Study Of Stratigraphic Classification And The Sedimentary System Of Late Jurassic In Northwest Area Of Sichuan Province

The studied area is located in Ya’an-Guangyuan area of the northwest of YangtzePlatform, which belongs to the contiguous zone of the nappe tectonic belt of LongmenMountains and Sichuan Basin. And the section outcrops of Late Jurassic have beenrevealed well. In this paper, based on the measured sections, the sedimentary phasemodel graph of the Late Jurassic in northwest area of Sichuan Province has beenestablished with a deeper analysis of the sedimentary system in the study area throughthe methods of lithostratigraphic, biostratigraphic and chronostratigraphicclassification.In this paper, seven representative measured sections and observed sections inChengdu and Guangyuan stratum region were selected. According tolithostratigraphic classification methods and principles and combined with previousstudies, this paper proposes a reasonable partitioning scheme of lithostratigraphicwhich divides the Late Jurassic stratum of study area into three formations that theycould be distinguished from bottom to top as Suining Formation（J3s）,PenglaiFormation（J3p）and Lianhuakou Formation（J3l）. Among these formations, PenglaiFormation（J3p）and Lianhuakou Formation（J3l）are simultaneous sedimentation withdifferent faces. And the lithologic characteristics of each lithostratigraphic units,stratigraphic thickness, contact relationship and lateral variation are also described indetails in this paper.In this area, according to distribution and stratigraphic assemblages of fossils inLate Jurassic, five biological assemblage zones were established. They areAclistochara urniformis-Aclistochara lufengensis assemblage zone, Porocharamaedleri-Aclistochara. Yuannanensis assemblage zone, Darwinula-Djungarica-Eolim-nocythere assemblage zone, Eosetheriopsis dianzhongensis assemblage zone andMartinsonella assemblage zone. And the chronostratigraphic attribution of eachLithostratigraphic units are defined by area comparison of time limits and horizons of eachbiological assemblage zone.Five biostratigraphic Assemblage-Zones of trilobites have been establishedaccording to the fossil association and stratigraphic distribution, whichareEoredlichia—Parabadiella Assemblage-Zone， Eoredlichia—WutingaspisAssemblage-Zone， Palaeolenus—Paokannia nanjiangensis Assemblage-Zone，Hoffetella—Redlichia Assemblage-Zone and Chittidilla—KunmingaspisAssemblage-Zone. The chronostratigraphic classification of each lithostratigraphic unit was discussed and determined based on the biostratigraphic Assembelage Zonesand regional correlation.According to a comprehensive analysis of lithological characteristics,sedimentary structures and fossils and other data of each section of the study area, thesedimentary system of each measured has been analyzed. Thus, the stratum of studyarea of northwest Sichuan Province in Late Jurassic has been divided into onesedimentary system group, which is a continental sedimentary system group. Thecontinental sedimentary system group falls into three types as alluvial fan facies,fluvial facies and lacustrine facies. The alluvial fan facies falls into root-fan subfacies,middle-fan subfacies and marginal-fan subfacies. The fluvial facies falls intomeandering river subfacies and braided river subfacies. The lacustrine facies falls intolake delta subfacies, shore-lake suhfacies, shallow lake subfacies and semi-deep lakesubfacies. Based on the analysis of stratigraphic classification and comparison andsedimentary system in study area, the sedimentary facies model diagram in upperJurassic of the northwest Sichuan Province has been summarized in this paper.