Menkadun Fm
Type Locality and Naming
It was named by Mu Enzhi in 1973. The naming section is located in the Xiumuo Village at the No. 715 Maintenance Squad of the Dingri-Nyalam Highway, Tibet.
Synonym: （门卡墩组）
Lithology and Thickness
Lower part is alternating layers of grey-green and grey-black siltstones and shales, containing concretions. Middle part is composed of grey-black microcrystalline limestones, bioclastic limestones, calcareous sandstones, siltstones, poststones (= fine-grained sandstone) and shales. Upper part consists of dark-grey siltstones, sandy shales, and is rich in argillaceous, siliceous, calcareous and ferruginous concretions. It is 710 m in thickness.
Relationships and Distribution
Lower contact
The basal part of the formation composed of grey-green siltstone, shale and poststone rich in concretions is in a conformable contact with the alternating layer of the dark-grey medium- and thin-bedded limestone and quartz-sandstone from the top part of the underlying Nieniexiongla Fm
Upper contact
Regionally, the schematic strat column indicates the next younger unit as Gucobingzhan Fm
Regional extent
It is distributed extensively in the areas of Jilong, Dingri, Nyelamu, Dingjie, Ganbar and Yadong, with a consistent lithology, yet a greatly changeable thickness varying in a range from 710 m to 4000 m.
GeoJSON
Fossils
In the Menkadun area it yields abundant ammonites, bivalve and Belemnoidea fossils, with the ammonites comprising Virgatosphinctes contiquus, V. denseplicatus, V. pompeckji, V. himalayanus, V. multifaceiatus, V. frequens, V. subfrequens, V. haydeni, Haplophylloceras pinque, Uhligites kraffiti, Pterolytoceras exoticum, Aulacosphinctes moexikeanus and Prorasenia quenstedti; and with the bivalves comprising Buchia rugosa, B. piochi and B. lindstroemi.
Age
Depositional setting
The formation represents normal clastic and carbonate deposits of shallow-sea and continental-shelf origin.
Additional Information