Benxi Fm

The type section lies in Niumaoling (Mt.) between the Xindong and Mayicun gullies about 6 km west of Benxi City, Liaoning Province. It was named by Zhao Yazeng in 1960.

Claystone, Sandstone and Limestone. It is composed of shale and sandstone intercalated with thin-bedded marine limestone or thin coal seams. There is usually a member of ferriferous purple-colored shale at the bottom, which often forms irregular roost-like iron beds and shale containing bauxite or bauxite beds. In the type section: Lower part consists of 15 m-thick purple shale intercalated with bauxite beds; Middle part, of yellow sandstone and sandy shale with shale and thin coal seams, 74.6 m thick; Upper part, of yellow shale and fine-grained sandstone with bauxite shale, containing 4 limestone beds, which are ascending: the Mayi limestone, Xiaoyushi limestone, Benxi limestone and Niumaoling limestone, about 60 m thick. The lithology is relatively stable, all the intercalated limestones are thin-bedded or lenticular, dark-gray to black, mostly clayey. The coal seams intercalated are rarely mineable.

Iron ores and bauxite shales are often seen at the bottom of the formation. The former are called “Shanxi-type iron ores”, mostly roost-like or lumpy; the latter usually form important bauxite deposits with abundant reserves and high quality.

Thickness of this formation varies greatly: reaching 145−300 m in the Taizi River reaches, being about 495 m at Hulusitai of the Helan Mts., only 60 m in Shandong, 30−40 m in northwestern Shanxi, 40−50 m in Taiyuan, less than 30 m in southeastern Shanxi, only ~10 m in western Beijing, about 60 m at Datun, Peixian County, Jiangsu Province, and decreasing to ~10 m around Shuicheng, eastern Henan

The Lower part of the formation contains 18 species, 12 genera of Brachiopods, belonging to late Early Carboniferous−early Middle Carboniferous. The black shale in the roof of layer-G bauxite at the Bottom of the formation yields Plants Sublepidodendron tangshanense, Archaeocalamites scrobiculatus, Cardiopteridium spetsbergense, Rhodeopteridium hsianghsiangense, etc., and spore-pollen Verrucosisporites kaipingensis, Knoxisporites stephanophorus, Lycospora pusilla, Triquitrites protensus, etc. totaling 51 species of 36 genera (corresponding to late Visean−early Namurian A in western Europe).

In the Benxi Fm of Liaodong underlying the corresponding horizon, fossil Fishes are also found (Liu Xiaofeng and Fan Guoqing, 1993), dominantly elements of the orders Crossopterygii and Dipnoi under class Osteichthyes, possibly also Placodermi. Also collected from the horizon are Plants Lepidostrobus grabaui, Sphenopteris changduensis?, Drepanophycus sp. as well as charophytes Trochiliscus ingricus and Eochara wichendam, which are aged Devonian to Early Carboniferous. At the bottom of the formation in the Zibo Coalfield and Yangquan Coalfield of Shanxi Plant fossils such as Linopteris brongniartii are found. The formation in Hebei and Shanxi yields the Corals Arachnastrea kaipingensis-stereolasma monophylloides-Neniscophyllum kansuense assemblage, conodonts Neognathodus roundyi-Streptognathodus parvus assemblage, etc. At the bottom of the formation in the Pingsuo Coalfield and Yongxia Coalfield of Shanxi, Linopteris neuropteroides, etc. are found. At Hulusitai of the Helan Mts., the formation contains Plants Linopteris brongniartii, L. neutopteroides, Neuropteries gigantea, etc. and Brachiopods Choristites norini, C. yanghukouensis, C. cf. mansuyi, Neospirifer aff. orientalis, etc. Also found are Ammonoids Bilinguites sp., Gastrioceras sp., etc., as well as Fusulinids Eostaffella minutissima, Profusulinella chernovi, etc. At Heidaigou, Jungar Banner, Inner Mongolia, this formation also yields Fusulinids Fusulina quasicylindrica and Fusulinella bocki, Brachiopods Choristites cf. yanghukouensis, etc., plants Neuropteris gigantea, etc.

It is interpreted as a littoral to intertidal facies. When this formation was deposited, the North China platform began to subside. The seawater, except in the Qilian Sea on the western border that invaded along the Helan aulacogen and in the sea region of the eastern side that invaded by way of the Fuzhou Bay of Liaoning and Linyi of Shandong, might be still connected with the Yangtze Sea in the south. The “ferralitic rock” of the formation is deposited in a littoral plain environment; the “bauxite bed G” occurs in the upper part of subtidal zone of lagoon beach and the intertidal zone, and the dense massive bauxite belongs to the supratidal product. Mudstones, coal seams and carbonaceous shales commonly occur in the roof of ferralitic member, usually with plant fragments and fossil roots, which are products of the swamp facies.