The Quaternery Transgression And Paleoclimate-Monsoon Evolution Of Eastern Changjiang Delta, China

Changjiang delta is located on the eastern coast of China, closely linking the largest Pacific Ocean of the world and the summit of Tibetan plateau of the Asian continent. Since late Tertiary, with the rapid uplift of Qing-Zang plateau, the whole china mainland had experienced the dramatic topographical reversal, which results in a series of large change in paleo-environmental evolution, including the formation of eastern-directed flowing river system, initiation of Asian monsoon, transgression and regression on the marginal sea, etc., Due to the important geographic position on the coastal zone, the Quaternary borehole recovered from the Changjiang delta can record the significant information on global change induced by the late Tertiary Reversal processes. The present study on the basis of numerous petrological analyses will shed light on better understanding of coupling relationship between regional tectonic and paleoclimate interaction, which had played a critical role in controlling stragraphic architecture and paleo-environmental reconstruction in the eastern Yangtze coast of China.Our present study is chiefly based on a 360 meter-long borehole recoverd from the Eastern Changjiang delta plain. Sixty-tree samples were taken for clay mineral analysis and geochemistry respectively. Three hundred-thirty-five samples were selected for grain s ize analysis. One huderd-fifteen s amples w ere for micro fossils and pollen-spore analysis respectively. In addition, Two hundred-fifteen samples were done for paleo-magnetism, which helps date the Quaternary stratigraphy, transgression in association with the paleo-climate fluctuations.On the basis of paleomagnetism, lithology and microfossils examined in the borehole sediments, the Quaternary stratigraphy can be divided into 4 sedimentary faces, i.e. early Pleistocene alluvial fans, Mid-Pleistocene alluvial to fluvial facies, late Pleistocene drowned valley and Holocene delta facie.Our petrological analyses evidence that the early Pleistocene strata (289-153 m), consisting of several major sediment cyclicities, are dominated by the thick yellow and gray gravelly sand and silt. Sorting is extremely poor. Large-scale troughbeddings appear often in the sediments and no marine fossils occur throughout. These indicate alluvial fan deposits. The mid-Pleistocene strata (153-104 m) are rather thin and are dominated by the yellow coarse to fine sands and mottled muds. Sorting is relatively poor. Trough and undulate beddings are present. Foraminifera and marine ostracode appear sporadically in the upper sediment sections. These have reflected the alluvial to fluvial regime. The late Pleistocene strata (104-29.4 m) consist c hiefly o f a m ajor s edimentary cyclicity. The yellowish t o gray, c oarse t o medium sands occur as basal sediment section, and fine sand and silt appears at upper sediment section. Varieties of sedimentary beddings are seen in the sediments, including trough, wavy, and undulate, etc. Foraminifera and marine ostracoda occur throughout. These have indicated the drowned valley deposits. The Holocene strata (29.4-0 m) comprise the gray fine sand and muds. Sorting becomes better, cross beddings are seen, and marine fossils prevail. A stiff mud layer without marine fossils separates the late Pleistocene and Holocene sediment sections at core depth of 32.40-29.4 m. Clearly, these are the recent delta facies.Four major sea level events were revealed as Jiading transgression (early Mid-Pleistocene, 70ka), Taihu transgression (early Pleistocene, 100-80ka), Gehu transgression (Mid-Pleistocene, 60-20ka), and Zhengjiang transgression (Holocene, 10ka). Jiading transgression was evidenced b y sporadically distribution o f marine fossils, and is the weakest phase in our study. Drowned valley prevailed during Taihu transgression. Gehu transgression was the most significant event in the study area. Two cycles (60-40ka and 40-20ka) can be further defined by microfossils. Drowned valley and coast was dominated during 60-50 ka BP followed by weak regression during 50-4...