the upper, middle and lower wall; (2) overhanging or beyond vertical walls, including potential cave systems; (3) sediment aprons accumulating on the upper slope and reef wall; (4) debris blocks shed to the upper slope; (5) angular or fracture controlled grooves; and (6) gulleys and sediment pathways from the platform to the upper slope. This dataset is a unique 3D view of the heterogeneity that is present along an active steep-rimmed carbonate reef wall and provides insight for the development of fracture-controlled deformation often observed along ancient carbonate margins.