摘要
笔者通过对长江三角洲平原晚新生代钻孔SG7孔沉积物的粒度和磁性分析,揭示磁性矿物类型随时间的演变,探讨新构造运动、气候、海平面等对本区沉积物源、沉积环境的耦合作用。研究结果显示,上新世磁性强弱相间,且达到全剖面最强,磁性矿物为磁铁矿、磁赤铁矿、针铁矿和黄铁矿,反映本区气候暖湿,发育山间湖泊,沉积物主要来自周边白龙港玄武岩的风化产物;早更新世早期,磁性为全剖面最弱,以磁铁矿为主,反映在构造沉降作用下,发生物源改变,即主要来自贫铁的中酸性浅成岩或喷出岩,同时沉积物主要为冰期的河道相沉积,反映山间河流/冲积扇环境;早更新世晚期至晚更新世末,磁性显著增强且和沉积物粗细变化一致,粗颗粒沉积物中磁性矿物以变质岩来源的粗粒磁铁矿为主,细粒沉积物中磁性矿物见针铁矿、赤铁矿和黄铁矿,反映古地理环境逐渐向冲积平原、滨海平原演变;自晚更新世晚期出现细粒磁赤铁矿和磁铁矿,反映长江上游物源的加入。全新世沉积物磁性较更新世泥质沉积物显著增强,反映长江河口、三角洲环境。
The grain-size analysis and magnetic property measurements of late Cenozoic sediments of borehole SG7 in the Yangtze delta plain have revealed the evolution of donfinant magnetic minerals with time and coupling effects of neotectonic movement, climate and sea level changes on the sediment provenance and depositional environment. Results demonstrate that strong and weak magnetic properties alternated with the highest signal of the whole profile during the Pliocene. Dominant magnetic minerals are magnetite, maghemite, goethite, and pyrite indicating a warm/humid climate and an intermontane lacustrine environment in the area and that the sediments were mainly derived from the weathering product of the Bailonggang basalt in the surrounding areas. During the early part of the early Pleistocene, magnetic signals were weakest of the borehole, with magnetite predominating. This implies that the material sources changed due to tectonic subsidence, i.e. the material sources were mainly iron-poor intermediate-acid hypabyssal rocks and extrusive rocks. Meanwhile, channel deposits formed during glaciation are mainly preserved, reflecting an intermontane fluvial or alluvial fan environment. During the late part of the early Pleistocene to the end of the late Pleistocene, magnetic signals increased markedly and fluctuated with the grain size of sediments. Coarse-grained magnetite derived from metamorphic rocks dominates magnetic minerals in coarse-grained sediments, while goethite, hematite and pyrite are found in fine-grained sediments, which suggests that the paleogeographic environment gradually evolved to the alluvial plain and coastal plain. Fine-grained maghemite and magnetite appeared again during the late part of the late Pleistocene, indicating the input of the material sources from the upper Yangtze catchments. Magnetic susceptibility of Holocene sediments is obviously higher than that of Pleistocene fine-grained sediments, indicating Yangtze estuary/delta environments.
出处
《中国地质》
CAS
CSCD
2008年第4期670-682,共13页
Geology in China
基金
国土资源部与上海市人民政府合作项目"上海市三维城市地质调查"(基[2004]011-02)资助
关键词
磁性矿物
白龙港玄武岩
变质岩物源
长江上游物源
构造沉降
magnetic mineral
Bailonggang basalt
sediment source of metamorphic rocks
sediment source of the upper Yangtze catchment
tectonic subsidence
