Dangduo Fm
Type Locality and Naming
The type section is located in Dangduo gully, 24 km northwest of Diebu County, Gansu Province. It was named by Zhang Yan in 1961. Qin Feng and Gan Yiyan formally quoted it in 1976.
Lithology and Thickness
Siltstone, sandstone with iron beds. This formation is divided into three members.
The Lower member is composed of dark-gray thin- to medium-bedded micrite or sparite limestone, sandy shale and clayey siltstone.
The Middle member is composed of dark-gray silty shale intercalated with barringerite or siderite nodules and lenses.
The Upper member is composed of grayish-green, purplish-brown medium- to thick-bedded quartz sandstone, siltstone and many beds of chlorite-bearing lime-grain hematitic ferruginous rock.
The thickness is 80.6 m in total.
Relationships and Distribution
Lower contact
Disconformably overlies the Gala Fm (column 40a) or the Duola Fm (not in English Lexicon set)
Upper contact
It conformably underlies the Lure Fm.
Regional extent
The formation is well developed in west Luqu and Diebu areas, where the thickness ranges from 85 to 115 m with the thickest being 118 m at the Dangduo gully stratotype. Eastwards from Dangduo gully, it rapidly becomes thinner; and the iron-bearing clastic rock of its upper member directly overly the lower parts of the Lower Devonian or the Silurian, and the basal strata is commonly a calcareous breccia bed of 0.5 to 8.9 m in thickness.
GeoJSON
Fossils
The formation contains abundant fossils with coral reef and shelly beach deposits. There are five brachiopod zones distinguished from bottom to top: the Cymostrophia-Devonochonetes assemblage zone, the Otospirifer acme zone, the Euryspirifer-Rostrospirifer assemblage zone, the Acrospirifer-Parachonetes assemblage zone and the Athyrisinopsis uniplicata acme zone. In the upper part, the conodont Eognathodus bipennatus montensis is reported.
Age
Depositional setting
The lower part of the Formation was formed in a depositional setting that started with a shallow offshore open platform and then changing into platformal basin; and the upper part belongs to alternating neritic shelf, carbonate platform and marine-beach facies.
Additional Information
The formation contains an important iron-bearing bed in west Qinling area.