Synthesis of magnetostratigraphy of the Cretaceous System in the northwest Pacific: Results from the Nemuro Group and the lower Yezo Group

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This study provides a new magnetostratigraphic data from the lower Yezo Group and the Nemuro Group in Hokkaido, Japan. Compilation of this data set and those from elsewhere as Kyushu, Shikoku and Sakhalin of Russian Far East leads to a comprehensive, intra-regional correlation of the Upper Cretaceous strata in the Northwest Pacific, which can be calibrated to the geomagnetic polarity time scale in the late Cretaceous. The combined magnetostratigraphic and biostratigraphic correlation has assigned these zones to the Late Cretaceous geomagnetic polarity chrons including C31r through C33r and the Cretaceous long normal interval. Corresponding geomagnetic reversals have been documented from the Upper Cretaceous successions of the Western Interior of North America, in combination with high-resolution ammonite biostratigraphy and radiometric age dates. Biostratigraphy of the Great Valley Sequence in California is also well-defined, but there is only one reversed interval that can be correlated with polarity chron C33r. The most complete record of polarity reversals in South Sakhalin would provide an integrated reference scheme which can be of significant use not only to correlate dissimilar faunal assemblages of disparate regions in the North Pacific, but also to contribute to a global definition of Upper Cretaceous stage boundaries.