Dargonuma Fm

Xainza Co. The Dargonuma Formation was established by Wang Naiwen in 1983. The section measured for the designation is in Daguo of the Xainza County (86°40′E; 30°42′N). The reference section is in Jiega of Bangba district of the Ge’gya County, Xizang (81°46′E; 31°45′N).

In 1957, Wang Wenbin from the Qinghai Petroleum Prospecting Brigade under the Ministry of Geology and Mineral Resources called the set of limestone with sandy shale, conglomerate, sandstone and mudstone bearing Early Cretaceous bivalves as the Burga rock series in Burga of the Xainza County, which was assigned to Early Cretaceous. In 1983 the Xizang Regional Geological Survey Team of the Xizang Bureau of Geology and Mineral Resources called the limestone and bioclastic limestone of the upper Burga rock series as the Burga Fm, and sandstone, shale and volcanic rocks as the Zenong Gr in the report on the 1:1000000 Xigaze Map of the Regional Geological Survey, which was assigned to Early Cretaceous. In 1983 Wang Naiwen called the mainly intermediate volcanic rocks with tuffaceous sandstone, sandy limestone and tuffaceous shale bearing Orbitolina with a thickness of more than 3000 m as the Dargonuma Formation, which was assigned to Cretaceous or partially Late Cretaceous. In 1987 the Cretaceous in Wuma, Yuduo-Geji- Shiquanhe areas of the Gerze County was called in a descending order as the Jiangba Fm consisting of pyroclastic and clastic rock formation, the Yuduo Fm consisting of neritic carbonate rock with volcanic rock formation, and the Weien Fm consisting of continental-littoral clastic rocks and volcanic rock formation in the reports of 1:1000000 Rutog Map of Regional Geological Survey. In 1987, in the report of 1:1000000 Gadak Map of regional geological survey, the set of volcanic rocks with Orbitolina limestone or carbonate rock with volcanic rocks was called as the Jiega Formation in the Chalicuo, Yare, Sailipu, Jiega and Xiaguo area of Ge’gya County. They are of late Early Cretaceous - early Late Cretaceous age. In the Xizang Annals of Geology compiled in 1993, the Zenong Group from the report of 1:1000000 Xigaze Map of regional geological survey continued to be used, which was assigned to Early to Late Cretaceous. The set of volcanic rocks with bioclastic limestone, Orbitolina limestone and limestone deposited continuously on the Zenong Group are called the Jiega Formation in the report of 1:1000000 Gadake Map of the regional geological survey, but were assigned to the Albian stage of Early Cretaceous. It is considered in this lexicon that the Jiega Fm and the Dargonuma Fm are synonymous, and the Burga Fm is equivalent to part of the Dargonuma Fm (or the Jiega Fm). In order to follow the principle of nomenclature and on the implication of the original designator, the set of the strata in Xainza- Wuma of Gerze-Ge’gya-Shiquanhe is named as the Dargonuma Fm and assigned to late Early Cretaceous - early Late Cretaceous; while the strata under them are called the Zenong Gr of Early Cretaceous age.

The Dargonuma Formation is represented by intermediate volcanic rocks intercalated with limestone, sandy limestone, tuffaceous sandstone and shale. Lower part is built up by purple gray and gray white medium and thick-bedded massive andesite, green gray and brown gray siltstone, gravel-bearing grit and siliceous rocks with stratified andesite and phyllitic slate, intercalated with gray and light gray medium and thin-bedded quartzite and Orbitolina limestone, also including corals, bivalves, brachiopods, etc. Upper part is built up by purple gray thick-bedded massive andesite, black gray stratified basalt, andesite, dacite and rhyolite, intercalated with gray medium and thin-bedded Orbitolina limestone and bryozoans. The Dargonuma Formation is 500-6000 m thick.

The formation yields foraminifera Orbitolina sp.; bivalves Bournonia haydebni, Cuculaea cf. forbesi; Gastropods Nerinea coquandi etc. in Xainza; Foraminifera Orbitolina conica, O. cf. scutum, O. cf. aperta, Stomatoecha plummerae, etc. in Coqen; foraminifera Orbitolina spp., bivalves Ostrea sp., Pleuromya sp., Lophasp, Chama sp., Neithea (Neitheops) quadricostata, Biradiolites sp.; Gastropods Nerinea cf. pauli, N. cf. libanolica, Nerinella cf. dayi, Plesioptyxis aff. langshanensis, Ampullian xainzanensis, Conjusiscala sp., etc. in Ge’gya.