Regulation Of Sedimentary Distribution And Sedimentary Filling Process During Middle And Lower Jurassic In Western And Northern Sichuan, China

Western and Northern Sichuan are regions where contain the most abundant gas resourcesin Sichuan Basin. Beside Xujiahe Formation in Late Triassic, Jurassic is one of the mostimportant clastic layers for gas production with wide distribution, great thickness and shallowburied depth, thus it also be a great target for exploration and development of natural and shalegas. There are obvious differences in tectonic evolution between Western and Northern Sichuanin the Early and Middle Jurassic, which lead to more differences in the regulation ofsedimentary accumulation and distribution. With the development of exploration and research,it is necessary to solve the basic geological problems like evolution of the foreland basin, theregulation of sedimentary accumulation and distribution, sedimentary filling process within thesequence stratigraphic framework deeply, which is the guidance for further il and gasexploration. This paper use the theories and technical methods such as sedimentology, sequencestratigraphy, structural geology, sedimentary geochemistry to analyse formation and evolutionof Middle and lower Jurassic under the background of differential uplift in periphery orogeniczone by the idea guidance of “sedimention not only controlled by structure, but makeresponse to it”,all of which will show regulation of sedimentary distribution and sedimentaryfilling process with the development of basin evolution. Main contents and results are asfollows:(1) Based on the measuration of field section and description of drilling cores, combinedwith stratigraphic division and contrast, we have identificated the sequence boundaries on thebasics. Sequence stratigraphy division plan of Middle and lower Jurassic in Western andNorthern Sichuan is setted, it can be divided into2structural sequences and5third-ordersequences, and the sequence stratigraphic frame has been established.(2) Based on the characteristics analysis of the source area in the periphery of study area,using petrological characteristics analysis and sedimentary geochemical analysis and othermeans, in Western and Northern Sichuan, we have carried out a detailed analysis of thespace-time conversion of the provenance in Ziliujing Formation (Baitianba Formation) in earlyJurassic and in Qianfoya Formation (Xintiangou Formation) and Shaximiao Formation inMiddle Jurassic.a: During Ziliujing Formation (Baitianba Formation) deposition period, the provenancegiven of Western and Northern region mainly affected in both directions, from Longmenshan inwestern of Sichuan basin and from Micangshan-Dabashan in northern of the basin. The detrital materials provided by the two source areas are collision suture and fold-overthrust beltsediments, which belong to a recycled orogen source area. The difference is that the former oneis mainly quartz recycle source area,, and the latter one is a transitional and cuttings recyclesource area.b: During the sedimentary period of Qianfoya Formation (Xintiangou Formation), thewestern Sichuan region was predominantly affected by two directions of provenance. Xindu,Deyang, Zhongjiang regions were mainly influenced from the middle section provenance ofwestern Longmenshan. They have characteristics of high quartz, extremely low feldspar andlow rock debris. This indicates the material of collision suture and fold-thrust belt, belonging tothe recycle-orogenic provenance zone. The northern section, such as Jiangyou, Jiange,Guangyuan regions, was chiefly affected by provenance zone from the northern section ofLongmenshan, with high quartz, increasing feldspar, and low rock debris, and indicates thematerial of collision suture and fold-mudstone belt, belonging to the tectonic setting oflandmass. The northern Sichuan region was predominantly affected by two directions ofprovenance. Wangcang, Nanjiang regions were mainly influenced from Micangshan, with lowquartz, high feldspar, high rock debris, show the material of collision suture and the substanceranging from fold-thrust belts to mixed orogenic belt, belonging to the provenance zone ofcutting island arc to landmass. And northern Sichuan region, such as Tiexi, Wangyuan,Tongjiang, XuanHan, Dazhou regions, was principally affected by provenance zone fromDabashan. They have higher content of quartz and lower content of feldspar than Wangcangand Nanjiang regions. This illustrates the material of collision suture and fold-thrust belt,belonging to the recycle-orogenic provenance zone.c: During the sedimentary period of Shaximiao Formation, the western Sichuan region,especially the Longmenshan front area, was predominantly affected by the middle sectionprovenance of Longmenshan. It has rich