Yan'an Fm

Ordos Basin. It was named by Wang Shangwen as the Yan’an System in 1950. The naming section is located on the Xingzi River to the west of Yan’an City, Shaanxi Province.

Synonym: （延安组）

Lower member is known as the Baotashan Sandstone Member which is composed mainly of grey-white and reddish-pink massive gritstones with oblique beddings and fine-grained sandstones, with its basal part consisting of micro-conglomeratic gritstones, and with its upper part consisting largely of fine-grained sandstones intercalated with grey-black argillaceous siltstones and shales. Upper member known as the Zaoyuan Member represents mainly an alternating layer of grey and grey-green sandstones in association with grey-black and dark-grey shales and mudstones, with a total thickness of 250 m. Containing abundant coal beds, with a thickness of 100-300 m, and with a maximum thickness being of 600 m.

It yields plant fossils such as Coiniopteris hymenophylloides, C. tatungensis, Baiera furcata and Phoenicopsis angustifolia; bivalve fossils as represented by Ferganoconcha sibirica, Tutuella crassa and Sibireconcha anodontoides, as well as spore-pollen fossils.

The strata of this formation is suggested as fluvial-lacustrine deposits in the naming locality. Except for the vast area of the central part of the above-mentioned basin enclosed with the Fixian, Qingyang, Huachi, Wuqi and Zichang counties and Yan’an City, in other regions the formation is interpreted as fluvial-facies, delta-facies and flood-plain-facies together with swamp-facies deposits.