On the Peritidal Carbonate Cyclicity and Its Origin of the Ordovician M5 Section in Central Ordos Basin

Key words: 
• peritidal carbonate sedimentation /
• simple offlap /
• staggered offlap /
• autocyclicity

Abstract: The term peritidal coined by Folk is a useful general name for the spectrum of nearshore and shoreline depositional environments and facies. Ancient peritidal carbonate lithofacies are characteristically organized stratigraphically into meter- to decameter-thick, shallowing-upward successions; their vertical stacking is a valuable record of the dynamics of carbonate platform development. There are currently three models used to explain how a shallowing-upward succession forms, 1) as a prograding wedge, 2) as a simultaneously aggrading sheet or, 3) as a mosaic of tidal flat islands. A prograding wedge is generated by the tidal flats prograding in two styles, simple offlap and staggered offlap. A simultaneously aggrading sheet accretes vertically to sea level and the whole platform becomes sequentially intertidal and then supratidal. Tidal flat islands nucleate and accrete by aggradation and progradation and shift in response to hydrographic forces.Much discussion exists currently as to what causes the rhythmic stacking into thick stratigraphic packages of ancient shallowing-upward successions. Two possible mechanisms are suggested,i,e., allocyclic and autocyclic mechanisms. The driving force behind the autocyclicity is the dynamics of sedimentation on the platform, while the allocyclicity emphasizes the factors of subsidence and eustacy that cause relative sea level change. However, the two mechanisms are not necessarily mutually exclusive.The Ordovician M5 section in central Ordos basin consists of a rhythmic repetition of several shallowing-upward peritidal carbonate successions deposited in epeiric sea. Based on the sedimentary structure and lithologic features of studied area, with the consideration of paleotectonics and paleogeography of the central basin, thirteen carbonate sedimentary-diagenetic microfacies are recognized that make up six sedimentary facies. The facies changes vertically and laterally. The paper presents the possible mechanism of the rhythmic package. Considering the geological settings of the studied area, Autocyclicity may interpret the formation of the repetition of peritidal carbonate shallowing-upward successions as the result of the peritidal prograding through simple offlap and staggered offlap.