Middle Jurassic sedimentary facies and palaeogeography of the Yima Basin on the southwestern margin of the North China plate

The Yima basin located on the southwestern margin of the North China plate is the sole recoverable Mesozoic coal field in Henan. Exemplified by the Middle Jurassic coal measures, the stratigraphic division, sedimentary facies and palaeogeographic analysis are made on the basis of lithological, geochemical and sedimentological data. The Middle Jurassic coal measures belong to the Aalenian-Bajocian, when four sedimentary facies are developed, including the alluvial fan, braided stream, delta and lake facies. The Aalenian-Bajocian sedimentary facies and palaeogeographic maps are constructed with the aid of the single factor and multifactor mapping methods. The palaeogeographic units tend to display the variations of the alluvial fan, delta and lake facies, and represent a retrogradational process in the Yima basin throughout the coal-forming periods.