ON THE FEATURES OF THE INDOSINIAN MOVEMENT IN THE LOWER-MIDDLE YANGTZE VALLEY
Ju Kuixing ( IGMR, Nanjing )
The Early Mesozoic strta in the Lower-Middle Yangtze valley are widespread and shown a complete cycle of sedimentation from marine to continental sedments. The lower Triassic beds are represented by a transgression phase composed of calcareous shale and limestone. The Middle Triassic beds are represented by a regressional and transitional series composed of dolomite, anhydrite, marls and siltstone from marine to continental sediments. The Middle-Upper Triassic beds are continental sediments composed of red siltstone. The Upper Triassic and the Lower-Middle Jurassic beds are continental clastic rocks with coal series. Within the Triassic beds their contact are conformable except in the west part (Puqi, Jiayu in Hubei) where the Middle-Upper Triassic red siltstone is covered disconformably by the Upper-Triassic coal series. However there is unconformity between the Upper Triassic and the Lower Jurassic in the east part of this region, but it is not clear in the west. The distribution of the Lower Triassic rocks is closly related with the Palaeozoic rocks and they always situate in the core of syclines of the linear and tight folds composed of the Palaeozoic and Lower Triassic rocks. The Middle-Upper Triassic rocks are distributed closely with Lower-Widdle Jurassic rocks and form the broad and the gentle synclinal basins, and always situated on the northern side of the above-mentioned folds. There continuous sedimentary rocks are separa ted or disjuncted in distribution although they occupy in the structure of the same tectonic system. The basic features of the Indosinian movement in this region are elevating and subsiding, upwarping and downwarping. The west part of this region once elevated in the early Late Triassic while the east part subsided continuously, and the east part of this region elevated in the period between the Late Triassic and the Early Jurassic while the west part subsided continuously on the whole. So, the tectonic movement in this region migrated from west to east, and the sedimentation in the downwarp area migrated from soth to north. The Lower-Middle Yangtze valley located in the zone between the two massif Jiangnan landmasses on the south and Huaiyang shield on the north. The Indosinian movement in this area was caused by the compres-sive stress of northward migration of the south massif, which was very slow and persistently and so was the upwarping and the downwarping in their development.