| Based on predecessor's researches of paleostructures,paleogeography, paleohydrodynamics and paleoclimate as well as the sedimentary facies of single wells and outcrops, authors discovered that Qinling Trough and the Central Uplift controlled the sedimentary features and evolution in the southern part of Ordos Basin during Early Ordovician period.The southern part of Ordos Basin is divided into two main sedimentary areas, the south western and the northeastern areas, owing to the barrier role of the Central Uplift. A basin,a slope and a platform margin developed in the southwestern and a lagoon in the northeastern. Lower Ordovician Majiagou Formation is composed of six members (abbreviated as Ma1 to Ma6), in which Ma1, Ma3 and Ma5 are evaporite including dolomite, anhydrite and halite and Ma2, Ma4, and Ma6 are limestone with dolomite. A new model, the Majiagou barrier-lagoon sedimentary model, is further put forward in order to interpret the sedimentary features of Majiagou Formation. This model includes two main characters. the one is that the Central Uplift plays an extreme role in the exchange of seawater between the lagoon and Qinling sea as a barrier; and the other is that the model is divided
into three dynamical phases on the basis of an integrated cycle: the limestone in wide Barrier-lagoon during trans gression to high sea level (phaseⅠ), the mixed limestone/evaporite succession in concentrated lagoon during regression (phaseⅡ), and the anhydrite and halite in barrier lagoon during low sea level( phaseⅢ). |
