Sedimnetary Deposits of Longtan Group and Late Palaeozoic Tectonic Framework in the Eastern Yangtze Plate China

Longtan Group in Early Upper Permian is mainly composited of clastic deposits bearing coals, wide spreaded in Lower Yangtze area, East China and extends in NE-SW direction. Its mian sedimentary body showed a clear delta sequence in some geological sections. The ancient current of Longtan Group sandstones flown in NW-SE directions without exception. The thickness of the group in the east part of Yangtze Plate is thicker than that in the west part and the thickest geological section up to 1000m which occures in the east part (e.g. in Binhai County, Northern Jiangsu); and it quickly thinned to several (ens meters westward. Correspondingly, the thickness of sandstones in Longtan Group thinned from east to west, and the tock associations regularly changed from mudstone- sandstone association in the east part, through sandstone-mudstone association in the middle, to carbonate-mudstone association in the west part. Plagioclase contents of the sandstones are richer in the east than in the west. Doubtlessly. Longtan Group is a clastic wedge that pinched out westward, i.e., a compressed molasse. One of the important characters of the sandstones is the abundance of tourmalines, its average content is about 510.2lg/ t, and a lot of detritus of metamorphic and magmatic rocks. Longtan Group sandstones can be identified as detrital graywack by their chemical compositions. The grain framework composition and the chemical composition of sandstiones indicate that the deposits of Longtan Group were accumulated under a tectonic setting com bining Atlantic type rift with a inactive convergence margin according to several author' s tectonic crimination models such as Dickinson et al.Bhatia, and so on. Evidently, an old orogenic belt must be presented in the eastern source area and extended near S-N directional strike. The present authors would like lo suggest that it is called Donghai Orogenic Belt. A series of thickness sections clearly show that, the intra-basin structure If Longtan Group is uneven, there are several NE-SW derictional synsedimentary uplifts and depressions in it, which are alternated with each other in the space and differentiated in sedimentary characteristics and thickness. The framework of uplifts alternating with depressions can he identified by the sedimentary characteristics of Late Devonian and Early Carbonifeous deposits in this area. Thus, the tectonic framewock had been sprouted since Late Devonian and developed into the most typtcal stage during Longtan Epock, which is considered to be the represent of the tectonic framework of whole Kale Palaeozoic in this area. The authors concluded that Longlan Group sedimentary basin is an impactogen that caused by crust txtension in the area and was created under the compression condition from Donghai Orogenic Bell.