Abstract:

Turbidity current deposits developed widely in Guizhou and Guangxi du-ring the Middle Triassic.Terrigenous materials came from island are in thesoutheast part of Guangxi,this is based on that:1.The turbidites in the southeast occured anterior to that in the northwest;2.The proximal facies in the southeast changes into distal facies in the northwest;3.Volcanic rocks decreasein thickness and even pinch out from southeast to northwest;4.The flow direc-tion of the flute castes point to NNE but not point to EW in the proximal facies;5.The scale of the flute castes changes gradually small from southeast to northwest.The quartz contentes are from 43 to 59%,SiO₂ ranges from 70.46 to 72.04%,ratio of K₂O to Na₂O is less than 1 in the turbidite rocks,which isin accord with graywack of middle quartz content classified by Crook(1974),but it is in the range of high value.This shows that their formation condition maybe distent with oceanic crust but close to continental crust.In addition,the tur-bidity current deposits,ranging from 3000 to 5000 m in thickness throughout the study area,distributed extensively about 70000km² in Guizhou,Guangxi and Yunnan,which are bounded by a marginal reef barrier in the north and avolcanic island arc in the south.It is shown that such sediments were formed in back-arc basin but not fore-arc trench.It is because the turbidites belong to back-arc basin that the sediments con-tain intercalated beds of volcanic rocks.More over,the closer to the islandarc,the greater the thickness.Since the continental crust of the back-arc basin changed thinner under the micro-spreading processes,mantle materials thus spil-led over to develop into basic,untrabasic intrusive bodies and slump conglo-merats that are resulted by faults.