RESEARCH INTO THE PERMIAN AND TRIASSIC VOLCANICLASTIC TURBIDITE OF NANPAN RIVER SEG-A UNIQUE TURBIDITE MODE WITHOUT SUBMARINE FAN
Hou Fanghao Huang Jixiang ( South-western Petrpleum Institute )
Nanpan River seg is located in the junction of Yunnan, Guizhou and Guangxi Provinces and its area is about 100, 000 km2. Besides isolated carbonate platforms which are different in size and roughly distribute in the WE direction, a set of volcaniclastic turbidite and chemigenic or biochemigenic silicilith associated with it were extensively deposited in this area from late Maokou Stage to Early Triassic epoch. Its maximum thickness is up to 1, 400. There is still a dispute about the lithologic character, naming, distributive regularity, eruptive mechanism and depositional mechanism of the volcaniclastic turbidite and silicilith. On the basis of the results of the field and laboratory work over four years, the author has pointed out that it is a set of andesitic volcaniclastic turbidite that erupted along the rift belt on sea floor and belongs to an ash flow type. According to the regional distributive regularity of the section types of different lithologic fabrics it can be divided into three types. ( 1 ) Volcanic lasfic avalanche-turbidite type ( 2 ) Dust cloud-suspension depositional type ( 3 ) The super type of volcaniclastic turbidity current and calciclastic turbidity fan. Nanpan River seg was located in Yunkai-Wuyi back-arc expending belt in late Paleozoic epoch. On the folede basement of early Paleozoic epoch, the activity of the seg became so strong that the crust thinned and back arc was expending. Thus paleogeomorphologic prototype which showed the form of alternation of trenches with platforms revealed itself. Under the great influence of Dongwu movement, the basement rift movmeent in NW, NE and NS directions happened in this area and this paleostructure was formed from the graben type of trenches alternating with the carbonate platforms that belong to horst type. Soon after eruption and intrusion of basic magma, in the trenche,there frequently happened intermediate ash flow tuffs eruption and after the middle Triassic the whole area began to subside. Then the seg mainly received calciclastic gravity flow sediments that came from adjacent carbonate platforms and terrigenous clastic turbidity sediments from the east part of Yunkai Mountain. In addition, there was a bit of the turbidity sediments resulting from acidic magma eruption during early Ladinic stage. The regional strong folding of the second episode of Indo-China movements made the sedimentary depositional history come to end. Two types of turbidite without submarine fan were summarized by Walker (1978). The first is the exogeosynclinal situation, the second is in the predeltaic areas on the craton. Nanpan River seg was located in back-arc expending belt from late Maokou stage to Early Triassic epoch. Actually, the graben type trench in this area is an exogeosyncline. The difference between them consists in material source, turbidity sediments in this seg from ash flow type tuffaceous materials erupted in the trench. So it is a unique turbidite mode without submarine fan.