Magnetostratigraphic interpretations from Leg 198 sediments were restricted to upper Miocene and younger sections at all sites because cores from older sediments gave erratic magnetization directions. Despite making tens of thousands of measurements on hundreds of APC cores and using alternating field demagnetization up to 20-mT peak fields, the Oligocene to Cretaceous cores produced inclinations and declinations with poor correlation between different holes at any given site. The problem is thought to be the highly deformable nature of these sediments, which allowed disturbance and distortion of the sediments during coring, recovery, and/or splitting.
Measurements from Neogene sediments generally yielded an interpretable polarity record, typically showing polarity chrons from late Miocene to Pleistocene time (Fig. F43). Of these records, Sites 1209 to 1212 on the southern Shatsky Rise yielded a magnetostratigraphic sections of 50100 m, going back in time to C3An (~6 Ma). The outstanding record is that from Hole 1208, on the central Shatsky Rise, where a drift deposit with high sedimentation rates was cored. This hole gave an extraordinarily expanded magnetic stratigraphy extending back to C5An in the mid-Miocene (Fig. F43).
Shipboard measurements were also made on archive-half sections from the igneous sill section drilled at the bottom of Hole 1213B. The results are unusual because two of the sills gave high positive inclinations (50°70°) and the third yielded negative inclinations (~30°). If it is assumed that all of the sills were formed within a relatively short geologic time span, these results imply that two of the sills are of normal polarity and the third is of reversed polarity. The steep positive values are inconsistent with the low predicted paleolatitude of Shatsky Rise during Late Jurassic and Cretaceous time.
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