Daohugou biota-bearing lithostratigraphic succession on the southeastern margin of the Ningcheng basin, Inner Mongolia, and its geochronology

Abstract

A complete and successive Late Mesozoic lithostratigraphic succession is exposed on the southeastern margin of the Ningcheng basin, Inner Mongolia; however, this succession and the age assignment of the biota contained therein are still controversial. The discovery and measurement of the lithostratigraphic section containing the Daohugou biota indicate that the Late Mesozoic stratigraphic succession in the study area is composed in ascending order of the Middle Jurassic Jiulongshan and Tiaojishan formations, Late Jurassic Tuchengzi Formation and Early Cretaceous Yixian Formation. The lower part of the Jiulongshan-Tiaojishan formations consists dominantly of sedimentary rocks, yielding the Daohugou biota, while its upper part, intermediate-basic rocks- characterized by the ages of 152Ma- 164-165Ma by SHRIMP zircon U-Pb dating. The stratigraphic succession of the Jiulongshan-Tiaojishan formations is disconformably overlain by the Late Jurassic Tuchengzi Formation, and both are in turn unconformably overlain by the Early Cretaceous Yixian Formation. The geologic age of the Late Mesozoic strata bearing the Daohugou biota is earlier than that of the Jehol biota and is the Middle Jurassic product. Now the strata are together assigned to the Jiulongshan-Tiaojishan formations.