The Grain Size Distribution Of Magnetic Minerals And Its Implication For Provenance From FengJing Core Of The Yangtze River Delta In The Late Genozoic

Since the late Cenozoic, the tectonic movement in term of mainly the uplift of the Tibetan Plateau and expansion of the West Pacific marginal seas has caused the geomorphology and river system in China significantly. The Yangtze River Delta, linking the highest plateau and the largest marginal basin in the world, becomes the most sensitive area of land-ocean interaction. Controlled by the new tectonic movement, paleoclimate and sea level fluctuations, the late Cenozoic stratigraphy and sedimentary environment have became quite complex. With the tectonic subsidence since the late Cenozoic, the drainage basin and the sediment provenance expanded. Thus it's very necessary to study the sedimentary environmental evolution and provenance change for the Yangtze coast, in order to determine the bedrock, source location and pathway of transportation to reveal the tectonic controlled evolution of the Yangtze River.The study core FJ is from the southern plain of Yangtze River Delta, located in Fengjing town, Jinshan region, Shanghai. The core depth is335m, drilling throughout the whole late Cenozoic sediments and to the bedrock of Jurassic. Firstly, author decided the sedimentary environment evolution based on paleomagnetism stratigraphy, lithology and grain size analysis. Then author analyzed room temperature magnetic properties and decided the magnetic mineral assemblage and change of domain size based on the result of magnetic measurement. Finally, author examined the magnetic contribution of different particle-size fractions through measurement of size-fractionated magnetic susceptibility. By these studies, author tend to examine the change of sediment provenance for the Yangtze coast evolution throughout the late Cenozoic, which will also provide a new indicator for stratigraphic comparison and promote the environmental magnetism study in the fluvial, lacustrine and coastal environment.The major research results of provenance evolution of core FJ sediments in the Yangtze River Delta since the late Cenozoic is as follows:Pliocene and early stage of the Early Pleistocene (before2.14-2.01Ma B.P.). Sediments were characterized by thick sediments of mud-gravel confounding lower magnetic susceptibility, and higher values of other magnetic parameters. Magnetic properties indicated that there were more incomplete antiferromagnetic minerals and less ferrimagnetic minerals, and the magnetic domain was mainly the form of SD. But the magnetic minerals distribution in different particle-size fractions is uniform. Author suggest a sedimentary environment of foreland basin or low sinuosity meandering river with strong hydrodynamics and sediment provenance mainly from the adjacent mountain. Combined with the characteristics of magnetic minerals, acidic igneous rocks in Western Maoshan and Tianmu Mount may be the main source.Early to mid stage of the Early Pleistocene (from2.14-2.01MaB.P. to1.87-1.67MaB.P.), the gravel component declined obviously. Magnetic susceptibility was still low, and other magnetic parameters became weak. SD ferrimagnetic mineral content significantly reduced. The magnetic mineral differentiation in coarser sediment fractions decreased, which not only implied the sediment transport distance increased, but also prove that the sediment source have changed. The sedimentary environment experienced the process from meandering river to the alluvial plain, accordingly watershed area increased. But the weak magnetic properties reflect the bedrock of low magnetic mineral content.Mid to late stage of the Early Pleistocene period (from1.87-1.67Ma B.P. to about0.97-0.9Ma B.P.), sediment became further fine, but magnetic susceptibility reached the maximum. MD ferrimagnetic minerals are mainly concentrated in the fine-grained sediments, while less in coarser fractions, which indicated increased sediment transport distance and new sediment source with rich MD magnetic minerals. It is presumed that sediments from the metamorphic rock in the middle and lower of the Yangtze River have arrived at the core location at this time period.The end of Early Pleistocene to Holocene (since about0.97-0.9Ma B.P.), sediments were the finest and the magnetic parameters returned to low value. The ferromagnetic minerals overall decreased and were dominated alternately by SD and MD particles. The phenomenon that magnetic minerals in the very fine sand fraction reduced substantially, and more concentrated in the silt fractions indicated the change of sediment source once again. More SD magnetic minerals may be from the basalt in the upper of Yangtze River. Since the Middle Pleistocene, with the evolution gradually to a coastal and shallow marine environment, distal upper reaches of the Yangtze River have deposited in the area.