Fill Models of in the Oiangtang Composite Foreland Basin in Oinghai—Xizang Plateau,China

Key words: 
• fill model /
• composite foreland basin /
• Qiangtang /
• Middle Jurassic to Earlv Cretaceous /
• Qinghai-Xizang Plateau China

Abstract: Middle Jurassi。-Earlv Cretaceous Qiangtang foreland basin is located between Jinshajiang suture belt and Bangong-Nu-jing suture belt.It is an important petroliferous basin in the north of Qinghai-Xizang Plateau.This had been basin elongates in E-Wdirection with 800 km long and 200 km to 300 km wide.Both of the northern and southern margins of the basin are largemsuture belt and linked to thrusting of large suture helt,so the Qiangtang foreland basin belongs to composite foreland basin according to Jordan's classification of the foreland hasin(1988).~ The foreland basin had been filled with 3 000-6 000 m thick marine sediments of Middle Jurassic to Earlv Creta-ceous.The spacial changes of the stratigraphy indicate that the basin texture looks like a symmetric boody and it can he subdivided into three tectonic paleogeomorphic units from north to south e.g. northern hasin center uplift and southern basin.The sedimentary thickness in the north varies from 3 000 to 6 000 m, and from zero to 1 000 m in center uplift and from 3 000 to 5 000 m in the south. The thickness changes of filled show that the occurrence of the Mesozoic filling stratig-raphy is in the north and the south of the basin thicker than in the center uplift of the basin.There are two centers of suhsi-dence of the basin the front of suture belt. Both of them are located in the front of the suture belt of the south and north foredeep belt.The basin is one of common foreland basins between suture belt and belongs to typical symmetric foreland basin.This kind of basin geometry is related to basement uplifts and thrusts in the cratonic edge of the mountain helt wihich allows huge svnsedimentarv molasse sediments to he preserved. Middle Jurassic to Early Cretaceous tectonic sequence is bounded by unconformitv and composed of the Quemocuo for-mation Buqu formation Xiali formation Suowa formation and Xueshan formation which is more than 3 000 m in depth.A tectonic sequence is a body of genetically-related strata isolated by unconformitvy and it had been deposited in a basin-formed stage responding to a thrusting episode.The provenance and constituent of the clasts in sandstone and conglomerate indicate that there are three provenances in the foreland hasin including Jinshajiang suture helt Bangong-Nujiang suture belt and the center uplift of the later was formed in the late Triassic.There are three marginal fades belts in the basin rec- ognized two of them distribute in the front of suture helt one of them distributes in the both side of center uplift.The sedi-mentarv fades can he symmetrically subdivided into four fades belt from center uplift to north or to south namely center up-lift foreland slope fore-deep and the foothill of thrust belt.The statistic measured data of cross-bedding,rippled mark and flute mold indicate that there are two palaeocurrents systems and one drainage distribution pattern in the hasin the cen-to uplift was drainage division. The uplifting amplitudes of the center uplift can provide information on the thrusting strength of both suture belt.The highland of the center uplift characterised by fan,fan delta conglomerate mark that Jingshajiang tectonic belt or Bangong-Nujiang tectonic belt were thrustcd scvcrclv to produce lots of tectonic load which led to rise the center uplift severely whereas the topography of underwater uplift in the center uplift characterised by carbonate hank and ramp mark that the Jingshajiang tectonic belt or Bangong-Nujiang tectonic belt was in the course of static times.So the evolution of the basin can he divide in to five stages by the emplacement of large thrust sheets on both side of the basin by Bangong-Nujiang suture belt Jingshajiang suture belt.