THE PHYSIOGRAPHY AND PREDICTED FACIES OF THE OUTER CONTINENTAL SHELF AND UPPER CONTINENTAL SLOPE OF THE NORTHEASTERN GULF OF MEXICO FROM MULTIBEAM MAPPING
Fields of small-scale bedforms are found scattered on the outer shelf but the largest bedforms are found associated with the southward- and eastward-facing flanks of the shelf-edge delta fronts. Almost all the bedforms are asymmetric in profile with wave heights generally <2 m and wavelengths that range from <100 to >500 m. The outer shelf bedforms indicate a NE-trending transport whereas sediment banked behind ridges and reefs suggest a sediment transport direction to the SW. The large asymmetric bedforms along the flanks of the shelf-edge delta fronts define a northward- and westward-flowing geostrophic current that has been steered by the morphology of the protruding delta fronts. It is not known whether this current is relict or modern.
Co-registered multibeam acoustic backscatter (95 kHz) has been correlated with the 100-kHz angular-response curves of Jackson (Applied Physics Lab, Univ. of Washington, Seattle Technical Report APL-UW TR9407, 1994) to predict the surficial facies of the entire mapped area. The acoustic backscatter clearly shows a rock-like response of the reefs and relict barrier islands as well as the relatively high backscatter from the MAFLA sand sheet. However, in addition, large areas of relatively low backscatter suggest that, north of the De Soto Canyon, Mississippi River muds have been transported far to the east. South of the De Soto Canyon, the acoustic backscatter of the outer shelf-upper slope is controlled by the presence of relict barrier islands and what appear to be deep-water reefs.