长江三角洲地区晚新生代沉积物岩石磁学特征及其物源指示意义

ROCK MAGNETIC PROPERTIES AND SOURCE INDICATIONS OF LATE CENOZOIC SEDIMENTS IN YANGTZE DELTA AREA

对长江三角洲地区7个晚新生代钻孔沉积物的岩石地层和磁化率特征进行详细对比,并进一步选择浦东机场孔(PD孔)进行了详细的岩石磁学测试,讨论了该地区的晚新生代沉积物物源的演变。结果显示沉积物磁性的4次显著变化:1)各钻孔上新世磁性强弱差异明显;2)早更新世早期本区普遍表现出弱磁性;3)从早更新世中晚期至全新世,磁性强度一般随粒度粗细变化波动,即砂质沉积磁性强,泥质沉积磁性弱;4)晚更新世晚期以来,细颗粒沉积物的磁性较明显增强。磁性参数分析显示上新世和早更新世早期磁性矿物晶体颗粒较细,早更新世中晚期-晚更新世以粗晶粒磁性矿物为主,全新世细晶粒磁铁矿再次占主导。我们认为上述磁性变化反映构造沉降控制下现代长江三角洲地区物源区的不断扩大。

Lithostratigraphy and magnetic properties of sediments from 7 Late Cenozoic boreholes（ SG6 ,SG7, SG8 ,SG10, SG13, J18A, and PD）drilled in the Yangtze delta area were analyzed to reveal the sediment source and its change in this area. Paleomagnetism was measured for all boreholes with the sampling intervals of 1.0 - 1.5m. The sediments of PD were further measured for rock magnetic property with a mean sampling interval of ca. rim. The results show that gravelly sand and hard clay dominate the sediments deposited during Pliocene to mid-Pleistocene. Fine-grained soft sediments mainly occur after Late Pleistocene. Results also show that magnetic susceptibility changes significantly among Pliocene sediments in 7 boreholes. The strongest magnetic susceptibility is found in SG7, exceeding 200, which is believed to be contributed by the local bedrock of basalt. By contrast, magnetic susceptibility is generally weak in other boreholes. Palaeotopography is suggested to control the spatial distribution of magnetic properties and the associated local sediment source. During early stage of Early Pleistocene, magnetic signals become extremely weak in all boreholes,suggesting a uniform sediment source. During the middle and late stages of Early Pleistocene to Holocene, magnetic susceptibility fluctuates along with the change of grain size of sediments. It is stronger in sandy sediments and weaker in muddy sediments, reflecting new sediment source of rich magnetic minerals. Since the Late Pleistocene, magnetic susceptibility of fine-grained sediments enhanced obviously. This indicates the increase of fine-grained magnetic mineral that is possibly contributed from the upper Yangtze drainage basin. Rock Magnetic property analysis of PD demonstrates that magnetite is the major magnetic mineral and its crystal particle size is smaller in Pliocene and the early stage of Early Pleistocene,and bigger during the middle and late stages of Early Pleistocene to Late Pleistocene. Fine-grained magnetite dominated Holocene sediments again. We believe that these magnetic changes reflect the sediment sources of the present Yangtze delta area which kept on expanding during Late Cenozoic era under the control of the tectonic subsidence.