Lithofacies Paleogeography Evolution In Qilian Orogenic Belt During Cambrian

This thesis is based on study of sedimentary rock and igneous rock of Qilian orogenicbelt Cambrian layer-Xiangshan Group, Dahuangshan Formation, Heicigou Formation,Xiangmaoshan Formation, Maojiagou Group, Shengou Group, Liudaogou Formation.According to Sedimentology theory. The thesis subtly analysis Cambrian section and sediment dynamics in this zone, drawing a conclusion that subregion sedimentary model and depositional environment. The eassy anew divided into stratum regime according to geochemistry and geotectology new progress and draw the the early Cambrian and middle-upper Cambrian tithofacies and paleogeography map.Early Cambrian:In west of ordos paltaocontinent, Carbonate formed affected by ordos paltaocontinent. Understratum of Dahuangshan formation generate in Yongchang-Shandan where is located Alashan landmass edge. Rift valley and volcanic strata are probably generating in Baiyin. In Changma Miaoyugou, Wudaogou, West zone of Qilian mountain,carbonate platforms and shelf clastic rocks that showing neritic facies generate. Addationally,in Guodikeng mountain, Middle-Cambrian stratum overlap Sinian stratum directly. So this zone should be paltaocontinent or continental shelf mean that sea-land alternatly emerge.Sedimentary record is absence in middle Qilian mountains, So mid-Qilian mountains may be paltaocontinent. Limestone that showing sublittoral platform is generating in south of Qilian mountains.Middle-upper Cambrian:Sea deepen in east Qilian mountains, the first subgroup, second subgroup and third subgroup of Xiangshan Group are generated. Structure characteristic of sedimentary rock indicate this zone should be foreslope facies. Blong to alluvial fan. Littoral neritic facies become foreslope facies in Yongchang-Shandan(named Zoulang in chinese),showing that sea deepen. Rift valley go on, concomitantly volcanic strata and sedimentary strata generating in Baiyin, Tianzhu, Qilian, Changma and so on. Specially, siliceous mudstone representative of abyssal facies exist. Seawater had spreaded Changma showing sea area become larger, seawater deepen. Volcanic strata exist and covered directly Jixian period’sdolomite of in Datong and Huzhu, middle Qilian mountains zone. Pillow lava is in volcanic strata. Lajishan valley begains to take place. Upper Cambrian, volcanic activity happened tempestuously in Lajishan located south of Qilian mountains, creating many volcanic bombs and pahoehoe that representative volcanic eruption facies. The constituent of conglomerate in sedimentary strata is volcanic rock.