Analysis On Lithofacies Palaeogeography Of The Key Sequences Of Stratigraphy In Southern North China

Southern North China in the south edge of North China plate, to some extent whose environment is different from northern North China, has similar coal-bearing section to Yangzi plate. It is thought its special environment is related to tectonics at the south edge. Therefore it is benifitial to rebuild its palaeogeography, thereby we could deeply understand basin evolution, and theoretically support the oil exploration.In this article, based on sequence palaeogeographical methode, we established sequences framework of Permo-Carboniferous, and identified 3 super sequences and 7 sequences. Then 4 key sequences including sequence 1,sequence2,sequence3&sequence6, were selected to reconstruct their palaeogeography. In the period of sequence1 it is found that, the transgression mainly comes from northeast, and the terrain is high in the south and low in the north. Yet the transgression end quickly and only influence part of the area, in which Xuzhou have the highest sea level. Meanwhile, offshore alluvial system is developed in the northern Huaiyang, where transgression mainly comes from south. Compared with sequence1, transgression area of sequence2 extend and orientation is reversed from northeast to southeast, then accordingly terrain of basin appeares high in the north and low in the south. Difference between sequence1 and sequence2 confirme a transgressive inversion surface existed between sequence1 & sequence2. During sequence3, the configuration of palaeogeography kept the same with that of sequence 2, sea in the south and land in the north. However, depocencer begin to carry forward to south and an extensive regression begin in the later of HST, so that coal-forming follow in large scale. After then(Sq4-Sq6), the area became delta deposited, but occasionally affected by short transgression to develop tidal flat & embayment facies in delta deposit. In the period of sequence6, distribution of fluvial-delta plain -delta front facies is belted from north to south, which illustrates delta progradation goes ahead south, and depocencer lies in Xuzhou and south of Huainan.On the basis of palaeogeographical reconstruction of key sequences, transgressive inversion surface is proved between sequence1 & sequence2, which together with long-time transitional delta deposit in Sq4-Sq6 is related to the compressive depression caused by subductio of Yangzi plate. It is concluded that both global sealevel rising and plate boundary tectonics lead to the special palaeogeographical evolution of Neopaleozoic in the southern North China, and the two factors cooperate together and dominate alternately. In the earlier days(Sql-Sq3), the global sealevel rising lead to epicontinental backfilling, and transgression orientation is reversed by tectonic actitivities. At later stage(Sq4-Sq6), long-time delta equalizing deposit is mainly formed by compressive depression, and is partly influenced by transgression of global sealevel changes at the same time.