Xiaochaka Gr
Type Locality and Naming
The type section is located in Xiaochaka, Shenzha County, Tibet. It was named by the Tibet regional geological survey team in 1985 and first publicly cited by Rao Rongbiao et al in 1987.
Synonym: Xocaka Gr, Xiaochaka Fm (spellings). Xiaorongka Group (as used in Jurassic Lexicon entry for the overlying formation); In 1985, Wu Ruizhong et al. named the Upper Triassic in the northern Qiangtang as the Juhuashan Fm, taking the Juhuashan section east of the Gomuxue Mountain in the Shuanghu area of Tibet as the named section, which has the same meaning with this group.
Lithology and Thickness
This group consists of carbonates and clastic rocks. Lower part is gray thick limestone, black limestone and marl, 315 m thick. Upper part is a rhythmic layer of gray, gray-green, brown-yellow and other variegated coarse sandstone, fine sandstone, sandy shale and marl with a thickness of 1163 m.
Relationships and Distribution
Lower contact
The basal thick-layered limestone of the Xiaochaka Gr is in conformable contact to the underlying top-most sandstone of Kangnan Fm
Upper contact
The limestone and marl at the top are in conformable contact with the overlying Yellow Mudstone of the Lower Jurassic (= probably the lower part of Qushai Fm ?).
Regional extent
The Group is distributed in the north of Shenza County and the south of Ruolagang in Tibet. In the area of Xiaorongka, Beinuocuo, Laxongcuo and Changliangshan in Shenzha, the group overlaps on different strata of Paleozoic and is in pseudo-conformable contact with the overlying Jurassic strata. The lithology can be divided into lower limestone, a middle-variegated sandstone and an upper bioclastic limestone, with a total thickness of 2500-3580 m.
GeoJSON
Fossils
The lower part contains bivalves Palaeocardita langlongensis, Entolium cf. quotidianum, Indopecten sp., I. serpaticostus, Plagiostoma nuitoense; Brachiopods: Amphiclina intermedia, A. taurica, Rhaetinopsis pentagonnalis, etc; Corals: Thecosmilia sp.
Age
Depositional setting
It is interpreted as shallow Marine environment.
Additional Information