Xujiahe Fm
Type Locality and Naming
The Xujiahe Formation originates from a set of coal-bearing strata from the second member of the original "Xujiahe Formation" in the northern Sichuan Basin to the quartz conglomerate of the Baitianba Fm. The type locality is located in the Xujiahe area a dozen miles north of Guangyuan, Sichuan. In 1931, Zhao Yazeng and Huang Jiqing changed the "Guangyuan Coal Measures" of Lichhofen in 1872, named Xujiahe Coal Measures, and the age is Jurassic. In 1964, Chen Chuzhen called it Xujiahe Formation, and the age was set from the Late Triassic Norian to the Rhaetian. In 1974, the Mesozoic Emei Meeting in Southwest China re-defined the Xujiahe Formation and divided the first member of the original Xujiahe Formation into another named Xiaotangzi Fm. The upper boundary is between the quartz conglomerate of the overlying Jurassic Baitianba Fm and the underlying Upper Triassic Xiaotangzi Fm (the first member of the former Xujiahe Formation) (Marine member), most of them overlapped over the old strata and were dated to the middle and late Triassic.
Lithology and Thickness
The Xujiahe Formation is mainly composed of a set of coal-bearing strata composed of black carbonaceous shale and gray sandstone, and it could be divided into 5-6 members. Members 1, 3, and 5 are mud-and-coal-bearing shale intercalated with sandstone; Members 2, 4, and 6 are sandstone intercalated with shale and coal measures. However, the development degree of each member is different. Generally, the fourth member is divided into upper and lower subgroups with quartz sandstone intercalated with glutenite (=gravel-rich) as the boundary.
Relationships and Distribution
Lower contact
At the western bottom of the basin, the Formation is in pseudo-conformable contact with the underlying Xiaotangzi Fm as breccia; at the eastern bottom of the basin, it overlaps over the old strata.
Upper contact
The contact between the top of the Formation and the overlying Baitianba Fm is mostly pseudo-conformable.
Regional extent
Widely distributed in Sichuan Basin, Shiqijin River in the west of the basin is the thickest, up to 3000 m. The thickness at the Pengxian Cifeng field is 2211 m, and that at Dayi Wuzhongshan is 2037 m. The thickness decreases to the east and gradually loses; and the thickness also becomes thinner north to south, the lower part is well developed, and the upper part is eroded.
GeoJSON
Fossils
Plants: Dictyophyllum nathorsti-Clathropteris meniscioides assemblage; bivalves in the upper and lower part: Yunnanophorus boulei-Trigonadus keuperinus assemblage.
Age
Depositional setting
It is interpreted as river alluvium-lake deposits.
Additional Information