High-resolution side-scan sonar mapping of reef-like mounds and sediment texture variations on the Mississippi-Alabama outer continental shelf

Abstract

Numerous reef-like carbonate mounds are found along the outer continental shelf of Mississippi and Alabama. They are believed to be relict reefs which formed at a time of lower sea-level during the last ice age. In an effort to classify the mounds based on morphology, and to examine the nature of their surrounding surficial sediments, five datasets were collected from four study sites on the outer continental shelf. They are: 1) high-resolution side-scan sonar images, 2) high-resolution subbottom profiles, 3) sediment grab samples, 4) current velocity data and 5) sediment trap data. Using GIS software, the size and shape of the mounds at each site were investigated with the intention of developing a characterization of the different types of mounds present. Mound sizes range from less than a few meters to nearly a kilometer in diameter and in height from approximately 2-15 m. There are three types of mound present: 1) low, carbonate hard bottoms, 2) irregular, spire-like mounds and 3) flat-topped mounds. Interesting features, seen on the side-scan records, are high backscatter "haloes" associated with many of the mounds. The haloes can be: a) symmetric around a mound, b) asymmetric, appearing on only one side of a mound or c) a linear, high backscatter "tail" that begins at the mound and trends southwest. Sediment grab samples show that the high backscatter areas have a higher gravel content than the surrounding lower backscatter areas. Here, we interpret these tails as indicators of current direction during storm events, such as hurricanes, which are common in the Gulf of Mexico. The storms appear to create high velocity (i.e. ~1 ms⁻¹) currents that move in a southwesterly direction. An interesting observation is that the direction of the currents does not appear to be dependent on the approach direction or geometry of the storm. The high speeds of the currents could, potentially, create eddy currents on the southwest side of the mounds which may then lead to erosion of the holocene sediments and, also, to the development of gravel lag deposits.