Wumishan Cyclic Markov Chain Model And The Fisher Diagram Analysis

The Mesoproterozoic Wumishan Formation at the Jixian section of Tianjin is a succession of 3300-m-thick carbonate strata formed in a period of about 100 Ma (1310 ±20~1207 ±10 Ma). In this succession of strata, the carbonate meter-scale cyclic sequences belong to peritidal type with an approximately symmetrical lithofacies-succession are best developed.The wide development of 1:4 stacking patterns shows that these metre-scale cyclic sequences are genetically related to the short-eccentricity cycles,which are called the Wumishan cyclothems. Many features such as the clear deepening and shoaling vectors of cyclothems, indicate that the Wumishan cyclothems are the product of autocyclic sedimentation governed by allocyclic high-frequency sea-level changes.The results of a Markov chain analysis reaffirm the existence of the lithofacies-succession model of the Wumishan cyclothems. The boundaries of the Wumishan cyclothems are marked by the instantaneous exposed punctuated surface,which leads to the discrepancy between the cyclothems and the parasequences of the sequence stratigraphy terminology system of Vail et al(1977).It is difficult to form a judgment that the time span of the Milankovitch cycles in the Precambrian is certainly equal to that of the Phanerozoic,but the regularly vertical stacking patterns of the seventh-order rhythms,sixth-order cyclothems and fifth-order parasequence sets still indicate their consistency with the duration of the Milankovitch cycles in the Phanerozoic. In Wumishan Formation,stromatolite biostromes and thrombolite bioherms are best developed,these biostromes and bioherms,together with tidal-flat dolomites and sandy-muddy dolostones,form Wumishan cyclothems with an approximately symmetrical lithofacies succession,belonging to a particular type of peritidal carbonate meter-scale cyclic sequences.The lagoon muddy shales with the caps of paleosoil layer constitute the top of Wumishan cyclothems,and a lot of fresh-water diagenetic marks were formed in tidal-fat dolomites,which suggests that the boundaries of Wumishan cyclothems are mostly instantaneously–exposed punctuated surfaces.In the light of the regular vertical stacking patterns in long-term third –order sequences,26 third-order sequences that can be incorporated into 6 second–order sequences can be discerned in Wumishan Formation.In order to study the changing extent of the long-term sea-level,the authors recognize 626 cyclothems in the middle and upper part of Wumishan Formation in which the outcrops are well developed.Considering the total thickness of Wumishan Formation and the general duration of its formation,the authors hold that the subsidence rate of the crust in the sedimentary period of Wumishan Formation was probably 2 ~ 4 meters in each interval of 100 thousand years.On the basis of this assumption,the Fischer plot might be used to calculate the changing extent of the third-order sea level.