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Abstract

The Orca basin is an intraslope depression on the continental slope, northwestern Gulf of Mexico, which contains an uncommon anoxic, saline brine. The 400-sq-km basin slopes at angles ranging between 3 and 14° from surrounding water depths of 1,700 to 1,900 m, to over 2,400 m. A 700-1 sparker seismic survey shows conformably dipping strata indicative of post depositional movement, as well as chaotic zones beneath the basin seafloor. The basin appears to be the result of salt tectonism. Sediments from the deep parts of the basin consist of dark anoxic clays with interstitial salinities of 250 ppt. Brine fills the bottom 200 m and provides a seismic-interface reflector at its surface similar to that of the Red Sea. Oxygen values are reduced to zero within the 260-g/kg saline brine and temperatures are slightly greater than the directly overlying bottom water. The bulk chemical composition of the brine is similar to that from the Red Sea whereas differences between the two in both heat content and geologic setting indicate different modes of origin.

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