Sequence Stratigraphy And Paleoenvironmental Changes In The Yellow River Delta Since Xianxian Transgressive,The Late Pleistocene

Delta is an important place where the terrigenous material in the upper reaches was carried by the river from sedimentary source to sink, and is the clastic dominated transitional facies. The formation and evolution of the processes are influenced by river, sea, climate and human activities,a large number of ancient climate and environment information are recorded, which is one of the important information sources to study the evolution of the ancient environment. Yellow River delta is a kind of geological body formed from the river-sea interaction, when Yellow River originates from the Tibet plateau and flows through the Loess Plateau and carries a large amount of sediment entering the sea. Due to changes of the way and estuary of Yellow River, Yellow River delta also changed. Modern Yellow River delta is a delta system formed after the diversion of the Yellow River since 1855. The area is not only an important place for agriculture and fishery production, but also the second largest oil field of Chinese which Shengli oilfield is located, rich oil and gas resources. Therefore, the research on the Yellow River delta is not only of great theoretical significance but also of great economic significance.This study was supported by a grant from Special Fund for Ocean Research in the Public Interest, named Standard acoustic sequences and typical geological hazards in the Yellow River delta(201005029-4). This paper choice of the modern Yellow River delta in the waters near the north as the study area, and GYDY geological core near Dongying Port waters was the source of the sediment samples, and 650 km sub-bottom profiling data from the waters of Yellow River delta to discuss the sequence stratigraphy; at the same time, sediment samples in plant bulk silicon as the main paleoenvironmental proxies, through the index which established by phytolith, warmth index, drought index and vegetation index of paleoenvironment analysis.In the study area, GYDY core sediment particle size and magnetization rate analysis showed that the Yellow River delta depositional sequence according to the time order of the division of the fluvial facies-transitional facies-tidal flat facies-neritic facies-delta phase five sedimentary facies belt, which sedimentary facies zone V, IV is continental zone, III, II and I is a marine.Through the shallow seismic data interpretation and comparison with the historical data, the study area can be divided into three more clear interface reflection R0, R1 and R2, based on which, the research area seabed strata can be divided from top to bottom for I, II and III three acoustic stratigraphical layer group. To level and wavy bedding layer group I sound reflection characteristics, layer II group to parallel reflection bedding, layers of group III to undulate and oblique bedding. The interface(R2), layer II and layer group I(R1) of the layer group III and the layer group II are all eroded. According to 14 C dating defines R1 erosional unconformity surface age of about 7.0 ka B.P., speculated that the Huanghua transgression; based on 14 C measurement in definition of R2 erosional unconformity surface age of about 27.0 ka B.P., suggesting the Xianxian transgression. Therefore, the strata in the study area basically covers and represents the since the late Pleistocene Xianxian transgression in the Yellow River delta near the sea subbottom development of specific geological time.The types of phytolith species are abundant in the sediments from GYDY core. Based on the study of the above sequence stratigraphy, in this paper, and the different combinations of features about phytolith, through the analysis of temperature index, drought index and vegetation type,based on cluster analysis method, these data have been used to divide phytolith assemblages into 4zones with different paleoenvironmental implications, Yellow River delta and adjacent sea area experienced the following paleoenvironmental history, cold-dry, warm, warm-humidity and warm-cool in sequence. By comparing curves of phytolith in the GYDY core, pollen concentration in the Qinhai Lake and δ18O in the Guliya ice-core. This results showed that phytolith characteristics in the GYDY core fits well with inferred climate change from the Younger Dryas event, 9.0~10.0 ka B.P. cooling events and 6.0~7.0 ka B.P. cooling events, so they can be used as a indicative index of studying paleoenvironment change.