Purco Fm
Type Locality and Naming
It was proposed by the Party of Regional Geological Survey, Xizang Bureau of Geology in 1985 and published by Guo Tieying et al in 1991. The type section is located on the northern bank of Yinshuihe River in Domar Township, Rutog (Dêrub) County, the Xizang Zizhiqu.
Synonym: (普尔错组) , Puercuo Fm
Lithology and Thickness
The Purco Fm, about 800 m in thickness, consists of gray, light gray medium-bedded limestone, bioclastic limestone intercalated with quartzose sandstone and siltstone.
The Purco Fm was once split into 3 formations, i.e., the Longmu Co Formation of Lower Silurian, Danmianshan Formation of Middle-Upper Silurian and the Xinxinghu Formation of Upper Silurian by Xia et al. (2009). They also studied the environment changes from open sea platform to tidal flat via blocking gulf and delta in Longmu Co area. However, the ages of the newly named formations were speculated without any explicit evidence. Therefore, the authors retain the Purco Fm until new evidence supports the idea to disintegrate the formation.
Relationships and Distribution
Lower contact
It rests conformably on the Yinshuihe Fm of the Upper Ordovician (not in Ordov lexicon). Regionally, the next older unit is the Xialazi Fm of the Upper Ordovician.
Upper contact
It is disconformably overlain by the Middle Devonian
Regional extent
It is exposed in the area of Yinshuihe River — Wanquanhe River, shaped in nearly latitudinal narrow band of about 40 km long and upmost 5 km wide at the middle sector. The clastic rock increases considerably from Yinshuihe River northwards to Shouxinhu Lake and the granularity changes from fine-grained to coarse-grained as well; slightly metamorphosed lithic sandstone and phyllite may locally appear; the thickness of quartzose sandstone and lithic sandstone increases from 350 m to more than 900 m, and the total thickness from 800 to 1300 m.
GeoJSON
Fossils
The limestone yields rich cephalopods Dawsonoceras sp., Harrisoceroides sp., Geisonoceras cf. rivale, Kopanioceras sp., Harrisoceras sp., Kionoceras sp., Arionoceras submoniilform, Temperoceras (Orthoceras) tritionuum; trilobite Encrinuroides meitanensis, Scotoharpes meitanensis, Latiproetus donqiaoensis; brachiopod Striispirifer sp., Eospirifer cf. subradiatus and crinoid columnal, etc.
Age
Depositional setting
Additional Information
Chen et Zhang (2010) re-measured the sections on the northern bank of the Yinshuihe River and on the northern bank of the Shouxinghu Lake of the Lazhuglung area of Doima, Rutog, NW Xizang, respectively. They divided the Silurian rocks in that area in ascending order into Yinshuihe, Shouxinghu, Purco and Yeniupo formations, of which the Yeniupo Formation is a new lithostratigraphic unit they established. They argued that the Yinshuihe and Shouxinghu formations are Llandovery in age, rather than the Late Ordovician, the Purco Formation is Wenlock in age and the Yeniupo Formation is the Ludlow-Pridoli in age. The idea is significantly different from the idea previously accepted. The different subdivisions between the Longmu Co area and the Lazhulung area, indicate the complexity of the Silurian rocks itself and the contradiction between different experts. Although the authors believe the fossil evidence Chen et Zhang (2010) provided, the subdivision and correlation of the lithographic units in this area should be proposed more cautiously.