Sequence Stratigraphic Framework Research Of The Mesoproterozoic Nankou Group In Eastern Yanshan Area

The Yanshan area is the most developed region of the Meso- and Neoproterozoic stratum in north China, the whole area has continuous outcrops, complete stratum and steady deposition. The Jixian section, Tianjin, is the ideal section to study Meso- and Neoproterozoic deposition because it has the most complete strata and the stratum thickness are gigantic. The Neoproterozoic stratum in Jixian section is dominated by carbonate mixed with clastic rock and clay rock which belongs to lithoral facies, tidal-flat facies and submarine shelf facies. The Nankou Group includes Gaoyuhzuang Formation and Yangzhuang Formation. The Gaoyuzhuang Formation is dominated by carbonate and the contact relationship between Dahongyu Formation is parallel unconformity. The lower part of Gaoyuzhuan Formation is characterized by various stromatolite; the middle part include a great number of manganese; the upper part is marker by various nodules. The Nankou Group can be divided in to four second-order sequences according to their fabric features of facies succession:The first second-order sequence of Nankou group (S5) consistent with the first member of Gaoyuzhuan Formation (or the Guandi subformation), it's lithologic character is variational: the lower part is a set of quartz sandstone, the thickness is 1~3 meters; the upper are mainly intertidal zone or supratidal zone deposits, which include stromatolitic dolomite with terrestrial clastics or argillaceous dolomite with dolomitic sandstone or dolomitic siltstone interbeded. And futher, the S5 can be divided into 6 third-order sequences (S.5.1-S.5.6). The second member of Gaoyuzhuang Formation (or the Sangshu'an subformation) consistent with the second second-order sequence of Gaoyuzhuang Formation (S7), the lithologic character of the lower part is intertidal zone or supratidal zone manganous dolomite and manganous dolomitic silty dolomite; the upper part is subtidal zone dolomicrite. S6 can be divided into tow third-order sequence S.6.1 and S.6.2. The third second-order sequence (S7) consistent with the third member of Gaoyuzhuang Formation (or the Zhangjiayu subformation), the lithologic character is subtidal zone to shelf facies dolomitic limestone and argillaceous dolomite. There are organic clastic in texture; have no stromatolitic but develop particular molar tooth (MT). The third second-order sequence can be divided into 3 third-order sequence (S.7.1-S.7.3) according to cycle character. The forth member of Gaoyuzhuang Formation (or the Huanxiusi subformation) consist the forth second-order sequence (S8), which dominated by tidal-flat facies mass stromatolitic dolomite, the S8 can be divided into 2 third-order sequence (S.8.1-S.8.2) further. The Yangzhuang Formation itself consist a second-order sequence (S9), it is also the fifth second-order sequence of Nankou Group. The Yangzhuang Formation is dominated by interbedding dolomite and dusty purple-purple dolomitic mudstone, the particular color and lithology indicate confining lagoon facies deposits. According to the cycle feature, the Yangzhuang Formation can be divided into 4 third-order sequence (S.9.1-S.9.4). In the study area, the strata becomes thinner from Jixian section to east and south, the strata thickness at Jixian section is more than 2100 meters but only approximately 1000 meters at Xinglong section, Kuancheng section and Qianxi section. At Qinglong section and Luanxian section, the thickness is several hundreds meters and eventually pinch at Funing section.