Gala Fm
Type Locality and Naming
The type section is located in Dangduo gully, 24 km northwest of Têwo County, Gansu Province. It was named by the Xi’an Institute of Geology and Mineral Resources and the Gansu No 1 Team of Regional Geological Survey in 1973. Qin Fong and Gan Yiyan formally quoted it in 1976.
Lithology and Thickness
Dolomite. The formation is dominated by dark-gray massive to thick-bedded algae dolomite and stromatolite-bearing and stratifera-bearing dolomite, intercalated with a few sandy dolomitic slate, dolorudite and microcrystalline dolomite. The top is dominated by siltstone and fine-grained sandstone, intercalated with thin- to medium-bedded sandy cryptocrystalline dolomite and sandy marl. It is 989.3 m in total thickness.
Relationships and Distribution
Lower contact
It conformably overlies the Shangputonggou Fm.
Upper contact
It conformably underlies the Dangduo Fm.
Regional extent
The formation, exposed in the area from Luqu (Ma’ngê) to Têwo counties in western Qinling Mts is consistent in lithology and thickness. However, it is 513-598 m thick in Luqu to the west, 857-990 m thick from Putonggou to Dangduogou, and then eastwards from Dangduogou it sharply thins to only 91 m at Yiwagou; and further eastwards this formation is absent at Rangdag and Zhugqu, and the outcropping thickness is only 12 m at Jiaogonggou in Wudu County.
GeoJSON
Fossils
Fossils are rare, and include corals: Chalcidophyllum, Lyrielasma, Siphonophrentis and Favosites lazutkini, Squameofavosites etc. in the middle and lower parts; and conodont Polygnathus declinatus, ostracod Moelleritia elongata and bivalves etc. in the upper part.
Age
Depositional setting
It is interpreted as a semi-restricted platform tidal flat facies.
Additional Information
Four depositional cycles are differentiated in this formation and every cycle includes several smaller cycles, each of which comprises supralittoral, intertidal and subtidal zones.
The subtidal zones are mainly composed of laminated dolomite with burrows and complete individuals of ostracods;
The intertidal zones contain stromatolites, oncolites, algal pellets and bird's-eye structures, with alternating light and dark bands;
The supralittoral zones possess involution structures, gypsum vugs, erosion surfaces and crack structures.