Geological division of the rocks at southeast of Ulaanbaatar (Gachuurt-Nalaikh), central Mongolia

Abstract

This paper describes lithology and radiolarian fossils from a chert slab in the Gachuurt-Nalaikh area, southeast of Ulaanbaatar, central Mongolia, and discusses the geological division of the rocks in this area. The rocks in this area consist of clastic rocks with chert slab intercalations. Although the rocks of this area have been previously divided into the Gorkhi, Altan-Ovoo, and Orgioch-Uul Formations, there are no significant structural and lithological gaps across the boundaries between these formations. Therefore, there is no reason to divide the rocks of this area into the three formations, and these three formations should be regarded as a single geological unit. A conformable succession of Devonian radiolarian chert, siliceous shale, mudstone and sandstone in ascending order, which is generally regarded as ocean plate stratigraphy (OPS), were found from this area. The repetition of chert and clastic rocks with OPS within this area might show the stacking of tectonic slices of the chert-clastic rock sequence within an accretionary complex. This suggests that the rocks of this area should be regarded as the northern extension of the Gorkhi Formation of an accretionary complex, though most of the rocks of this area have been previously assigned to Carboniferous shallow marine system.