An Analysis of the Formation of the Japan and Chishima Basins by a Paleomagnetic Study of South Sakhalin.

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The study area was the region near Bykov City, about 50 km north of Yuzhno-Sakhalinsk City in South Sakhalin, USSR. The area, 20 X 10 km in areal extent, is underlain by Cretaceous through Neogene deposits, with well-exposed Upper Cretaceous sections along the Naiba River and its branches. We spent there 30 days for geological survey and rock sample collection. One week was taken for field surveys of Tertiary sections near Yuzhno-Sakhalinsk, Kholmsk and Kolsakov Cities.An intensive geological mapping was conducted for the Naiba River sections which have long been known as a type locality of the Upper Cretaceous of Northwest Pacific. The ammonite biostraitgraphy established by this study has revealed that the age of the Cretaceous strata ranges Aptian to Maastrichtian. The Cretaceous system is overlain unconformably by coal-rich Paleogene and Neogene formations.Samples for paleomagnetism were collected, using portable rock drill, at 118 different levels of the Cretaceous sections in the Naiba area and at 68 horizons of the Tertiary sections in the Naiba and the other regions. A total of 1224 core samples were drilled in place, from which 2160 specimens were prepared for paleomagnetic measurements. About one fourth of the Tertiary specimens have been measured so far with a cryogenic magnetometer, after progressive alternating field or thermal demagnetization. Intensities of natural remanent magnetization were fairy week, ranging 10^<-4>to 10^<-3> A/m. Only one third of the specimens possessed stable magnetization component. Unfolded directions of the Paleogene specimens deflected eastward by 30^ﾟ to 60^ﾟ in declination, while those of the Neogene did not. The systematic change in declination with time is consistent with that reported for the Tertiary deposits in Hokkaido. It seems therefore that both South Sakhalin and Hokkaido suffered a clockwise rotation in the Tertiary.