Stratigraphic division of Neogene Gansu Group in Wuwei Basin, northeastern margin of Qilian Mountains

A set of grayish-white, brick-red sandy fine-grained conglomerate, conglomerate-bearing coarse-grained sandstone is developed in Wuwei Basin, northeastern margin of Qilian Mountains, which was confirmed as Neogene Gansu Group by regional geological survey and stratigraphic correlation.The Gansu Group was divided into upper and lower formations here, named Guoyuan Formation and Fengle Formation, respectively, based on the borehole and surface stratigraphic sections, borehole paleomagnetic chronology, lithologic characteristics and sedimentary environment analysis of Gansu Group.The Fengle Formation is characterized by Early-Middle Miocene fan delta-lacustrine sediments consisting of orange-yellow sandy fine-grained conglomerate, muddy siltstone, mudstone intercalated with grayish-white feldspathic quartz sandstone, with the paleomagnetic age of ca.11.18~8.25 Ma.The Guoyuan Formation represents fluvial sediments, composed of yellowish silty mudstone and brick red medium-coarse-grained conglomerate.Its lower boundary is in parallel unconformity contact with the Fengle Formation, and the top boundary is in angle unconformity contact with the Lower Pleistocene Yumen conglomerate.Its sedimentary age is Late Miocene to Pliocene (8.25~2.58 Ma) as paleomagnetic dating.Therefore, the Fengle Formation and Guoyuan Formation of Gansu Group in Wuwei Basin are two formal lithostratigraphic units, which can be regionally correlated with the Neogene stratigraphy in the adjacent areas, and unify the Neogene stratigraphy system in the northeastern margin of the Tibetan Plateau, providing the stratigraphy basis for the Cenozoic geology in the northeastern margin of the Tibetan Plateau.