Division and age of the Mid-Upper Silurian Xibiehe Formation in central Inner Mongolia

The Xibiehe Formation is a stable littoral-neritic sequence in an Early Paleozoic orogenic belt on the northern margin of the North China plate. With a clear-cut top and base, it is unconformably underlain by an Early-Mid Ordovician island-arc sedimentary sequence and is in turn overlain unconformably by the Lower Devonian Qaganhebu Formation and Upper Carboniferous Amushan Formation. According to the lithostratigraphic features, four mapping units may be distinguished; they are the Sandstone-Conglomerate Member,Limestone Member,Sandstone-Slate Member and Bate Obo Reef. On the basis of the biostratigraphic study,two coral and brachiopod assemblage-zones are established. Thus this formation is determined to be Mid- Late Silurian strata, i.e. Wenlockian- Pridolian sediments.