Hekou Fm

Guangxi District Survey Team comprehensive group named in 1975. The named section is located in Bafeng-Hekou, Li Zhouxu, Tianlin County, Guangxi; the reference section is located in Baise Baikang, Guangxi.

Synonym: Zhonghekou Fm. The Middle Triassic strata in western Guangxi have been successively called "Pinghe relationship" (Xu Ruilin, 1933), "Pingheguan Formation" (Zhao Jinke, 1962), and "Pingheguan Group" (Guangxi Geological Research Institute, 1962). Guangxi Petroleum Census Brigade (1961), when studying the Lanmu section in Donglan County, named the Middle Triassic Latin Stage strata as "Lanmu Group". The Guangxi District Survey Team (1975) also used the named section as the basis to create the name Hekou Formation, and determined its age as the Middle Triassic Ladinian. Yunnan-Guizhou-Guizhou Petroleum Geology Institute (1983) named the lower part of this group in Yangqigou, Qubei County, southeastern Yunnan as "Lanmu Fm" and the upper part of the 355 m formation containing early Late Triassic fossils as "Hongwan Fm"). It is considered that the upper part of this group is a set of continuously deposited turbidites, and it is still appropriate to collectively call it the Hekou Formation.

It is sand and mudstone interbedded or mudstone intercalated with siltstone, fine sandstone and tuff. It can be divided into two sections, the lower part of each section generally has a set of sandstone as a stratification mark, and gradually changes to mudstone and calcareous mudstone intercalated with tuff lenticular body upward. The structure and structure of turbidite sedimentation are generally developed, except for the lower part or the bottom, where sandstone or siltstone is dominant and forms small and medium rhyolite with mudstone, and the development of scouring phenomenon, envelope lamellae and oblique lamellae at the bottom, while the middle and upper part are basically dominated by mudstone, with only local thin layer of siltstone or fine sandstone.

The lower part produces ammonoids: Protrachyceras deprati, P. douvillei, Trachyceras sinense, Megaphyllites cf. jarbas, M. sp., Rimkinites sp., Celtites sp.; bivalves: Daonella lommeli, D. indica, Posidonia wengensis, Halobia rugosoides, H. planicosta, H. kui, H. subcomata, etc.; distally produced ammonoids: Paratibetites clarkeii, Cyrtopleurites sp.; and bivalves: Halobia rugosa; in addition, this group also contains brachiopods and corals.

This group is a deep-water terrestrial source clastic turbidite facies.