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Piercement structures intruding deep-sea sediments have been observed on seismic profiles from many places in the Atlantic, Indian, and Pacific Oceans. Although these features may be salt diapirs in some areas close to the continental margins, they also are present in some areas where DSDP drilling has shown the absence of a salt layer. High-frequency seismic records (3.5 kHz) from the Canary basin, the Somali basin, the Nares Abyssal Plain, and the Bering Sea made over these piercement structures show doming, faulting, and other disturbances of uppermost Pleistocene turbidite layers, so it is unlikely that the apparent piercement structures are acoustic artifacts created by defocusing of the seismic energy over basement highs. Because the piercement mechanism appears to ave acted contemporaneously with the sedimentation, this mechanism most likely is related to a sedimentary phenomenon such as the loading of terrigenous sediments on the pelagic sequence. Igneous intrusion also could explain some of the observations, but this process would be active only over a short period of time and there is no evidence of widespread Pleistocene igneous intrusions in the ocean basins. This does not exclude the possibility that igneous intrusion could act as an initial trigger of the piercement mechanism. The cause of the effective density inversion which would appear to be necessary for continuing sediment diapirism is unknown because of a lack of data on in-situ physical properties of marine sediments.
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