The Distribution Characteristic And Difference Of Sedimentary System Of Jurassic In The Western Sichuan Foreland Basin

Guided by sedimentology, sequence stratigraphy, geochemistry, seismic stratigraphy andso on, integrated researhed the outcrops, cores,logging data,seismic data and geochemistrydata during Jurassic in the western Sichuan foreland Basin，combined with thin sandstonesection analysis，sedimentary structure mark analysis，sandstone grain size analysis，heavymineral analysis, cathodoluminescenc analysis, trace elements analysis and so on, thecharacteristics of stratigraphy, provenance, sedimentary system, sequence stratigraphy hadbeen researched, the rugulation and difference of every sedimentary system had been detailedresearched. The main achievements as follow were obtained:1.According to cathodoluminescenc analysis, heavy mineral analysis, trace elementsanalysis, paleocurrent analysis and so on, founding that during the Shaximiao Stage of MiddleJurassic there were two places provided sediments, which were metamorphic rocks from thenorth part of Longmenshan and acidic rocks from the middle part of Longmenshan. Duringthe Suining and Penglaizhen Stage of Later Jurassic，the sediment mainly came form themiddle part of Longmenshan, rarely came from the north part of Longmenshan.2.According to the outcrops and cores, logging data and seismic figures, combined withrock association, sedimentary fabric, section structure and evolutionary series,4sedimentarysystem were recognized, alluvial fan sedimentary system, fluvial sedimentary system,deltasedimentary system and lake sedimentary system.The distribution characteristics ofmeandering river delta and braided river delta has been revealed,and the sedimentary modelbuilt. meandering river delta developed during the Lower Shaximiao Stage to menber2ofUpper Shaximiao Stage, other time, braided river delta developed.3.According to the outcrops and cores,combined with logging data and seismic figures,5typical sequence surfaces were found,which were surface of unconformity, surface of erosion,surface of onlap,transition surface of rock character and facies and flooding transgressivesurface. And then8third-order sequence and18forth-order sequence were recognized,thecharacters of each third-order sequence detailed researched, and third-order sequenceframework had been built.4.On the basis of sequence texture division, the short-term base-level cycles’characteristics of different facies(sedimentary) studied, and sumed up the distribution characteristics of short-term base-level cycles.There were5short-term base-level cyclestexture, which were type A, type B1, type B2, type B3and type C. Type A mainly developed inthe Fluvial-Delta plain sedimentary system, Type A to type C could be found in the Delatforntier sedimentary system, the developed sequence texture type determined by the distancefar from the coastline, the far from the coastline,the more occurrence of type B3or type C.Prodelta-shallow lake sedimentary system mainly developed type B3and type C, especiallytype C developed frequently.5. Sequence of sedimentary filling dynamics has been discussed. During the progradationstage, only ascending hemicycles sediments or ascending hemicycles sediments more thandescending hemicycles sediments was the mainly texture of base-level cycles within the Deltaplain, only descending hemicycles sediments or ascending hemicycles sediments less thandescending hemicycles sediments was the mainly texture of base-level cycles within the Deltafrontier and prodelta. During the aggradation stage,bese-level cycly rarely changed, texture ofbase-level cycles fully developed, which types mainly Type B or Type C. During theretrogradation, bese-level cycle sustained tiny rised, the texture of base-level cyclescharacteristics showed, only ascending hemicycles sediments or ascending hemicyclessediments more than descending hemicycles sediments.6.On the baisi of studying the characteristics of depositional system and sedimentarysequence, the regional sequence-based lithofacies-paleogeographic maps of Jurassic in thewestern Sichuan Foreland basin are compiled by choosing different sequence cycles as theisochronous mapping unit.,This paper demonstrats in details the characteristics of lithofacies aleogeography and theirevolution during different fourth sequence cycles development from MSQ8to MSC18duringthe Upper Shaximiao Stage of Middle Jurassic to Penglaizheng Stage of Upper Jurassic.The difference and distribution characters of different sediment system during the differentstage was discussed in detail.