Diachronism of the Cambrian strata on the North China platform

The Cambrian Mantou, Maozhuang, Xuzhuang, Zhangxia, Gushan, Changshan and Fengshan Formations of the North China platform are characterized by purple argillaceous dolostones and dolomitic mudstones, shales, oolitic limestones and edgewise limestones which were formed in the lagoon sabkha, oolitic shoal and ramp environments. The sequence stratigraphic approach of some sections such as Jingjiatan section in Western Hills of Beijing, Xuankongsi section in Tangshan, Jiagou section in Suxian and Dongshan section in Huainan has led the author to the following conclusions. (1) All the strata outlined above are diachronous; but it is impossible to define the widespread strata which were formed in the same sedimentary environments as the equivalents deposited in the same periods. (2) The individual Cambrian formations within the North China platform were formed not simultaneously, but one after another in response to sedimentary facies migration. All the sabkha lagoons, oolitic shoals and ramps are relatively small; they often shifted with changing transgressions. (3) The thickness of the formations doesn't mean the amplitude of subsidence during the same period; it depends mainly on the duration and depositional rates of the sedimentary facies in one region. The duration, in turn, is determined by that needed for the equilibrium between sea level rise rates and depostional rates in the same area. The distinctive durations and depositional rates are responsible for the varying stratigraphic thickness in different parts; (4) The biological associations from the Cambrian strata of the North China platform may be constrained by their living environments. Their living bodies may be alive in different environments, whereas their fossils can be only preserved in the marginal zones of these environments. (5) It is incorrect to use the faunas as the only indicators for the recognition of stratigraphic "diachronism" or "isochronism".