Biostratigraphy, paleomagnetism and sedimentology of Late Cenozoic sediments in northwestern Hokkaido, Japan
Four Neogene sedimentary basins in northwestern Hokkaido (latitudes 44°-45° N.), the northernmost island of Japan, were studied to establish a combined microfossil-paleomagnetic sedimentologic stratigraphy. The measured section near Atsuta encompasses a shoaling depositional sequence from upper bathyal depths in the lower part to stagnant, nearshore conditions near the top of the sequence. Magnetostratigraphic evidence and age-diagnostic diatoms indicate the section to be Late Miocene age covering the interval from early Epoch 5 through Epoch 6, and possibly to late Epoch 7. The second measured section located north of Shosanbetsu exposes sediments derived from a volcano-clastic source and deposited on a continental slope with oversteepened slopes. Rich diatom floras from the sequence are assignable to the Denticula seminae var. fossilis-D. kamtschatica and D. kamtschatica Zones. A magnetic signature containing one reversal was observed in the upper part of the section, although the data seem to indicate the lower section to be dominantly normally magnetized. In the northwest Pacific deep-sea sequences, the zonal boundary of these two diatom zones lies between the Nunivak ( = "b") and "c" Events of the Gilbert Epoch, thus the distinct magnetic reversal observed in the upper part of the sequence is correlated with the top of the Nunivak Event. Neogene strata developed near Wakkanai City, the northwestern tip of Hokkaido, appear to have been deposited during a time interval of predominantly reversed geomagnetic polarity. Diatom floras suggest a correlation of these strata with the Denticula kamtschatica Zone which in turn corresponds, in the northwest Pacific deep-sea sequences, to the lower Gilbert Series below a horizon midway between the Nunivak and "c" Events. Diatom rich sediments of the Shimo-Ebekorobetsu area belong to the Yuchi Formation and comprise two assemblages assignable to the D. kamtschatica Zone and the superjacent Denticula seminae var.fossilis-D. kamtschatica Zone. From northern Honshu through Hokkaido to Sakhalin, in the western Pacific coastal region, beds containing the large pecten Fortipecten takahashii (YOKOYAMA) constitute a marker horizon useful for inter-regional correlation. In its southern range of distribution, F. takahashii is a diagnostic species in the lower part of the Tatsunokuchi Formation from which diatom floras assignable to the D. seminae var. fossilis-D. kamtschatica Zone are described. The F. takahashii bed occurs in the Shosanbetsu section, lying within the same diatom zone as the Tastunokuchi Formation and is in a reversely magnetized interval above the Nunivak Event (3.9 m.y. B.P.). The Neogene marine sediments of northwestern Hokkaido were deposited in the back-arc basin and reveal evidence of active arc magmatism at the time of deposition. In this tectonically active back-arc region, sediment-collecting basins shifted from area to area, accumulating sediments only for a relatively short period of time when spurts of active subsidence occurred in a given sedimentary basin.
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Also Published In
- Bulletin of the National Science Museum. Series C, Geology & paleontology