quartz and lacking feldspar, shows the tectonic settingof recycle-orogenic provenance zone, belonging to the material of collision suture andfold-thrust belt. In the north of western Sichuan region, like Jiangyou, Jiange regions, waschiefly influenced from northern section provenance of Longmenshan. They havecharacteristics of high sedimentary rock debris, low metamorphic rock debris and extremelylow volcanic debris. This debris shows the tectonic setting of landmass, belonging to thematerial of collision suture and fold-thrust belt. Zhongjiang and eastern regions wereprincipally influenced from the provenance of Micangshan-Dabashan in northern Sichuanregion, with rich feldspar and poor quartz; indicate the tectonic setting of provenance zone ofcutting island arc, belonging to the material of mixed orogenic belt. Micangshan front area innorthern Sichuan, like western region of Nanjiang, was mainly affected by the provenance ofMicangshan, with high feldspar, low quartz and low rock debris. This indicates the material ofcollision suture and the substance ranging from fold-thrust belts to mixed orogenic belt,belonging to the provenance zone of cutting island arc. Dabashan front area, such as Tongjiang,Wangyuan, Dazhou regions, was chiefly affected by the provenance of Dabashan, with highquartz, high rock debris and low feldspar, indicates the material of collision suture and thesubstance ranging from fold-thrust belts to mixed orogenic belt, belonging to the mixed provenance zone of cutting island arc, mixed zone and the recycle-orogenic belt.(3) According to outcrop and drilling cores, logs and other data analysis identified fivesedimentary system, including: alluvial fan depositional system, river depositional systems,lacustrine delta system, fan delta depositional systems, lake sedimentary system. on the basis ofthird-order sequences and tertiary sequence system tracts for the mapping unit, the preparationof western Sichuan, northern Sichuan region early and middle Jurassic epoch Tectonicsequence based lithofacies-paleogeography maps. Through sediment distribution and evolutionof different periods a depositional model is established to explain each stage of evolution: theearly Jurassic fan delta plain-fan delta front-the front fan delta-shallow lacustrine sedimentarystage; The Early Jurassic in lake sediments, the late Delta for small-scale-large-scale shallowlacustrine sedimentary stage; the early middle Jurassic alluvial fan-River-Delta-lake sedimentsstage; the early and late middle Jurassic, alluvial fan-river-a large-scale Delta-lacustrine depositphase.(4) Differential uplift in periphery orogenic zone is responded by time-space evolution ofsediment. In the early time of Early Jurassic, sediment of recycling orogenic belt are maindeveloped in Western and Northern Sichuan in the period of tectonic calm with planarsedimentary distribution and no obvious center of subsidence. Then in the early of MiddleJurassic, Dabashan was affected by downcoast expandation of Hannan uplift, Micangshanfolded and rised underwater, which led center of subsidence in Western Sichuan of late Triassicmigrated to the front of Dabashan, the thrust nappe phase started. The sediment in NorthernSichuan has characters of orogenesis uplift, while sediment in Western Sichuan was similarwith forward. In the middle and late of Middle Jurassic, with stronger development of thrustnappe movement in Dabashan, the center of subsidence was gradually move to the front ofDabashan, and sediment in Northern Sichuan even some region in the middle of WesternSichuan had characters of orogenesis uplift, all of which showed that the whole Dabashan hadstarted to rise, while sediment in Western Sichuan was also similar with forward.(5) Based on Differential analysis of sedimentary filling in the sequence stratigraphicframework, this paper appromately resumes evolution process of foreland basin under thebackground of compound orogeny with searches features of regional structure movement. Inthe early time of Early Jurassic, fan delta sedimentary system was developed in Western andNorthern Sichuan which affected by distal effect of tectonic movement in Qinling. Then in themiddle and late of Early Jurassic,, Lacustrine occupied larger part of searching area in tectoniccalm period. And in the early of Middle Jurassic, delta sedimentary system expanded into thebasin, because Micangshan uplift and Dabashan went forward made structure in searching areaactive. In the middle of Middle Jurassic, Dabashan was continued to go downcoast and deltasedimentary system expanded fuether. Then in the late of Middle Jurassic, with continueddowncoast going of Dabashan, structure movement was in strongest phase in foreland basin,and the sedimentary thickness was largest with piedmont depression dropped deeper.