THE PHYSIOGRAPHY AND PREDICTED FACIES OF THE OUTER CONTINENTAL SHELF AND UPPER CONTINENTAL SLOPE OF THE NORTHEASTERN GULF OF MEXICO FROM MULTIBEAM MAPPING

Extensive high-resolution multibeam-echosounder mapping of the outer continental shelf and upper slope of the NE Gulf of Mexico reveals a physiography more complex than was previously known. The outer shelf is dominated by a series of large shelf-edge deltas, with either reefs and hardgrounds or relict barrier islands preserved on their surfaces. The outer shelf south of Alabama and Mississippi is covered by relict reefs, patch reefs, pinnacle reefs and hardgrounds ,whereas relict barrier islands are found on the outer shelf off NW Florida and the panhandle of Florida. Moats are a common feature associated with bathymetric highs such as ridges, reefs, pinnacles and barrier islands. Outer shelf-upper slope failures are rare, the one exception being a series of large shelf-edge failures off Mississippi.

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.