PRELIMINARY RESEARCH ON TECTONIC BACKGROUND AND MATERIAL SOURCE AREA OF TURBIDITE DEPOSIT FORMATION OF MIDDLE-UPPER TRIASSIC IN YUNNAN, GUIZHOU AND GUANGXI PROVINCES

The marine turbidite deposit rock of Middle Triassic is widely distributed in Napanjiang River of the Yunnan-Guizhou-Guangxi boundary and the Youjiang River region of western Guangxi. It is abundant, dark-gray coloured, medium-or-thin-bed, cyclotheric and clastic graywacke, its thickness is 3,000-5,000m and its distribution area is about 80,000km². The distribution of marine and terrestrial interdi-gitational bearing-coal deposit in Upper Triassic is limited at the edge zone of the platform. There was turbidite deposit in the early Carnic stage.In the study of the turbidite within the area, the external platform area of turbidite basins and adjacent oldland are recognizied as main material source areas. So the turbidite in Middle Triassic of the Nanpanjiang River and Youjiang River is supposed as multi-material source and multi-centre deposits. This paper suggests that the tectonic background of turbidite genesis be related to the rise of the new Hunan-Guangxi-Guangdong structural system of Middle Triassic in eastern Guangxi which is the synorogenic deposit of this system.The source of turbidite deposit comes from growing structural system instead of adjacent old land. So the turbidite within the area is related to geosyncline. The Yunnan-Guizhou-Guangxi sea area in Early Triassic is a part of the sea basin of Southern China. It is a platform area from Kaiyuan-Qiubei, Yunnan to the northwest part of Zhenfeng-Anshun-Guiyang, Guizhou; the Nanpanjiang River-Youjiang River area in the southwest part is a structural unit of subsidence. The Early Triassic deposit in this area consists of greyblack, thin-plate marls with rich-contained Ammonoidea, mud shales with silicious, thin intercalated layers and volcanic debris rocks, its bed thickness is 100-200m, showing the deposit feature of hungry basin. Volcanic debris comes from volcanic island arcs in East and Southeast Guangxi. In Early Middle Triassic, after large scale eruption of volcanos and intrusion of magma, Southwest Hunan, East and Southeast Guangxi and West Guangdong areas rose and became extending toward north and connecting with the Jiangnan old land so as to form a new Hunan-Guangxi-Guangdong structural system. On the west side of this system,the Nanpanjiang River and Youjiang River area subsided relatively,and deposited turbidite with the thickness of several thousands meters. Main mineral elements of turbidite consist of quartz (56%), feldspar ( 3.6% ) , debris ( 14% ), chlorite and hydromica clay matrix ( 19.8% ). The chemical constituent of turbidite consists of SiO₂ ( 71.22% ) , Na₂O ( 1.48% ) , K₂O ( 1.05% ) and Fe₂O₃/FeO< 1 , in accordance with the chemical constituent of graywacke in the other parts of the -world. In the research of recongnizing source area of turbidite, according to plenty of developed trench model structure, it is measured that the direction of water current is 260°-320°, dominantly 300°, which shows the turbidite flow running from east to west.In Qiubei,Guangnan of Yunnan and Xilin,of Northwest Guangxi,i.e. the west of the turbidite basin, the proportion between mudstone and sandstone is 1 : 1, and it changes gradually into 1 : 2 eastward to Doaglan, Guangxi, which reflects that the location of the source area is in the east side and the farther it is from the material-area, the less sandstone deposits are. The chemical constituent of turbidite and the kinds of heavy minerals all indicate that turbidite deposits come from the matrix area of granite. The constitutional type of debris shows a strong movement of magma intrusion and a high relief in the source area. In the platform of external turbidite basin and the edge zone of Maguan Oldland, the Middle Triassic series consists of limestone and dolomite coexisting with turbidite deposits of the Nanpanjiang River and Youjiang River area. In the turbidite deposits of this area, there is neither any massive sands tone of middle-fan, thick bed pebble sandstone, nor conglomerate deposits of the innerfan. So it is not suitable to predict turbidite deposits of the Nanpanjiang River a