A preliminary study of the cyclic sequences of composite sea-level changes in the Mesoproterozoic Wumishan Formation in Jixian,Tianjin

The Mesoproterozoic Wumishan Formation in Jixian,Tianjin represents a succession of 300-m 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.