Jienu Gr
Type Locality and Naming
It was named by Regional Survey Party under the Tibetan Geological Bureau in 1983. The naming section is located in the area of Jienu of Duoba District, Bango County, Tibet (E 89°40′, N 31°55′).
Synonym: （接奴群）
Lithology and Thickness
Composed of grey-white, grey, grey-green and purple coarse-, and fine-grained clastics, intercalated with volcanic rocks. Lower part consists of grey medium-bedded greywacke, metamorphosed quartz-sandstone intercalated with andesite, andesitic breccia, limestone, bioclastic limestone, shale and dark-violet thick-bedded and massive conglomerates, intercalated with pebble-bearing gritstone. Middle part consists of grey medium-, and coarse-grained sandstones, intercalated with pebble-bearing sandstone, yielding abundant bivalves, Gastropods, hexacorals and ammonites. Upper part consists of grey silty shales intercalated with thin-bedded sandstone and siltstone. Total thickness of 1187 m.
Relationships and Distribution
Lower contact
No basal parts of the group have been found. But next older regional unit is the Xiala Fm of Early Permian (according to schematic stratigraphic column) => major hiatus.
Upper contact
No top parts of the group have been found. But next higher regional unit is the Xiaqiongco Fm (according to schematic stratigraphic column)
Regional extent
The group is distributed extensively in the area of Coqin-Shenza-northern margin of Bango Co-southern margin of Lunpola Basin, with a consistent lithology and an increase in muddy composition; with volcanic rocks missing locally, its thickness being of 1150-3700 m. In most sections there have been found neither the top nor the base of the group, and only at the Rongguo section of Gaize County the basal part of the present group is found to be in an unconformable contact with the limestone of the underlying Xiala Fm (P1), with its top part being in a disconformable contact with the sandy shale of the overlying Songpo Fm in the area of Chuanba, Gaize County.
GeoJSON
Fossils
In the area of Jienu the group yields such bivalves as Arstarte subcardiformis, Gonlomya trapezicostata, Grammatodon (Indogrammatodon) virgatus; Gastropods: Nerinella dayi; corals: Montlivata cupuciformis; and ammonites: Protetraganites sp.
Age
Depositional setting
Additional Information