Seismic and Visual Observation of Seepage-Related Structures on the Continental Slope, Northern Gulf of Mexico

Abstract

ABSTRACT Two seismically similar structures have been the subject of submersible observations. The Green Canyon example is a conical mound 35 m of relief in water depths over 300 m. Eruptions of gas bubbles from the caldera-like crest create clouds of sediment into the water column which in turn settle along the flanks. This structure is clearly a mud volcano. The Mississippi Canyon example is an elliptical mound less than 20 m of relief and located in 700 m of water. Extensive benthic communities are observed along with broken and tilted blocks of authigenic carbonate. This structure is diapiric in nature. INTRODUCTION High resolution geophysical profiles have defined numerous acoustically amorphous, "mud mounds" on the continental slope in the northwest Gulf of Mexico (1). These structures are related to seepage of gas and fluids from depth along fault planes (1,2). They range in size from a few feet to hundreds of feet in relief (1,2,3). These morphological forms have been compared to terrestrial mud diapirs and mud volcanoes (1,4). On land mud diapirs and mud volcanoes are two different manifestations of the same process. In many places these two types can be found in close proximity (4). This can now be verified to be true in the marine environment. This paper will show two seismically similar structures in the Gulf of Mexico but visual observations reveal one is an active mud volcano and the other a mud diapir. Some general comparisons will be made which may help to differentiate these two forms on seismic data. Data The seismic data discussed in this report are from high resolution "geohazard" surveys. These surveys are routinely conducted prior to drilling operations in u.s. offshore waters (5). Typical deepwater surveys include echosounder, 3.5 kHz subbottom profiler and a sparker or watergun type source. Survey grids are generally 300 by 900 m. The visual observations were made from the Johnson Sea Link (JSL) I and II of the Harbor Branch Foundation. The dives were part of a program conducted by the Coastal Studies Institute of Louisiana State university and funded by NOAA (6). GREEN CANYON AREA - HUD VOLCANO Subbottom profiler records (3.5 kHZ) define an acoustically amorphous mounded structure in the Green Canyon area (Fig: 1). This mound is located along a major fault just north of a large carbonate capped bank (Fig. 2). The water depth at the base of the mound is approximately 335 m. The relief of the structure is just over 35 m (Fig. 2). The amorphous zone, approximately 450 m in diameter, is in abrupt contrast to the acoustically well stratified sediments surrounding it (Fig. 1). The amorphous area is circular in plan view but is centered along a major northwest- southeast trending fault. Secondary faults of less throw radiate out from the mound (Fig. 2). Submersible observations in 1989 (6) revealed a steep sided conical mound with a circular, caldera-like depression at the crest. The rim was 2 to 3 ft. in relief (Fig. 3) and consisted of firm silty clays.