Author(s): Brian E Tucholke
Reflecting horizons which have anomalously high amplitude and which are conformable to the seafloor at about 500 to 600 m subbottom have been reported in two locations off the United States east coast--one along the crest of the Blake Outer Ridge, and another beneath the upper continental rise off New Jersey and Delaware. Detailed mapping of these horizons shows that: (1) the horizons cut across bedding planes in the sediment; (2) subbottom depth of the horizons increases with increasing seafloor depth and thus with decreasing seafloor (bottom water) temperature; and (3) the horizons are restricted to areas where sediment strata dip landward; such anomalous horizons are uncommon within the normal seaward-dipping continental-rise strata. Deep-sea drilling into or close to the anomalous horizon on the Blake Outer Ridge (Sites 102, 103, 104) recovered methane-rich sediment. Pressure/temperature conditions within the sediment column in both areas of anomalous horizons are appropriate for formation of gas hydrates to several hundred meters depth, thus suggesting that a zone of gas-hydrates overlies the anomalous horizons. A plot of the hydrate/gas phase transformation of the methane/seawater system in the sedimentary column, using geologically reasonable values for seafloor temperature and for thermal gradient and sound-velocity in the sediment, shows a good correlation between the depth of the phase change and the minimum depth of the anomalous reflecting horizons. The horizons therefore are thought to represent an imped nce contrast caused by the downward change from gas hydrate to gas in the sediment. Landward-dipping strata and the gas-hydrate layers in the areas exhibiting anomalous horizons appear to form traps for free gas, whereby the gas-hydrate layer blocks seaward gas migration and the dipping strata restrict landward migration.