Yantang Fm
Type Locality and Naming
The type locality is located in Heiyantang, Yanyuan County, Sichuan. Named by The Second Division of the Survey Team of Sichuan First District of the Geological Bureau in 1971. The original Yantang Formation refers to a set of siltstone-dominated strata located between the Lower Triassic Qingtianpu Formation and the Middle Triassic Baishan Formation in eastern Sichuan. The age is determined to be the early Middle Triassic. In 1978, Regional Stratigraphic Table of the Southwest Region, Sichuan Province assigned the lower limestone section overlying the Baishan Formation to the Yantang Formation. In 1992, Wang Zunzhou et al. classified the clastic rocks in the lower part of the Yantang Formation into the Qingtianbao Fm (Qingtianpu Fm), and restricted the lithostratigraphic range of the Yantang Formation to only include the limestone in the upper part of the original Yantang Formation. We followed the usage of Regional Stratigraphic Table in this book
Synonym:
Lithology and Thickness
The Yantang Formation is mainly composed of a set of clastic rocks and carbonate rocks. Lower part is gray, grayish green medium-thick layer fine-medium-grained lithic sandstone and silty mudstone, mudstone. Middle part is grayish green, yellowish green medium-thick layer, massive fine-grained lithic sandstone and dark purple, yellowish green silty mudstone, calcareous mudstone, muddy siltstone thick-unequally interbedded, intercalated with gray massive tuffaceous silty limestone. Upper part is dominated by gray, dark gray thin-thick layers of microcrystalline limestone and dolomitic limestone, intercalated with bioclastic limestone, muddy limestone and sandstone. The limestone content gradually increased from bottom to top from the middle. The total thickness is 1252～1451 m.
Relationships and Distribution
Lower contact
Conformable contact between the grey feldspar sandstone in the bottom of this Formation and the underlying purple-red pyroclastic rocks of the lower Triassic Qingtianpu Fm.
Upper contact
The top is marked by the conformable first appearance of dolomite at the base of the overlying Baishan Fm.
Regional extent
Distributed in eastern Sichuan, the lithology and thickness vary greatly. From Yanyuan Krabi and Heiyantang to the north and to the east and west, the clastic rocks gradually decrease, and the carbonate rocks gradually increase. In the Yanyuanwei City and Baishan areas, except for the lower part (260 to 280 m thick) which is clastic rock, the upper part (thickness greater than 1000 m) is dominated by limestone with thin layers of clastic rock, with a total thickness of 1272-1488 m. To the north, at the bottom 16.6 m, there is clastic rocks interbedded with limestone lens, and the upper part is all carbonate rock, with a total thickness of more than 900 m.
GeoJSON
Fossils
The fossils contained are mainly bivalves, can build Costatoria goldfussi mansuyi-Asoella illyrica assemblage, a small number of gastropod fossils of brachiopods can also be seen; the upper part contain ammonoid Leiophyllites cf. confucii.
Age
Depositional setting
It is interpreted as shallow sea sediments.
Additional Information