Magnetic properties and relevant minerals of late Cenozoic sediments in the Yangtze River delta and their implications

Abstract：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 dominant 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.