The Basic Facies Sequence Model And Significance Of The Wumishan Cycles

The Mesoproterozoic Wumishan Formation in Jixian, Tianjin represents a succession of 300m thick carbonate strata, in which stromatolite biostromes and clotted bioherms are best developed. These biostromes and bioherms, together with tidal-flat dolomicrite and muddy dolostone, form circum tidal flat meter-scale carbonate cyclic sequences with symmetrical facies sequences and fabrics. These cyclic sequences are overlain by the lagoon dolomitic mudstones and shales. Generally speaking , three to five orderly stacked meter-scale cyclic sequences may form one fifth-order parasequence set. Occasionally , five seventh-order rhythmites are also recognized in some of the meter-scale cyclic sequences. Therefore , the seventh-order rhythmites , meter-scale cyclic sequences and fifth-order parasequence sets are correlative in origin with precession of equinox cycles, short eccentricity cycles and long eccentricity cycles , respectively. Viewed from the ordered stacking patterns of the meter-scale cyclic sequences in the long-period sequences , twenty-six third-order sequences may be identified in the Wumishan Formation. These third-order sequences may also be incorporated into six second-order sequences. In order to study the changing extent of the long-term sea-level , the authors recognized 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. According to the Fischer plot, some meaningful conclusions can be reached and many useful data can be obtained on sequence stratigraphy of Precambrian.