SAGER, WILLIAM W.¹ and IAN R. MACDONALD^2
1Oceanography Department, Texas A&M University, College Station, TX
²Geochemical & Environmental Research Group, Texas A&M University, College Station, TX

ABSTRACT: Side-Scan Sonar Survey of Halokinetic and Hydrocarbon Seep Sediment Modifications on the Louisiana Slope

We surveyed 2400 km² of Louisiana slope seafloor in areas of known hydrocarbon seeps using an 11-12 kHz digital, long range, side-scan sonar. Our objective was to image acoustic anomalies caused by seep-related sediment alterations and to assess whether seeps can be located using sea surface sidescan sonar. The survey area includes several well-documented seeps, most inhabited by chemosynthetic organisms. Additional ground-truth data were obtained by coring or inspection using the U.S. Navy submarine NR-1. The sonar data show numerous features related to halokinesis and seepage: mud volcanoes, authigenic carbonate mounds, brine lakes, hardbottoms, gassy sediments, faults, and sediment flows. Active faults are traced by surface expressions and comprise an interconnected network between diapir crests and bounding intra-salt basins. Several large sediment flows emanate from the faults. Mud mounds, such as the "Bush Hill" chemosynthetic site, are common. Some show a central "dead spot" from which no acoustic backscatter is detected. All deadspot mounds visited by submarine contain brine pools. We suggest that dead spots result from specular reflection off the brine-water interface. In the deeper area surveyed, seep-related features are notable, but halokinetic sediment-movement features are pronounced. Diapir flanks are often fault-bounded, with numerous gullies. Sediment flows extend from diapir flanks onto basin floors. Many appear related to sediments destabilized by salt movement. The rich detail in the sonar images enables interpretations of seafloor conditions and processes that are applicable to site engineering evaluations and their regional contexts.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas