GRAVITY-FLOW DEPOSITS IN THE UPPER DEVONIAN TONGYUSI FORMATION,SHANYANG,SHANXI.

The Liuling Group situated between the Shangdan fault and Shanyang-Fengxian fault in the Qinling orogenic belt represents a succession of EW-trending thick clastic rocks and claystones interbedded with carbonate rocks. The gravity-flow deposits dominated by the grain-flow deposits and turbidity-current depsoits are well-developed in the middle and upper portions of the Upper Devonian Tongyusi Formation. The grain-flow deposits are composed of massive conglomerates, gravelbearing coarse-grained sandstones and massive coarse-grained sandstones. The conglomerates and gravel-bearing coarse-grained sandstones tend to display normal grading, inverse grading and inverse-to-normal grading, and the pebbles from them to show imbricate arrangement. The turbiditycurrent deposits comprise a sequence of rhythmically bedded moderate-to thin-bedded sandstones, siltstones and slates. The sandstones and siltstones are characterized by sharp basal contacts and gradual top contacts, typical flute casts, graded bedding, parallel bedding and ripple cross-bedding. Individual beds are steady and may be traced for a long distance. The formation of the gravityflow deposits can be explained by submarine fan models. Different types of the gravity-flow deposits may reflect the features of various parts of a submarine fan. The palaeocurrent data show that the regional palaeoslopes point to the north rather than to the south. Like the Devonian sediments south of the Shanyang-Fengxian fault, the typical continental margin sedimentary formations recognized in the Liuling Group are also interpreted as the northern ancient passive continental margin sediments of the Yangzi plate and then together with the Yangzi plate, subducted towards the North China plate during the Hercynian-Indosinian, and lastly at the end of the Indosinian, intense intercontinental collision orogenesis occurred. Thus the Liuling Group constitutes an important part of the Qinling continental orogenic belt.