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Drilling at ODP Sites 642 and 643 revealed a 250 m-thick section of diatomaceous Pliocene to Miocene sediments on the outer Vdring Plateau, eastern Norwegian Sea. These biogenic silica-rich sediments have a significantly lower satu rated bulk density than the surrounding sediments, causing a decrease in acoustic impedance, which is seismically ex pressed as a negative polarity reflection. Variations in sonic velocity, the other key parameter in seismic analysis, is only of secondary importance in creating impedance contrasts in our study. Synthetic seismograms were produced from ship board physical property measurements corrected for in situ conditions. These synthetic seismograms are in good agree ment (< 6 m) with high-resolution air-gun seismic profiles shot during extensive seismic surveys on the Vdring Plateau and provide the basis for correlations between the seismic record and the borehole. The ability to unambiguously iden tify seismic horizons associated with biogenic silica deposits on the Vdring Plateau will permit the mapping of Miocene productivity patterns.

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