MIDDLE TRIASSIC GRAVITY-FLOW DEPOSITS IN WESTERN GUANGXI

In the Indosinian geosyncline of western Guangxi, the Middle Triassic is extensively distributed in Bose and Tianlin, Guangxi province. Its lower part ——Anisian Stage——is called the Baifeng Formation, while its upperpart—— Ladinian Stage——is named the Hekou Formation. The rocks all belong to terrigenous clastic rocks, more than 9,000m thick. According to the study in recent years, this suite of deposits resulted from gravity flows. On the basis of the sediement support mechanism, three types of gravity flows may be distinguished: debris flow, grain flow and turbidity current. Debris-flow sediments are only found in the vicinity of the Sino- Vietnamese border south of Napo county, Guangxi. The sediments are litholog- ically muddy conglomerates with low maturity. There are altogether 4 beds, having a total thickness of 1, 432m and a controlled length of 10--50km. The rocks are composed of blocks, gravels and mud matrix of differing ages. The gravels and blocks vary greatly in size, shape and composition, which indicates that they were formed by rapid transport and deposition of the materials close to the provenance. Grain-flow sediments consist of conglomerates and occur as lenses. Their contact with turbidites is abrupt. The gravel composition includes more than 20 kinds of rocks such as radiolarian siliceous rocks, marbles, sandstones, granites, metamorphic rocks and volcanic rocks. Turbidites are widespread in western Guangxi. They consist of fine-grained sandstones, siltstones and mudstones and exhibit the typical Bouma sequence. Divisions A and B are represented by fine-grained sandstones and exhibit normal grading; on their sods there often occur flute casts, groove casts, saltation casts and load casts. Division C is marked by siltstones, in which convolute bedding, climbing bedding, ripple bedding and sluice structure are developed. The turbidites have an average grain size of 3.2--3.7φ. The cumu- lative probability curve is made up of the suspention transport part and the saitation part, with the environmental discriminant value y＜9. 8433. According to the changes of the Bouma sequence in various areas and the measurements of the direction of the flute and groove casts, it is inferred that the gravity flows at that time ran from south to north and that there ex- isted a steeply northward dipping continental slope. So the North Vietnamese oldland might be the provenance of the gravity flows.