Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Mud mounds are common features at cold seeps in many areas of the world. They are abundant in the northern. Gulf of Mexico, where they occur at the numerous hydrocarbon seeps on the continental margin. Although the mounds are widely known, their geophysical signatures and geology have not been widely studied in detail. In this study, we examined mounds in two areas of the upper Louisiana slope using geophysical data of several types: 11-12 kHz side-scan sonar, 3.5 kHz echo-sounder profiles, near-bottom 2-12 kHz chirp sonar profiles, and three-dimensional (31)) multichannel seismic data. In side-scan images the mud mounds appear as ovoid high backscatter spots associated with larger, irregular high backscatter zones along faults and fault complexes. Three different sonar responses were noted: (1) a more-or-less uniform appearance, (2) a central higher backscatter spot surrounded with a lower return halo (bulls-eye), and (3) a more-or-less uniform high backscatter area with an acoustic dead-spot near its center. Near-bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics, implying that the long-range acoustic signature of a given mound results from the integration of seafloor characteristics. In these records, anomalous seafloor echo types include strong seafloor reflectors (hardbottom) or buried hardbottom, acoustic wipe-out (attenuation), and acoustic turbidity (prolonged echoes). Cores from acoustically anomalous seafloor areas indicate that a variety of seep-related material has been introduced into the near-seafloor sediments. The prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features. Multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath. Seafloor seismic amplitude maps from these mounds are either generally uniform, with a positive or negative anomaly, or display a ring of higher reflectivity around the mound periphery. Positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging. The ring feature is thought to result from the preferential concentration of seep-debris in the mound periphery relative to the center. Some mounds, which show no seismic reflection anomaly from the seafloor, appear inactive. Altogether, observations from this study are consistent with the mud mounds being formed by sedimentary volcanism. The data imply they are Holocene features that have a life-cycle that includes hardbottom formation, which progressively crusts over the fluid vent, en ding its activity. (C) 2003 Elsevier Science B.V. All rights reserved.