Yinga Fm
Type Locality and Naming
The type section is located in between the Xiongencuo area and the Yinga Pastureland to the south of the Lagu Village of the Bomi County, Tibet. It was named by Yin Jixiang in 1984.
Lithology and Thickness
Conglomerate and Clastics. Representing essentially a suite of clastic rock series, composed commonly of interbeds of grey and weathered yellow-grey pebbled sandy slates and clayey silty slates, intercalated with medium- and fine-grained quartzite and conglomerates, with its upper part containing occasionally thin-bedded micrite. There occur 9 layers of pebbled sandy slates and polymictic conglomerates, which are irregularly scattered in the clayey and silty groundmasses. The gravels are coming largely from quartzite and quartz-sandstone, lesser from black slate, occasionally from black-grey metamorphosed sandstone and limestone. The diameter of gravels is commonly of about 2 mm; only a small amount of gravels can reach as great as 5 mm in diameter, with the slate fragments of 8 mm in diameter being found occasionally. The pebbles are largely subangular. A few of the pebbles in the middle and lower parts of the formation are larger in their diameter and amount than those in the upper part of the formation, but at other horizons they demonstrate their uniformity and an obscure bedding. The fine clastics in the groundmass represent essentially sands and silts, with the major components being quartzes, which are poorly sorted and poorly rounded. The lithoclastics represent commonly coarse sands, comprising siltstone, fine-grained sandstone, quartzite, chert and slate, and with the clayey matter serving as cementing material. The cementation is completed by a basement style. All the cementing material had universally been silicified carbonatized and chloritized, locally being metasomatized by ferruginous cement. A single layer of conglomerate or polymictic sandy conglomerate is commonly only 0.5 m-thick, and occurs in most cases at the base of a quartz-sandstone layer or in the form of a single intercalation within the pebbled slate, which makes up only a small proportion. The composition of gravels is the same as for the pebbled slate, with the diameter of gravel being of 2-4 cm, very few being of 10 cm, and the form of gravels being rounded subrounded, and with the smaller-diameter gravels being largely well-round to subangular. Of the slates there occurs mainly the sandy slate. The exposed extent of the formation is of 3-4 km (!).
Relationships and Distribution
Lower contact
Uncertain relationship to the other strata (e.g., whether on either Xiongencuo Fm or Zadongcuo Fm of Permian or Lagu Fm of upper Carboniferous) – See Additional Information.
Upper contact
Uncertain relationship to the other strata (e.g., whether on either either Xiongencuo Fm or Zadongcuo Fm of Permian) – See Additional Information.
Regional extent
GeoJSON
Fossils
Lacking in fossils
Age
Depositional setting
Pebbled slate is interpreted as being glacial marine deposits throughout the Sibumasu and Tibet microcontinents.
Additional Information
Upper contact:
Uncertain relationship to the other strata (e.g., whether on either either Xiongencuo Fm or Zadongcuo Fm of Permian) – See Additional Information.
Regional extent:
GeoJSON:
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Fossils:
Lacking in fossils
Age:
Inferred that the age of the formation should belong to the early stage of the Middle Permian Epoch IF the Yinga Fm is situated under the Xiongencuo Fm (Xiong'encuo Fm).
Age Span:
Beginning stage: Sakmarian
Fraction up in beginning stage: 0.0
Beginning date (Ma):
Ending stage: Artinskian
Fraction up in ending stage: 0.5
Ending date (Ma):
Depositional setting:
Pebbled slate is interpreted as being glacial marine deposits throughout the Sibumasu and Tibet microcontinents.
Depositional-pattern:
Additional Information
The structure of the Permian System in the area from the Lagu to the Yinga Pastureland is fairly complicated, so there are different views on whether the whole stratigraphic sequence represents a great reversed stratigraphic sequence (?), or whether the subdivision of the particular sequence concerned into the upper, middle and the lower parts (namely into the Zadongcuo Fm, the Xiong’encuo Fm and the Yinga Fm) is tenable, that is to say whether the horizons containing pebbled slates in the upper and lower parts of the formation represent a repetition of one and the same stratigraphic unit? Yin Jixiang would think that the Permian strata in the Lagu area represent a reversed stratigraphic sequence, which includes in ascending order the Yinga Fm, the Xiong’encuo Fm and the Zhadongcuo Fm.
In addition, a different view has been put forth whether the pebbled slate in the lower part contains a part or the whole stratum be designated to the upper Carboniferous Series. Jin Yugan et al. (1977) would think that in accordance with the common plan of subdivision of the Permian System in the Gondwana, the Permian System here is composed of three lithological formations, namely in ascending order the Pebbled slate-bearing (glacio-marine), the Sandstone-and-slate-bearing, and the essentially Limestone-bearing formations.
From Dong-Li Sun, 1993 " On the Permian biogeographic boundary between Gondwana and Eurasia in Tibet, China as the eastern section of the Tethys", Palaeo-3, 100:59-77: " The Zhadongco Fm is overlying the Xiongenco Fm carbonate unit and consists of pebbly slate of 1000 m thick. A fault can be observed between them. Therefore, this pebbly slate sequence was interpreted as reappearance of the underlying Yinga Fm."