The Key Coal-forming Period Lithofacies And Paleography Recoering Of The Carboniferous-permian Around Bohai Gulf Area

In the paper, through multidisciplinary methods by sedimentology,sequence stratigraphy, geophysics etc, base on the anlysis of sedimentological methods, field abservation, lithologic characteristic of drills and log interpretation, ,in the Late Palaeozoic stratum, the study area develops four depositional systems: the large-scale tidal flat depositional system, the barrier lagoon depositional system, the River dominated delta in shallow water depositional system, rivers-lacustrine depositional systems and 30 depositional facies,identified 8 sequence contact surfaces, according to the surfaces which distinguishes, dividing the late Paleozoic Era stratum of the Bohai Gulf area into 7 sequences, and in accordance with their sequence formation characteristics to analyze of the sequence 1 to 4 as a key sequence of coal sequence, and the transgression system tract of sequence 2, the highstand system tract of sequence 3, the high system tract of sequence 4, were listed as the key coal-forming period of the key sequences.In connection with change of sea level,climate and structural changes and other factors, the evolution of palaeogeography of sedimentary sequence 1 to 4 of Around Bohai Gulf area experienced the entire process of a landing sea basin formation, development, prosperous, decline and transformation . The pattern of lithofaeies palaeogeography has had the corresponding change along with evolution. In the depositional stage of sequence 1, the objected area mainly developed eluvial facies, carbonate sedimentary facies, barrier -lagoon system.In the depositional stage of sequence 2, the objected area mainly developed carbonate sedimentary facies, tidal flat depositional system and in the north edge of the study area developed river dominated delta in shallow water depositional system. In the depositional stage of sequence 3, the study area mainly developed tidal flat depositional system and barrier -lagoon depositional system, only in some section carbonate sedimentary facies developed. In the depositional stage of sequence 4, the study area developed river deposit system and river dominated delta of shallow water depositional system and. river depositional system.The transgression system tract depositional stage of sequence 2 mainly developed carbonate sedimentary facies and tidal flat depositional system. The highstand system tract depositional stage of sequence 3 developed carbonate sedimentary facies and tidal flat depositional system. The highstand system tract depositional stage of sequence 4 mainly developed fluvial dominated delta depositional system, including not only delta plain but also submerged delta flatland intrafacies, prodelta facies not well developed.In short, in the Late Palaeozoic as one important section of continental sea ,Around Bohai Gulf area has experienced primarily evolution of the barrier lagoon depositional system to the large-scale tidal flat depositional system and to the delta deposit system and river depositional system, finally, to the river- lacustrine depositional system...