Abstract:

The Mesoproterozoic Wumishan Formation in Jixian, Tianjin represents a succession of carbonate strata with 3,300 m in thick, in which Wumishan cyclothems with approximately symmetrical lithofacies successions are the best developed. They are particularly meter-scale cyclic sequences of peritidal type. Based on the analysis of types and association of genetic units of lithofacies of the Wumishan cyclothems, the general and sedimentary models of the Wumishan cyclothems were established. The magnetic susceptibility of the genetic units and its relationship to the high frequency cyclothems were discussed. It is believed that Wumishan cyclothems were formed in the sedimentary environment change, i.e. water depth changed from shallow to deep,then to shallow, and to be instantaneously exposed at last. So it could be considered as a real sedimentary cycle. According to the vertical stacking patterns of the genetic units of lithofacies of the Wumishan cyclothems, it is concluded that the Wumishan cyclothems were probably formed by autocyclical deposition that was controlled by high frequency sea-level change related to the Milankevitch cycle.

Key words: Wumishan Formation, Wumishan cyclothem, cyclic sedimentation, Mesoproterozoic, Jixian of Tianjin