INTERNATIONAL CORRELATION OF MAGNETO-BIOSTRATIGAPHY OF JAPANESE CRETACEOUS

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A series of normal and reversed polarity zones have been found from [he Upper Cretaceous sedimentary strata in Hokkaido of Japan and south Sakhalin of Russia. Combined magnetostratigraphic and biostratigraphic correlation has enabled us to assign these zones to the Late Cretaceous polarity chrons including the Cretaceous superchron.A polarity sequence in the Izumi Group in southwest Japan consists of the lowermost horizon of reversed polarity and a thick succession of normal polarity with two intervening horizons of reversed polarity. Based upon previous biostratigraphic age constraints, the Pattern of these polarity zones matches a polarity change from Chrons 33r through 31r during the early Campanian to the earliest Maastrichtian stages. Thick successions of normal polarity were recognized in the Upper Ezo Group in central Hokkaido and the Bykov Group in south Sakhalin. The normal polarity horizon of the Ezo Group is overlain by a short reversed horizon, and they yield abundant fossils of Cenomanian to Campanian age. It is thus most likely that the normal polarity can be assigned to the Cretaceous normal polarity superchron C34n and the overlying reversed zone assigned to subsequent Chron 33r in the early Campanian. The thick horizon of normal polarity in south Sakhalin is overlain by at least three zones of reversed polarity, including Chrons 33r in the early Campanian and Chrons 32r and 31r in Maastrichtian.All these reversed polarity chrons have been documented for the first time from the Cretaceous systems of the Northwest Pacific. The successive record of polarity reversals and their correlation to faunal assemblages that commonly occur in Japan and other areas in the Far East provide an integrated reference that should be of value for linking local biostratigraphic zonations in the North Pacific and serve as another basis for calibration in the quest to establish a global definition of Upper Cretaceous stage boundaries.