Yuanyangyan Fm
Type Locality and Naming
It was named by Jin Chuntai et al (1989). The type section is located near Yuanyangyan by the roadside from Xinggou to Luding in the eastern part of the Erlangshan Mts. area in west Sichuan Province.
Synonym: (鸳鸯岩组) . Lenji Bed and the Foeryan Bed (see Additional Information)
Lithology and Thickness
It consists of dark gray-black thin-bedded chert intercalated with siltstone, calcareous sandstone and grayish black shale, thick-bedded pelitic siltstone and micritic sandstone. It is 256 m thick.
Relationships and Distribution
Lower contact
The formation is probably conformably underlain by the Erlangshan Fm of the Upper Ordovician.
Upper contact
The formation is conformably overlain by the Luoquanwan Fm.
GeoJSON
Fossils
Graptolites
Age
Depositional setting
Additional Information
Lithology and Thickness:
It consists of dark gray-black thin-bedded chert intercalated with siltstone, calcareous sandstone and grayish black shale, thick-bedded pelitic siltstone and micritic sandstone. It is 256 m thick.
Lithology-pattern: Chert
Relationships and Distribution:
Lower contact:
The formation is probably conformably underlain by the Erlangshan Fm of the Upper Ordovician.
Upper contact:
The formation is conformably overlain by the Luoquanwan Fm.
Regional extent:
The Yuanyangyan Fm is only exposed in the Erlangshan area in west Sichuan Province.
GeoJSON:
Fossils:
Graptolites
Age:
This is a formation of graptolite facies from the Glyptograptus persculptus-sinuatus transient zone to the D. convolutus zone. Except the base at present having been delimited to the latest stage of Ordovician, the remained parts belong to from Rhuddanian to early-middle Aeronian age. It is still to be clarified whether there is formation correspondent to the M. sedgwickii zone (Jin Chuntai et al, 1989).
Age span:
Beginning stage: Hirnantian
Fraction up in beginning stage: 0.5
Beginning date (Ma):
Ending stage: Aeronian
Fraction up in ending stage: 0.5
Ending date (Ma):
Depositional setting:
Depositional-pattern:
Additional Information
The Lengji Bed was proposed by Tan Xichou, Li Chunyu (1929~1935) (Please see Li Chengsan et al, 1940). The type section is located at Lengji, southeast of Luding in west Sichuan Province. Li Chengsan, Yuan Jianqi and Guo Lingzhi held that the Silurian strata defined by Tan and Li were not located at Lenji, but at the opposite side of Longbapu and at Foeryan, and thus renamed it the Foeryan Bed, which comprises the (Nautilus-bearing) limestone of the Ordovician in the lower part and the graptolite shale (containing very rich graptolite including the elements of Rhuddanian—early Aeronian age) in the upper part. These graptolite-yielding strata are, in fact, similar to the Yuanyangyan Fm. The above-mentioned two names, the Lenji Bed and the Foeryan Bed, have almost been forgotten because of having long been out of quotation.