Study Of Stratigraphic Classification,the Sedimentary System And The Ancient Climate Of The Paleogene Period In West Area Of Sichuan Province

The Paleogene Period develops well in the study area. In this paper, on the base of deailed sections, the sedimentary phase model graph of the Paleogene Period in west area of Sichuan Province has been established with a deeper analysis of the sedimentary system and climate in the study area by the lithostratigraphic, biostratigraphic and chronostratigraphic classification.8 representative sections in Chengdu stratum region were selected to study in this paper. According to the methods and principles of the lithostratigraphic classification, combined with previous studies, a reasonable partitioning scheme of lithostratigraphic was proposed. It divides the Paleogene Period stratum of the study area into three formations. They could be distinguished as Daxili Formation(KEd), Mingshan Formation(E1-2m)and Lushan Formation(E3l)from bottom to top. Among these formations, the upside Daxili Formation(KEd)、Mingshan Formation(E1-2m)and Lushan Formation(E3l) are simultaneous sedimentation with different faces. The lithologic characteristics of each lithostratigraphic units, stratigraphic thickness, contact relationship and lateral variation are described in details as well.According to distribution and stratigraphic assemblages of fossils in Late Jurassic, 3 biological assemblage zones were established: Limnocythere yaanensis-Cyprinotus subquadratus-Homoeucypris bacerusa assemblage zone, Peckichara longa-Stephanochara funingensis-Gyrogona qianjiangic assemblage zone and Pentapollenites-Rhoipites-Cadripites assemblage zone. The chronostratigraphic of Mingshan Formation is considered to be Palaeocene- Eocene series by area comparison of time limits and horizons of each biological assemblage zone. The first group of Mingshan Formation is roughly the same with the Palaeocene and the second is the same with the Eocene series, but Lushan Formation is mainly for the Oligocene which may include a small amount of later Eocene series strata. The Daxi conglomerate is regarded as the later Cretaceous to Palaeocene and its top boundary may to Oligocene by the ESR measuring age.According to a comprehensive analysis of lithological characteristics, sedimentary structures and fossils and other data of each section, the sedimentary system has been analyzed. Thus, the stratum of study area has been divided into one sedimentary system group, which is the continental sedimentary system group. The continental sedimentary system group has been divided into 2 types: alluvial fan facies and lacustrine facies. The alluvial fan facies has been divided into middle-fan subfacies and marginal-fan subfacies. The lacustrine facies has been divided into shore-lake suhfacies and shallow lake subfacies. Based on the analysis of stratigraphic classification and comparison and sedimentary system, the sedimentary facies model diagram in Paleogene Period of the west Sichuan Province has been summarized.Based on the comprehensive research of geochemistry, sedimentology, paleobotany and paleogeography, the ancient climate characteristics has been studied preliminarily. It suggests that from the Minghsan stage of Paleocene- Eocene to the Lushan period of Oligocene, the climate of the study area on the whole is arid tropical-subtropical climate and its heat and drought degree reduced from morning till night.