Huanhe Fm

Its base, marked by brown red sandstone with oblique micro-bedding, is separated from sandstone with large oblique beddings of the underlying Luohe Fm and lies unconformably under the latter.

Its top, marked by purple gray and gray mudstone with sandstone, lies unconformably under the Upper Pleistocene Malan Fm. Regionally, it is conformably overlain by the Aptian Luohandong Fm

The present formation occurs in the vast area of the southern and northern parts of the Shaanxi-Gansu-Ningxia basin. It crops out in Otog Qi, Otog Qianqi and Hanggin Qi of Inner Mongolia. It is characterized by purple red and gray blue sandstone with small oblique beddings, which may reach 705 m in thickness downhole. In Shaanxi it is distributed north of the line linking Yijun, Xunyi, Binxian and Ganyang, and extends to Huangling, Zhidan and Yulin. It is fine and persistent in lithology. Only in Caobigou of Ganyang, it contains conglomerate and gravel-bearing grit, with a decrease of thickness from north to south and from east to west, generally from 329 to 538 m. In Gansu, it is found in Huanxian, Zhenyuan, Huachi, Qingyang and Heshui, with the lithology becoming finer from northeast to southwest and with a thickness ranging from 257 to 734 m. It is of lacustrine deposition. In Hedaochuan of Huanxian, it lies conformably under conglomerate of the overlying Luohandong Fm. In Ningxia, it is observed downhole in Lingwu and Yanchi and its lower part is dominated by brown red -variegated gravel-bearing sandstone and conglomerate, while its upper part, fine in lithology, is dominated by mudstone, clayey sandstone and sandstone, with a thickness ranging from 478 to 584 m.