The Features And Evolution Of Sedimentary Facies From The Late Jurassci To The Neogene In Mid-Yangtze Region

Based on measured outcrop sections, combined with the comprehensive analysis of wells, seismic profiles, geochemical behavior and the paleocurrent direction, the types and the features of sedimentary facies, the features of the depositional system in different epoch are identified, and the cycles of sedimentation recognized and the depositional models constructed in the Middle Yangtze region.In the Middle Yangtze region, five sedimentary facies, i.e, alluvial fan, braid river, meandering river, delta and lakes are classified from Late Jurassic to Neogene, including the features of their subfacies and microfacies.Middle Yangtze region is tectonically the last stage of foreland basin in the Late Jurassic and filled with fluvial and delta deposit. In the Early Cretaceous, tectonic stress changed and formed rift basin in the parts of region. From margin to center in the basin, fan deposit and braid river deposit occur by the order. From the Late Cretaceous to the Oligocene, multiphase rift basin developed because of the significant changes of the structural setting: NS-trending compressional force converted by EW-trending tension. Further, multiphase fault-depression developed in the basin reflect the changes of the time and the trending of faults. In the early stage of fault-depression, heavy-bedded fan deposit developed at the edge of fault. By the sediment supplying, braided river, fan delta and subsea apron are formed at the actic region of basin. Meandering river and delta developed at the shelving of basin and lake developed at the center of basin. In the Neogene, depression basin is filled with meandering river and delta deposit until finished.The basin can be divided into six cycles of sedimentation: the Late Jurassic, the Early Cretaceous, the Late Cretaceous, the Palaeocene Epoch-the Early Eocene Epoch, the Middle Eocene Epoch-the Oligocene Epoch, the Neogene, and included eight secondary sedimentary cycles. And cycles of sedimentation are compared with the structural cycles of the basin. On the basis of the different depositional system distribution in the different period, five depositional model are constructed: meandering river depositional model with the combination of delta in the last stage of foreland basin, fan depositional model with the combination of braid river in the restricted in the rifted lake basin, the alluvial fan-fluvial-delta-lacustrine depositional model of the rifted lake basin, the subsea apron-fan delta-basin floor fan-salt lake depositional model in the rifted lake basin, and the meandering river-delta- lacustrine depositional model in depressed lake basin.