Study Of Stratigraphic Classification And The Variance Of Sedimentary System Of Cambrian Stratum In Guangyuan Area, Sichuan

The studied area is located in Guangyuan area of the north of Sichuan province,and belongs to the contignous zone of the nappe tectonic belt in the southeast ofLongmen Mountains and the foreland basin of western Sichuan. For this reason, thereare many stratigraphic units and more complicated sedimentary types of Cambrian.Based on the measured sections, the multi-stratigraphic classification of the Cambriansystem has been established with the methods of lithostratigraphic, biostratigraphicand chronostratigraphic classification. The characteristics of sedimentary systems andsedimentary variance are also discussed in this paper.A new stratigraphic division project was established in this paper based onanalysis of Cambrian charecteristies stratigraphic lithology and contrasting with manystratigraphic division projects. The Cambrian stratigraphic division system in thestudied area belongs to upper Yangtze sub-region in Yangtze region of Huanan realm,which is divided into three mic-regions that are Jiudingshan mic-region at theNorthwest and Chengdu mic-region at the South and the Micangshan mic-region atthe Northeast.In this area, five sections were measured and studied in detail in the threemic-regions in the studied area. The Cambrian section at Yangmu village inGuangyuan County in Jiudingshan mic-region was divided into the QiujiaheFormation（∈₁q） and the Youfang Formation（∈₁y）.The Cambrian section atShangsi village in Jiange County in Chengdu mic-region belongs to theChangjianggou Formation（∈₁c）. The Cambrian section at Sandui village inGuangyuan County in Chengdu mic-region was divided into the the ChangjianggouFormation（∈₁c）and the Modaoya Formation（∈₁m）.Particularly this section wasregarded as the stratotype of the Modaoya Formation for the first time. The Cambriansection in the Micangshan mic-region was divided into the Qiongzhusi Formation（∈₁q）, Xiannvdong Formation（∈₁x）, Yanwangbian Formation（∈₁y）, Shilongdong Formation（∈₁s） and Douposi Formation（∈₂d）. Lithostratigraphiccharacteristics, stratigraphic thickness, stratigraphic contact relationship, spatialvariation in lithosedimentary system are discussed in detail in this paper.Five biostratigraphic Assemblage-Zones of trilobites have been establishedaccording to the fossil association and stratigraphic distribution, which areEoredlichia—Parabadiella Assemblage-Zone， Eoredlichia—WutingaspisAssemblage-Zone， Palaeolenus—Paokannia nanjiangensis Assemblage-Zone，Hoffetella—Redlichia Assemblage-Zone and Chittidilla—KunmingaspisAssemblage-Zone. The chronostratigraphic classification of each lithostratigraphicunit was discussed and determined based on the biostratigraphic Assembelage Zonesand regional correlation.The sedimentary systems in the Cambrian sections have been analysed accordingto lithologic association, sedimentary structures and fossil charecteristies. TheCambrian sedimentary system was divided into the transitional sedimentary systemgroup and the marine sedimentary system group. The transitional sedimentary systemgroup included the fan delta sedimentary system, which are divided into fivesub-facies composed of near-source fan delta zone, mid-source fan delta zone,far-source fan delta zone, fan delta front and prodelta sedimentary system. The marinesedimentary system group is divided into coast with barrier sedimentary system andcarbonate platform sedimentary system and abyssal sedimentary system.In addition, based on the research of above-mentioned, the sedimentary basin andtectonic evolution of the studied area in Early to Middle Cambrian Epoch has alsodiscussed elementarily in this paper.