ABSTRACT

Examination of seismic data from more than 1,000 U.S. Minerals Management Service geohazard reports and core logs of 1,670 foundation boreholes (drilled to an average subbottom depth of 125 m) has revealed that gassy sediment sections are most abundant near the Mississippi River delta, in buried stream channels eroded during the early and late Wisconsinan, and in Miocene and Pliocene-Pleistocene depocenters on the shelf and upper slope. Of all the bore-holes we examined, 1,158 (68 percent) contained indications of gassy sediments.

The "average" Mississippi-River-delta-related gassy sediment section occurs at a water depth of 53 m; the top of the section lies at a subbottom depth of 10 m; its thickness is 12 m; and the sediment water content is 45 percent, with liquid and plastic limits of 73 percent and 24 percent, respectively. In nondeltaic areas of the continental shelf (areas west of 90°W), the average gassy sediment section lies at a water depth of 37 m, its depth below the seafloor is 37 m, its thickness is 8 m, the water content is 40 percent, and the liquid and plastic limits are 68 percent and 27 percent. In the upper continental slope in the north-western Gulf of Mexico, the average gassy sediment section is at a water depth of 265 m and 25 m below the seafloor, its thickness is 60 m, the water content is 40 percent, and the liquid and plastic limits are 62 percent and 32 percent.

The median areal extent of gassy sediment sections within the entire shelf and upper slope of the northwestern Gulf area ranges from 238 to 546 m. Although large patches of gassy sediments exist, some exceeding 10 km in size, most are less than 500 m.

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