A new Look at Gas Hydrate Occurrences on the Peruvian Continental Margin - ODP Site 1230

During Ocean Drilling Program (ODP) Leg 201, gas hydrates were recovered at Site 1230 on the Peruvian outer continental margin. Previous work at ODP Site 685, which is located in close proximity to Site 1230, suggested that gas hydrates occurred below 40 meter below sea floor (mbsf) in this region, but were concentrated in two intervals at about 107 and 165 mbsf. A combination of infrared thermal imaging data, observations of core disturbance patterns, high-resolution profiles of interstitial water chemistry, physical properties, and wireline logging data collected at Site 1230 supplements and expands these findings. Distinct horizons of gas hydrate at Site 1230 appear to be offset from those of Site 685 by approximately 20 m. This offset may be caused by a concentration of gas hydrate along oblique fault planes. One of the gas hydrate pieces recovered at Site 1230 was composed of several-millimeter thick layers oriented at a high angle to the bedding surfaces. We suspect that this gas hydrate was associated with such a fault plane, because small high-angle faults were common in Site 1230 sediments and because previous work has shown a link between faults and gas hydrate distribution in the sediment. At other continental margin sites with gas hydrate-bearing sediments, much of the hydrate is inferred to be disseminated in the pore space and most does not survive the wireline trip. This may also be the case at Site 1230. However, temperature observations, core disturbance patterns, interstitial water chloride concentrations, and velocity and electrical resistivity log data suggest that disseminated gas hydrate may occur throughout the recovered sedimentary section at Site 1230, starting below approximately 70 mbsf or at even shallower depth.