Paleocene Palynological Assemblage Of Shen Ke 1 Well In Jiangling Depression,Jianghan Basin And Its Geological Significance

The Jianghan Basin is located in the central and southern part of Hubei Province,with an area of 28000km~2.Continetal sedimentary strata of Cretacous and Paleogene are developed.It is a typical continental fault basin which is rich in sporopollen and ostracods.The tectonic evolution of Jianghan Basin can be devided into six stages,they are fault depression stage,break down stage,strong fault depression stage,break down stage,denudation uplift stage and depression stage.Continental clastic rocks of river-delta-lake system have been deposited for nearly 10000m.The Cretacous to Paleogene continental sedimentary strata developed well in the Jianghan Basin.The Shen Ke 1 Well is located in the Jiangling sag of Jianghan Basin,which is a continuous coring exploration well.In this paper,the sporopollen fossils of Honghuatao Formation and Yuyang Formation are analyzed by using the core data of Shen Ke 1 Well.The purpose is to restore the Paleogene paleoclimate environment in the Jianghan Basin,provide palynological and stratigraphic evidence for understanding climate change and vegetation evolution in the central and southern part of Hubei Province.At the same time,the Cretacous/Paleogene boundary is a boundary that records the evolution of major evolution events,such as dinosaur extinction and biological recovery about 66 million years ago.The extinction of about three quarters of the earth's organism during this period marked a great ups and downs in the earth's envrinment and biological evolution.Studying this boundary is of great significance to the study of the changes of the earth's envronment during the geological history.This paper attempts to explore the position of the Cretacous/Paleogene boundary in the Yuyang Formation from the perspective of palynology.The sporopollen fossils are mainly distributed in the Yuyang Formation and Honghuatao Formation,which is characterized by more gymnosperms and less angiosperims and ferns.The Honghuatao Formation in Jiangling Depression of Jianghan Basin has brown-red mudstone with a small amout of siltstone,fine sandstone and anhydrite.The Yuyang Formation has brown-red mudstone with a small amout of siltstone,fine sandstone and anhydrite,containing a small amout of gypsum and glauberite.Through analysis,two fossils assemblages were identified from bottom to top:1.Deltoidospora-Ephedripites-Ulmoideipites assemblage and 2. Pterisisporites-Ephedripites-Nanlingpollis assemlage.Assemblage 1 is located in the upper part of the Honghuatao Formation and the lower part of the Yuyang Formation,which contains fewer fossils.Assemblage 2 is located in the upper part of the Yuyang Formation,which contains relatively abundant fossils.The scarcity of fossils in Combination 1 may be due to the high salinity of gypsum-salt layer,which is not conducive to the preservation of sporopollen fossils,or to the occurrence of sporopollen extinction events.The age of the two assemblages is Paleocene.Unfortunately,the Cretaceous/Paleogene boundary has not been foud.Because the abundence and differentiation of sporopollen in the upper and lower parts of the Honghuatao Formation are not high,the upper cores of the Yuyang Formation rich in sporopollen fossils are selected for quantitive analysis.It is considered that the upper cores of the Yuyang Formation reflected a dry subtropical climate on the whole,and the sporopollen vegetation mainly consists of shrubs and coniferous forests.