Sumxico Gr
Type Locality and Naming
It was named by the Regional Geological Survey Party under the Geological Bureau of Tibet in 1987. The naming section is located at about 1 km north of the Sumxico, Duoma Region, Ritu County, Tibet (E 80°14′, N 34°18′ => TJ36 region).
Synonym: （松木希错群）
Lithology and Thickness
The group represents carbonate rocks, intercalated with clastic deposits, with a visible thickness of over 2223 m. Lower part is composed of dark-grey thick-bedded microcrystalline limestone, grey medium-, and thin-bedded quartz-sandstone, siltstone, fine-grained sandstone and bioclastic limestone. Middle part consists of brown-green argillaceous shale, grey medium-, and fine-grained quartz-sandstone and dark-grey medium-, and thick-bedded bioclastic limestone, microcrystalline limestone and oolitic limestone. Upper part is dark-grey thick- to fine-bedded microcrystalline limestone, intercalated with quartz-sandstone.
Relationships and Distribution
Lower contact
The group is short of its base in most areas; only in the Sumuna and Qipuchapu Pass regions the group is found to be in an unconformable contact with the underlying upper Carboniferous Chatir Gr. Regionally, the next older unit is the latest Permian limestone of the Jipucun Fm
Upper contact
The group is short of its top in most areas; only in the Daban area of the Jieshan Mts and at the Tianshan Mts of the Qiao’er region is the group is found to be in an angular unconformable contact with the Lower Cretaceous Tielongtan Gr (Cret column 5 => near TJ28 column).
Regional extent
It is distributed extensively in the areas of Sumxico, Daban of the Jieshan Mts, Bangdaco, Sumuna, northwest of the Qipuchapu Pass, Heweitan, and the Tianshan Mts of Qiao’er. Its lithology is consistent in various regions, but with minor changes in some areas due to the influence of tectonics. In the area of Heweitan it is composed of clastic rocks intercalated with limestone and shale, while in the Bangdaco region it is composed of limestone intercalated with sandstone, locally with gypsum beds, with a thickness of 110-1690 m. In most of the above-mentioned regions the group is short of its top and base, and only in the Sumuna and Qipuchapu Pass regions the group is found to be in an unconformable contact with the underlying upper Carboniferous Chatir Gr, and also is in an unconformable contact with the overlying Cretaceous Tielongtan Gr (Cret column 5) in the Daban area of the Jieshan Mts and at the Tianshan Mts of the Qiao’er region.
GeoJSON
Fossils
In the Sumxico region it is rich in bivalve fossils such as Prodocardia lycetti, P. cf. stricklandi, Amiodon cf. khoratensis, Crobula tanganyicensis, Modiolusglaucus, M. imbricatus, Ceratomya concentrica, Pholadonmya carinata, Chlamys (Radulopecten) tripperi and Homomya sp., as well as Brachiopoda fossils as represented by Burmirhynchia sp., Rhactorhynchia sp. and Kallirhynchia robusta.
Age
Depositional setting
The characteristics of the biota tend to indicate that the group represents deposits formed in a warm and normal shallow-sea environment.
Additional Information