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

研究区域位于苏联库页岛南部尤日诺-萨哈林斯克市以北约50公里的别科夫市附近。该地区面积20×10公里，被白垩纪覆盖，穿过新近系沉积，沿奈巴河及其支流的上白垩统剖面暴露良好。我们在那里花了30天的时间进行地质调查和岩石样品采集。对尤日诺-萨哈林斯克市、霍尔姆斯克市和科尔萨科夫市附近的第三系剖面进行了为期一周的野外调查，并对长期以来被认为是西北太平洋上白垩统典型地区的奈巴河断面进行了密集的地质填图。本研究建立的菊石生物应变图显示，白垩纪地层的年龄范围为阿普提安至马斯特里赫特。白垩系被富煤古近纪和新近纪地层不整合覆盖，利用便携式凿岩机在内巴地区118个不同水平的白垩纪剖面和内巴等地区第三系剖面的68个层位采集了古地磁样品。共钻取了1224个岩心样品，从中制备了2160个样品用于古地磁测量。到目前为止，大约四分之一的第三系样品在逐步交变磁场或热退磁后用低温磁强计进行了测量。天然剩磁强度在10^~(-4)~10^~(-3)A/m之间，仅有1/3的样品具有稳定的磁化强度。古近纪样品的展开方向在偏斜时向东偏转了30ﾟ~60ﾟ，而新近系样品的展开方向没有偏转。北海道第三系沉积物的倾角随时间的系统变化与已报道的一致。因此，南库页岛和北海道似乎都经历了第三纪的顺时针旋转。

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.