OCS Study MMS 95 - Data Center

view on the scale of hundreds of meters, but with a resolution of less than a meter andthis allows us to compare viewport-scale observations with broad scale seismic data .Our primary observations were made with two acoustic tools, a 25 kHz echosounderand a 77 kHz side-scan sonar. Both were used aboard the U.S . Navy nuclearsubmarine NR-1 to collect profile data in grids over four hydrocarbon seeps known tobe inhabited by chemosynthetic communities.We chose these particular seepsbecause they have been the subject of intensive interdisciplinary studies and industrygeophysical data are also available for all to some degree .This situation allowed us toexamine the middle scale geologic characteristics and to link the ultra small and broadscale data sets .In this report, we attempt to synthesize the geological andgeophysical characteristics of each of these four sites and then extrapolate thisinformation to the broader population of chemosynthetic community sites .3.1 .1 Study LocationThe four seep sites examined in the geological/geophysical study are all locatedon the Louisiana continental slope in the Green Canyon (GC) and Garden Banks (GB)lease blocks areas (Figure 3.1) . From east to west, the sites are GC234, GC 184/185,GB 386/387, and GB 424/425 (for brevity, we henceforth refer to the last three sitesas GC1 184, GB 386, and GB 425) . GC 234 is located along a portion of a tensionalfault between salt diapir ridges (Behrens 1988) . The other sites are all moundslocated either on the flank of a salt diapir (GC 386, GB 425) or on a fault radiatingfrom a salt diapir (GC 184) . The deepest seep site is GB 425 at 575 m, next is GB386 at 585 m, GC 184 at 540 m, and the shallowest is GC 234 at depth of 530 m .3 .1 .2 Geologic Causes and Symptoms of Hydrocarbon Seeps in the Gulf of MexicoHydrocarbon seeps are widely scattered throughout the ocean basins becausethe two main ingredients that give rise to seeps, hydrocarbon source sediments and3-2