Multibeam and Airborne Lidar Bathymetry for Morphometrics of the Caicos Platform Carbonate Shelf Margin, BWI

Abstract:

Carbonate shelf margins are dynamic depositional environments especially in icehouse climate conditions when high-amplitude, high-frequency sea-level changes can create significant heterogeneity in depositional facies transitions along steep dipping margins. Despite the importance, variability is typically oversimplified necessitating detailed characterization. Airborne LIDAR and echo sounder multibeam surveys were acquired to map the shape of the sea floor bathymetry along the leeward margin of the Caicos Platform. The airborne LIDAR dataset was acquired with different laser wavelengths to enable high-resolution mapping of the topography (0.35-m spacing) and bathymetry (1-m spacing) to a water depth of -30 m below sea level. Echo sounder multibeam surveys (200 kHz and 400 kHz) were used to image depths of -20 to -350 m below sea level with resolution of 1 to 2-m for the 200 kHz and 0.5 to 1-m for the 400 kHz.

Using the combined datasets, morphometrics (i.e., the quantitative analysis of form, size and shape) of the margin were characterized to provide insight to the complexity of shape of the shelf margin. 398 2D profiles were used to identify variability in the shelf margin profile including: (1) depth to platform top, (2) beyond vertical walls and overhangs, and (3) shape and dip of platform top to upper slope and upper slope angle. 3D characterization revealed distinct morphologic character 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 from the upper slope onto the reef wall; (4) debris blocks being shed down the upper slope; (5) angular or fracture controlled grooves; and (6) gulleys and sediment pathways from the platform to the upper slope. Nearly all of the morphometric variability occurs at depths of -130 m or less, suggesting that the role of the Last Glacial Maximum (LGM) may have a profound effect on the steep wall morphology.

This unique dataset challenges the notion of a singular archetypal profile for a region, both in modern and ancient shelf margin systems. Furthermore, the dataset is a unique 3D view of the heterogeneity in depositional facies transitions along an active carbonate reef wall and shelf margin and highlights the importance of the LGM in the variability.