The construction and significance of the Yanguoxia Formation on the southern margin of the Minhe Basin,Gansu

The well exposed Lower Cretaceous strata constitute a megacycle consisting of a succession of coarse-to fine-grained sediments in the Minhe Basin,Cansu.The Yanguoxia Formation is right located in the middle part of the megacycle.Vertically,the formation which is quite different from the overlying and underlying strata is dominated by the littoral and shallow lacustrine fine-grained clastic rock series and marked by subaqueous sedimentary structures and bluish grey marker beds which are represented by three maximum flooding surfaces.In this respect,the formation may have important geological significance in regional stratigraphic division and tectonic synthesis although laterally the lithology and lithofacies are inextensive and highly variable.