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MAGNETOSTRATIGRAPHIC STUDY OF LOOSE SEDIMENTS IN SOUTHERN YELLOW SEA AND ITS ADJACENT LAND AREA

Zhou Moqing Ge Zongshi (Institute of Marine Geology, Ministry of Geology & Mineral Resources, Qingdio, China, 266071)  
By analyzing 5 cores of loose sediment in the southern Yellow Sea and its adjacent land area, this paper has done a correlation of magnetostratigraphic classification of this region. Boundary between Brunhes normal epoch and Ma-tsuyama reversed epoch is located at hole depth of about 80 to99.5m both in the sea and in the north land area, but in the south land, this boundary is 270.4m deep due to much more rapid sedimentation rate during Brunhes normalepoch. Boundary between Matsuyama reversed epoch and Gauss normal epoch has been reached by neither hole QC1 nor hole QC2 in the Yellow Sea, because of the drilling limit. Only Jaramillo subchron(0.9ma B. P.) and the top of 01-duvai(1.7ma B.P.) were revealed respectively. Sedimentation rate during Matsuyama reversed epoch was slow in the land area, indicated by the sediment thickness of only 31.5 to 57.8m from south to north. Its lower boundary is located at the depth of 117.0 and 328.2m in the south and north respectively. Boundary between Gauss and Gilbert lies at 140.0m depth in the north land area, where Gauss sediment is only 31.0m thick. In the south, however, Gauss sedimentation had a much faster rate, and its boundary with Gilbert, at the depth of 460.15m(3.4ma B.P.), coincides with the baserock surface. Gilbert bounds the fifth zone at 190.5m deep in the north land, while the constantly uprising south area did not receive any sediment during this time. In the north, loose sediment meets the underlying Cretaceous(?) limestone at the depth of 400.35m, and the sedimentation began from 17ma B.P. Owing to tectonic movement, crust of the north part of the basin started to subside in middle and late Miocene, and sediment was accumulated then. By the time of middle Pliocene(3.4Ma B.P.), the movement began to spread in all parts of the basin. The subsiding center moved southwards, indicating a whole evolution development in the basin. In middle and late Miocene and early Pliocene, sediment developed only in the north part of the basin, bounded by the boundary between Gilbert and the fifth polarity epoch. The sediment surface is at the depth of 194.42m, serving as a sudden changed contact. Boundary between Pliocene and Pleistocene is that of Matsuyama and Gauss of 2.48ma B.P. This boundary is at 117.34m in the north and at 357.38m in the south of the basin. Boundary between middle and early Pleistocene is just the boundary between Brunhes and Matsuyama of 0.73Ma B.P., at the depth of 80-100m in the sea and north land area and at 261.12m in the south land area. Upper boundary of Black polarity event(0.108Ma B.P.) within Brunhes serves as the boundary between middle and late Pleistocene, lying at the depth of 30-59m in the sea and north land part, and at 111.33m in the south land. Boundary between Holocene and late Pleistocene is represented by Goteborg event. This interface is located at 10-20m deep in the sea and north land part, and at 54.14m in the south part. Magnetostratigraphic studies can be used as a basis for stratigraphic classification and correlation, but further researches are still needed, due to the differences between time and space of the two kinds of strata.
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