DEPOSITIONAL FEATURES AND PALAEOHYDRAULIC CONDITIONS OF THE FLUVIAL SEDIMENTS OF TOUTUNHE FORMATION,MIDDLE JURASSIC, IN SOUTHERN CHANGJI PREFECTURE,XINJIANG

Sedimentary Environments of the Toutunhe Formation in Southern Changji Prefecture,Xinjiang,have been systematically investigated by field observetion and using the techniques of thin-section examination,X-ray diffraction,scanning electronic-microscopy,cathodoluminescone micros-copy,stable isotope analysis,grain-size analysis,methematical statistics, etc.From southern to northern part of the studied area,Toutunhe Formation is represented by a succession of fluvial,deltaic and lacustrine deposits.At the Sulphur Valley section,the fluvial facies-association comprises four cycles each of which consists of a lower coarse member and an upper fine member. 11 lithofacies havebeen distinguished from these cyclic sediments: (1) Massive conglomerate facies (Gm),for which thicker beds situating at lower part of the cycle and streching laterally in a distance more than 10m are referred to lag deposits and thinner conglomerates laying in the scour hole on top of large-sea le tabular cross-bed sandstones are assumed to be chute fill.(2) Tabular cross-bed conglomerate (Gp) is produced by lateral migration of point-bar. (3) Large-scale cross-bed sandstone facies (St)- those with multiple cetts might be produced by the migration of sinuous-ridge dunes at the upper part of point bar and those with solitary set by filling-up of scour hole. (4) Large-scale tabular cross-bed sandstone facies (Sp) are produced by sand bar or scrollbar. (5) -cross-bedding sandstone facies (ε) with four-grade boundaries,It is evidence of lateral accretion of point bar. (6) Smallscalc cross-bed fine-sandstone facies (S1). (7) Ripple-bedded siltstonc facies (Sr) occur at the top of point-bar,on natural lavee and in oxbow lakes. (8) C-hannel plug silty mudstone facies (Cp). (9) Chute plug facies (Ccp) and (11) Pa-leosol facies (Psc) of mottled mudstone is intercalated with low-quality coal seams.The vertical facies-relationship and facies-model have been worked out by us-eing Selley's (1970) technique of transform matrix.The model is similar to that of the Old Red Sands-tone (Allen,1963) but even more complex.Inside the sand-bodies of Toutunhe Formation no simple upwards-fining sequence is found, instead, up- wards-coarsening often occurring on the basal scour surface of the conglomerate lenses. The presence of tabular cross bedds at upper part of the coarse m-embers and the occurrence of e-cross bedds suggest typical meandering river depo-its. Besides, the rapid facies-change, frequent scour structures and abundant conglomerate and mudstone lenses might suggest that the corresponing rivers were characterized by shallow water, high flow-velocity, unstable currents and ample detritus supplies. The Paleocurrent directions from measurements of cross-bedds, oblate gravels and loggs show a preferred azimuth between 260° and 61° which coincides well with the regional transport-direction North to Tien- Shan Mountain. Parameters of channelgeomorphology and hydraulics for cycle 1-3 have been worked out by employing the equations of Schumm (1960, 1963, 1968), Cotter (197 1), Eicher (1969) and Hack (1957). They suggest that during the period of cycle 1 to that of cycle 3, the corresponding rivers were getting deeper and broader and their channel sinuosity, length and curvature radius getting greater, but the channel slope getting smaller. These implicate a tendancy of increasing river scale, channel stability, annual mean discharge and suapended-load and decreasing flow velocity and bed-load. It is obvious that the rivers of cycle 3 were approching more to the lower reach than that of cycle 1 and 2, owing to the influence of climates and tectonic movements. But the innercycle-3 rivers evoluted in an opposite way, i.e., from former to later stage, the rivers were decreased in scale sinnosity and suspended-load but increased in slope and bed-load, which might be explained by an uplifting source area and a drier climate. It is presumed that the ancient Tou-tunhe river is similar to the modern Dane River in Great Britain. Both are of m-ixed-load channel types.