Study On Sequence Stratigraphic Framework Of Mesoproterozoic's Jixian Group In The East Of Yanshan Area

The dominating carbonate and the subordinate clastics and claystone are the major components of the Paleoproterozoic-Mesoproterozoic of the Eastern Yanshan region, which includes the Jixian Cunty of Tianjin, Xinglong County, Kuancheng Cunty, Qianxi Cunty to Qinglong Cunty and the Luanxian Cunty of Hebei. These three kinds of rocks, deposited in the subtidal zone, intertidal zone and supratidal zone, are the offshore facies, tidal-flat facies and submarine shelf facies respectively. The thicknesse of the Paleoproterozoic-Mesoproterozoic strata is 930 to 9300 meters, which is distinctly overlapped by the Fujunshan Formation of Cambrian and the Changlongshan Formation of Precambrian in local. However, there are successive outcrops, integrated strata, stable deposition and complete biological record in the whole discussing region. 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 is gigantic. The Jixian Group is the thickest and the most extensive exposed stratigraphic unit of Paleoproterozoic-Mesoproterozoic. The carbonate is the dominating rock-combination, while claystone is the secondly kind and the clastics is the least. The carbonates of offshore facies are kinds of dolomites, which are charactered by thick-beddings with unsmooth bed surfaces and small cross beddings, desiccation cracks, ripple marks and halite pseudocrystals. However, the carbonates of the shallow-sea facies are various calcite dolomites and limestones, which are thin to huge thick-beddings with micrite structure, smooth bed surfaces, little intraclast, and suture line. The claystones are dominated by shales of shallow-sea facies with illite and little glauconite and kaolinite, high organic content and the extremely straight beds. These characters reflect the reduction environment in the deep seawater condition. Through the application of the theory and the method of the sequence stratigraphic, the combination of the actual strata and the chronological stratigraphic data and the detail research of the outcrops, the Jixian Group should be divided into 9 second order sequences and 32 third order sequences in this paper. The Wumishan Formation is comprised of 6 second order sequences(S.10-S.15) and 26 third order sequences; the Hongshuizhuang Formation includes 1 second order sequence(S.16) and 1 third order sequence; the Tieling Formation is comprised of 1 second order sequence(S.17) and 2 third order sequences and the Xiamaling Formation includes 1 second order sequence(S.16) and 3 third order sequences. Taking the line from the Jixian County to the Xinglong County as the deposition centre, the Jixian Group is thin-out towards the eastern and the southern till the Qianxi County and the Kuancheng County and the thickness of the strata changes from approximately 4200 meters to 2200 meters. Furthermore, the thickness of the Jixian Group in the Qinglong County and the Luxian County sections are less than one thousand meters and entirely nip-out in the Funing County section. Base on these phenomena, the paleo-coast should be sited in the southeastern of the Qianxi County, the southern of the Qinglong County and the eastern of the Luxian County.