Sedimentary Facies And Evolution Of Lower Triassic Qingtianbao Formation In Beiya Area

The western margin of Yangtze plate had undertook a complete land—sea—land cycle in Triassic,the age of early Triassic is at the beginning of a large scale transgression after the Permian, resulting a regional set of thick clastic strata in this area.Li-jiang basin, wh-ich located in the most westrn of Yangtze plate,had Triassic strata bottom up consist of Qingtianbu Formation clastic, Beiya Formation and Zhongwo Formation platform carbonat-es,Songgui Formationcontinental-paralic facies molasses,successively, lower Triassic Qingtia-nbao Formation was consist of purple,chartreuse sand-conglomerate and mudstone in the1-ower and marl,nodular limestone in the upper, constitute a land—sea cycle. However,syste-matic sedimentary research showed that there were significant differences in sedimentary f-acies characteristics and evolution of lower Triassic between Lijiang basin and other regio-n of western margin of Yangtze plate.,.1.Lijiang basin had receiverd thick basalt eruption in Permian,forming up to5000m t-h ick basalt.The eruption of basalt had characteristic that early in marine and later in basin-mountain,paralice nvironment due to uplift.The late eruption was dominated by tuff and1-a st until to the beginning of early Triassic.2.After the cease of most basalt,the study region began to thermal subsidence,resulting large scale transgression.Qingtianbao Formation can be divided into flluvial fan face,braide d river face.delta face,tidal flat face and shallow sea face, successively,reflecting a complet e record of land—sea process.3.Study of sedimentary evolution have declosed that the basin had been dominated ear-ly by alluvial fan and later by delta as well as tidal flat at Qingtianbao age, respectively, the flluvial fan deposit,which changed into braided river in its late stage,was widely distri-buted and accounted for more than half the thickness of Qingtianbao stratum, the delta d-eposit showed strong control by tidal, the tidal flat deposit was thin and transited into s-hallow sea face quickly, reflecting a high rate of transgression.4.In early Triassic period, the provenance of Lijiang basin mainly came from Diancang shan oldland and adjacent basaltic highlands, which were to the southwestern of the basin, the ingredients of the provenance were composed main by debris,feldspar and single quart z,among those clastic,the debris included granitic,rhyolitic,basaltic and metasandstone. The Kangdian oldland,to the southeast,however, had offered few clastic material for Lijiang b-asin only in the early stage,its its detrial supply mainly injected into the Yanyuan basin and form a broad subaquatic flluvial fan.5.Large mount of clastic Injection from the Diancang oldland formed a broad flluvial fan advanced from southwest to northeast,whose proximal fan lied in Xiaoganhai vicinity, midfan cover Junmachang-Beiya-Pipashan region,outter fan reached Jidiping and Dadili.in the more northeastern of the basin.Hereafter,with the quick intrusion of sea water, Jidiping—Dadili area had submerged into tidal flat pioneerly,the time Junmachang—Xiaoganhai ar- ea submerged into intertidal zone Jidiping—Dadili area had already fall into subtidal mud—carbonate mixed flat.The flluvial fan—braided river—tidal dominate delta—tidal flat—shallow sea evolution of Qingtianbao Formation in Beiya area was differ from subaquatic flluvial fan—estuary—tidal flat evolution in Yanyuan area and river—estuary evolution in upper Yangtze plate This reflected that Lijiang basin had undertook a rapid subsidence in early Triassic, further conclusion can be obtained that westren margin of Yangtze block began to undertake lar-ge scale extension after the gradually cease of Permian basalt,this extension effects may i-ncreased from east to west.