Sequence Lithofaeies Palaeogeography And Molar-tooth Carbonates Of The Gaoyuzhuang-Wumishan Formation In Mid-Eastern Yanshan Area

Taking the marine carbonate rocks of Mesoproterozoic Gaoyuzhuang and Wumishan formation in Mid-eastern Yanshan area as example, by using the theory and method of astronomical periods cyclic sequences stratigraphy, sedimentology, lithofacies palegeography and event geology, basing on detailed survey, descriptive analysis and laboratory investigation of many field outcrop sections, the paper shows the characteritics of sedimentary facies and sedimentary model,the method of cyclic sequences stratigraphic division and correlation and the feature of cyclic sequences stratigraphic framework. With the combination of the key points in the areas, the paper works out sequence lithofaeies Palaeogeography map and studies sequence Palaeogeographic evolution and distribution of various Periods. Basing on above work, this paper meticulously depicts the typical morphology of Molar-tooth structure Gaoyuzhuang formation, also discusses the genetic mechanism and the formation environment of various molar-tooth structures and its impact on the evolution of paleoenvironmental significance.Five types of depositional system of Mesoproterozoic Gaoyuzhuang and Wumishan formation in Yanshan area, which are tidal flat depositional system, neritic shelf depositional system, ramp depositional system and deep-water basin depositional system. Eight sedimentary sequences can be concluded from the outcrops, platform Cryptomonas carbonates, platform tidal flat-beach bar, aulacogen slope carbonates, slump carbonates, carbonate granular flow sistem and carbonate dissolution and phase sedimentary sequences.Basing on detailed sequence boundary identification, meter-scale cyclic sequence analysis and the use of Fischer diagrams for accommodation space changes study, Gaoyuzhuang formation is divided into 14 third-order sequence(GSQ1-GSQ14), and further grouped into four super-sequence (second Sequence) (GSSQ1-GSSQ4). Similarly, Wumishan formation is divided into 27 third-order sequences (WSQ1-WSQ27), further grouped into four super-sequence (WSSQ1-WSSQ4).Supersequence lithofacies palaeogeography maps of Gaoyuzhuang and Wumishan formation in the study area are compiled on condition of supersequence used as drawing unit. This kind of map has some characteristics, such as isochronism, origin continuity and usability. It reveals the characterstic and evolution of lithofacies palaeogeography of different sedimentary period.Molar-tooth (MT) is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform, equant microspar. MT is both abundant and widespread in Mesoproterozoic and early Neoproterozoic strata, where void-filling microspar comprises up to 90% of individual beds and 5-25% of preserved carbonate. The temporal restriction of this fabric suggests a potential link between MT formation and the biogeochemical evolution of marine environments. Detailed petrographic relationships among MT crack morphology, distribution of MT microspar, and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated Proterozoic seawater.