Emeishan Basalt Fm
Type Locality and Naming
The type section is section is located at Emei Mt., Sichuan Province. It was named by Zhao Yazeng in 1929.
Synonym: Omeishan Basalt; Emei Mt. Basalt
Lithology and Thickness
Volcanics and volcaniclastics. This group of continental flood tholeiite and basaltic volcaniclastic rocks intercalated with a small amount of marine and continental deposits is commonly divisible into three members: The First member being composed mainly of black-grey to blackish-green breccia-rank and block-conglomeratic-rank coarse-grained volcaniclastics and lavas, which in the western proximal area are commonly intercalated with claystones yielding a small amount of plant fossils (phytolites); while the volcaniclastic rocks in the eastern part of the area of its distribution are predominated mostly by sedimentary volcaniclastic rocks with their clastics being possessed of chilled margins. The cements of the normal sediments commonly contain bioclastics of sponge spicules, echinoderms, foraminifera and Fusulinida, etc., with a thickness of 10-165 m, being thicker in the west and thinner in the east. The Second member is composed of black-grey to dark grey-green massive basaltic lavas, including tholeiite, dolerite, basaltic porphyrite, interstitial basalt and chilled clastogene basalt, intercalated occasionally with vitric tuff, with well-developed prismatic joints, being commonly possessed of blisters and amygdaloidal structure, with the cements in between the chilled clastogene basaltic fragments containing such bioclastics as sponge spicules, and with the basal basaltic conglomerate with a thickness of several meters to tens of meters being in a disconformable contact with the First member. The Third member is composed mainly of interbeds of grey and grey-green fairly thin-bedded fine volcaniclastics or normal fine volcaniclastics together with the lavas, with the coal-bearing clastic rocks occurring in the western part of the area. The cement of the sedimentary volcaniclastics and claystone interbeds yield brachiopod fossils, with its thickness getting smaller from west to east in a range of 300 m to several meters. Average total thickness of 700 m (but see variability under Regional extent).
Relationships and Distribution
Lower contact
Disconformable contact with the underlying Maokou Fm, in between which there occurred commonly tens of meters of Fe-Al-rich (ferralitic) claystone.
Upper contact
There are two types of its contact with the overlying strata. In the eastern part of the area there a gradual transition between it and the overlying Longtan Fm., with the final disappearance of basaltic lava or volcaniclastics serving as a boundary between them; while in the western part of the area the formation is in a disconformable contact with the overlying Longtan Fm or the coal-bearing clastics of the Xuanwei Fm, and with a clear boundary between them.
Regional extent
Distributed in East Yunnan and West Guizhou with a distribution area of 330 thousand km2, and with an average thickness of 700 m, but on the western margin of the distribution region its average thickness is of 1300 m, as well as of 5000 m at Binchuan, of 2700 m at Wuding and of 2850 m at Yanyuan in North Yunnan. There were commonly formed 3 to 4 cycles of development, with the red tuff occurring commonly at the end of a cycle as a “red top” of it. But in most areas of its distribution the thickness of the formation is lesser than 500 m. In the Lijian River area the lower part of the formation is intercalated with limestone which used to be known in other name as the Xipiaoluo Fm, while the upper part of the formation is composed of marine and continental depositional interbeds and coal seams which used to be known as the Yangjiaping Fm ("Regional Geological Memoirs of Yunnan Province", 1990), with a thickness of 715-3438 m. The formation is possessed of picrite in a belt extending from Yanyuan to Muli. In the area of Kam-Yunnan Fracture it is the ultra-basic effusive rocks that are predominant, while in the Mianning and Ganluo areas it is the basic effusive rocks that are predominant, with only a small amount of intermediate dacite being found there, with a thickness of 1000 m. In such marginal areas as the Gongxian County, Huaying Mt. and Daxian County the basalts are interbedded with siliceous rocks.
GeoJSON
Fossils
First member in the Jianshi area of Yunnan Province, and in the areas of Jijin, Qinglong and Shanding of Guizhou Province has yielded Metadoliolina sp. and Neoschwagerina douvillei fossils.
Age
Depositional setting
The volcanic episodes are interpreted as an eruption of a large igneous province.
Additional Information
Dating of the main eruptive phases of Emeishan large igneous province imply a correlation with the onset of the earliest Wuchiapingian episode of global cooling and with the beginning of Australian glacial episode P4.