Sporopollen assemblages of the core from Lianghucun Borehole in Dongting Basin and their climatic and stratigraphic implications
Bai Daoyuan1,2 Li Chang'an2 Zhang Wenqing3 Chen Duping1 Zhou Kejun1
1.Hunan Institute of Geology Survey|Changsha 410011； 2.Faculty of Earth Sciences； China University of Geosciences； Wuhan 430074； 3.Institute of Technologic Method of Hydrogeology & Engineering Geology|CAGS|Baoding|Hebei 071051
Lianghucun ZKC1 borehole is located in southeastern Anxiang sag of Dongting BasinQuaternary sediments(including bottom Pliocene deposits)with thickness of 294 m in the borehole are composed of alluvial and lacustrine deposits，and were composed of PlioceneEarly Pleistocene Huatian Formation，Early Pleistocene Miluo Formation，Middle Pleistocene Dongtinghu Formation，Late Pleistocene Potou Formation and Holocene depositsDetailed sporopollen analysis on the Quaternary deposits from ZKC1 borehole was conducted and 16 sporopollenzones were identifiedESR age，sporopollen assemblages and corresponding climatic feature indicate that the lower part of Huatian Formation was formed at the end of PlioceneAccording to the features of sporopollen assemblages and combined with tectonicsedimentary evolution and regional climatic settings，the climatic evolution of Dongting Basin since the end of Pliocene was revealed as follows：Sporopollenzone Ⅰ and Ⅱ indicate that the climate at the end of Pliocene was tropical and dryFrom early to late，The climatic variations during Early Pleistocene are cool and dry(sporopollenzone Ⅲ and Ⅳ)，tropical and humid interposed with cool and dry(sporopollenzone Ⅴ～Ⅶ)，cold and dry interposed with warm and humid(sporopollenzone Ⅷ～Ⅹ)，tropic and weakly humid(sporopollenzone Ⅺ and Ⅻ).During Early Middle Pleistocene with no sporopollen sample the climate was cold and dry，which is suggested by sedimentary environment of gravels from lower part of Dongtinghu Formation； during middle Middle Pleistocene the climate was tropical and weakly humid which is indicated by sporopollenzone ⅩⅢ； during late Middle Pleistocene with no deposits formed for rise，the climate was tropical and humid which is indicated by tropicalhumid eventAccording to regional climatic feature，the climate was dry in early Late Pleistocene with no deposits formed； during middle Late Pleistocene the climate was warm and weak humid which is indicated by sporopollenzone ⅩⅣ； there were no deposits formed for cold climate and sealevel droppedFrom early to late，The climatic variations during Holocene are tropical and weak humid(sporopollenzone ⅩⅤ)， and tropical and weak dry(sporopollenzone ⅩⅥ). This climatic evolution process coincide basically with climatic change indicated by CIA(chemical index of alteration)，and with Quaternary climatic variations of eastern ChinaThere exist a sporopollen boundary at depth of 140 m over which there were more pollen and spore then underThis variation was in all probability related with an important geological event，and the cause of formation await embedded research in future.