Stratigraphic division and correlation of the Yixian Formation in Aohan area, Chifeng, Inner Mongolia

The Aohan area of Chifeng belongs to "northern volcanic eruption belt of Ningcheng-Daogelangyingzi". Regional section measuring and large-scale geological survey has shown that the strata which are composed mainly of volcanic rocks in the Aohan area should be grouped into Yixian Formation, and can be further divided into four lithologic members in the following ascending order:The first member consists of polymictic conglomerate and coarse sandstone gravel; the second member consists of acidic rhyolitic volcanics; the third member consists of a series of andesitic volcanic rocks and sedimentary facies tuff gritstone; and the fourth member consists of trachydacite volcanic rocks with medium to high alkaline features. The LA-ICP-MS zircon U-Pb data of volcanic rocks from each lithologic member in the area indicate that the volcanic activity occurred at 127.7~121.6Ma, and this result can compare with things of the typical section of Yixian Fomation in northern Fuxin-Yixian basin. Rock assemblages and isotope dating results show that the strata in Aohan area have similar lower and medium members, but the upper strata are absent, as shown by a correlation with strata of the typical section of Yixian Formation.