Sequence palaeogeography and coal accumulation of inland faulting basin: an example from late Triassic Yangtze platform of China

To unveil the formation mechanism of key sequence boundaries of inland faulting basin and coal accumulation characteristics of coal seams in isochrohal stratigraphic framework, sequence stratigraphy, palaeogeographic recovery and other methods were used to research the sequence stratigraphy and coal accumulation in the example of Baoding Basn in the west margin of Yangtze Platform, and the authors advanced a coal accumulation model of Faulting basin on the basis of accommodation space changes in the background of palaeogeography of sequence framework. The results show that: normal lacustrine regression and forced lacustrine regression are the main driving forces for the formation of sequence boundaries of Faulting basin; basement subsidence is the main source of accommodation space of Faulting basin; and subsidence disequilibrium is the main cause for the difference in generation rate of accommodation space of Faulting basin. Coal accumulation in Faulting basin is obviously controlled by accommodation space changes in sequence framework and basin evolution. As Faulting basin evolves into depression basin, both subsidence rate of basin basement and generation rate of accommodation space decrease vertically, it appears as a progradational reverse-graded sedimentary sequence, coal accumulation in secondary sequence framework intensifies first and then weakens, and coal accumulation at the middle stage of highstand system tract is the best. During steady rifting period, minable coal seams were mostly developed in initial lacustrine flooding surface of fourth-order sequence and at the middle-late period of highstand system tract, and coal accumulation center lay in palaeogeographic unit of delta plain with moderate available accommodation space. During shrinking period, minable coal seams gradually migrated to the maximum lacustrine flooding surface and coal accumulation center lay in palaeographical unit of lacustrine with large available accommodation space.