Small hills, 10-20 km. across, rise 150-350 m. above the Sigsbee abyssal plain in the Gulf of Mexico The abyssal plain received a thick layer of silty gray lutite during the Wisconsin sea-level lowering where as the knolls are covered primarily by foraminiferal ooze and calcilutite; thus they stood above the abyssal plain for the period of time exceeding that required to deposit their sediment column. By extrapolation beyond the maximum C l4 date in cores from the top of the knolls, at least 200,000 years is estimated as required for deposition of the top-capping sediments. At least 20 structures are present, called the Sigsbee Domes, and as investigated by the seismic reflection profiler, most do not reach the ocean floor, and are judged to be piercement salt domes. Each of the buried domes underlie small knolls on the sea floor; the observed rate of distortion calculated above for a knoll 200 m. high is about 1 m. per 1000 years or 1 mm. per year.

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