DISCOVERY OF WATER-TRANSGRESS旧N CAUCE FILLING SAND一BODIES IN ANCIENT SEDIMENTS —AN APPROACH TO THE GENESIS OF CERTAIN UPPER TRIASSIC SAND一BODIES IN THE MIDDLE-WESTERN PART OF THE SICHUAN BASIN AND DISCUSSION ON WATER-TRANSGRESS旧N DELTA

Certaiin Upper Triassic sand-bodies in the middl一western Part of the Sichuan Basin possess the pasitive Grain-si rhythm with urdcrlying strata of terrestr-ial facics and ovcrlying strata of lacustrine facies. Their bedding is underdevel-oped and scouring unclear.They contain glauconite and have a grain-size prob-ability curve of three-section type, which reflects bidirectional flow ression cauce filling and conta-ins bisaltational components, quite similar to the peculiarities of the water-transgression cauce filling sand-bodies in recent stream-mouth regions. Theref-ore,they can be classified as this kind of sedimentation.They are sediments fi-lling cauce during the water-transgrained sediments formed at the intervals above the stream-mouth while the water surface of the basin was rising gradually, and the stream-mouth was retrogressing upstream.They have certain similarities and some obvious differences as compared with the river course sand-bodies and delta front stream-mouth bar sand-bodies. The mechanism is quite different, and so it is nccessary to differentiate them from each other.