Sequence-stratigraphic division of Cambrian in Lutonggou section, Chengde City, Hebei Province

The third revolution of stratigraphy was that of sequence stratigraphy, which uniquely focused on analyzing changes in facies and characteristics of strata. Lutonggou section in Hebei Province, characterized by sequential strata, evident and special sequence boundary and an ocean of sedimentary phenomena, is favorable for the sequence-stratigraphic division of the Cambrian in the North China Platform. Lithofacies in this section principally consist of micrite,oolitic limestone and dolomite, and sedimentary facies include tidal-flat facies, beach facies, ramp facies and shelf facies. The Cambrian strata in Lutonggou section can be divided into 9 third-order sequences according to the characteristics of sedimentary facies stacking patterns and long-periodic cycle change under the new chronostratigraphic system. Cambrian depositional succession, that was bounded by the third-order sequence"TST+CS+HST"formed by Series 3 Xuzhuang Formation, changed from the mixed tidal flat to the ramp carbonate deposits. At the same time, the sequence type changed from the succession dominated by the"TST+HST"to the drowning unconformity dominated by the"CS+HST". Furthermore, abundant stromatolite bioherms suggest a special depositional phenomenon that the skeleton- poor stormy sea in the late Cambrian was marked by the increase of microbial carbonate together with the radiation of metazoan. The sequence stratigraphy research on the Lutonggou section of Cambrian strata based on the characteristics of facies superposition and cycling will give a typical example for the sequence stratigraphy division of widespread Cambrian strata in the northeast of North China Platform, and also provide more ideas for the widespread tracking and correlation.