Dushan Fm

The lectostratotype section is located near Jiwozhai and Songjiaqiao Villages, east of the Dushan county (GPS 25°50′56.12″N, 107°34′32.74″E), Guizhou Province. The reference section is at Dahekou, east of the Dushan county seat, Guizhou Province. It was named by Ding Wenjiang in 1929 and quoted formally by Grabau A. W. in 1932.

Limestone. This formation is composed of three members (The tripartite subdivision seems to be only applicable to the Dushan and the Mochong--Bingwu areas of Duyun).

Lower - Jipao (Chipao) Member, consists of grayish-black medium- to thick-bedded limestone, and thin-bedded nodular limestone with reef limestone (coverstone, bufflestone and bindstone mainly) and rare interbed of fine grained sandstone. Several shallowing upward cycles starting from reef carbonate to Amphipora, fenestral limestones and culminating in mud cracked algal dolostone are recognized. A bed of breccia limestone presents in the top, 146.7 m thick.

Middle - Songjiaqiao (Sunchiachiao) Member, 88.7 m thick, is grayish-white, yellow brown thick-bedded quartz sandstone, gravel-bearing sandstone and purplish-red ferruginous quartz sandstone, intercalated with grayish green thin bedded siltstone and shale. Large planar cross bedding and oblique bedding are developed.

Upper - Jiwozhai (Chiwochai) Member, 100.8 m thick, is composed of gray to dark-gray thick-bedded to massive blocky bioclastic limestone with bioherms and fringing reefs, intercalated with grayish black carbonaceous clayey limestone and calcareous shale in the lower and light gray, grayish-black argillaceous limestone, siliceous limestone and carbonaceous shale in the upper, a thick dolomitic limestone, 23.5 m thick, is at the top.

[Figure Outcrops of the Dushan Formation in the type locality. A-Limestone of Jipao Member forming escarpment, north bank of Duliujiang River. B-Close-up of the Jipao Member, near Jipo. C-Close-up of Songjiaqiao Member. D- The layered or lenticular limestones formed by accumulation of transported organisms (massive stromatoporoids, rugose and tabulate corals) in Jiwozhai Member.(Photographs kindly provided by Guo W.)]

[Figure Outcrop of the Lower part of Jiwozhai Member, Dushan Formation in the type locality with a fringing reef (yellow rectangle, 7 m2) constructed by lamellar stromatoporoids (coverstone), Stachyodes and Thamnopora (bafflestone)and algae bindstone (After Huang et al., 2020).]

Abundant and diversified fossils are reported from Jipao and Jiwozhai Members including rugose corals, tabulate corals, brachiopods, stromatoporoids, chaetetids, and relatively scarce bryozoans, crinoids, and gastropods. Only plant fragments are seen in the Sonjiaqiao Member.

Jipao Member: Brachiopods Stringocephalus burtini, S. obesus, Bornhardtina transversa, Acrothyris kwangsiensis, Indospirifer sp., Emanuella sp. Corals: Hexagonaria beichuanensis, Pseudomicroplasma flabelliformis, Mesophyllum m. maximum, Disphyllum hsianghsienense,Temnophyllum majus, Grypophyllum convolutum dahekouensis, Mixogonaria jipaoense, Sociophyllum jumuwanense, Dendrostella tunshangensis, Columnaria spinosa.

Jiwozhai Member: In addition to those mentioned above it contains numerous stromatoporoids and bryozoans. The other Brachiopods are Emanuella takuanensis, E. plicata, Undispirifer undiferus, Spinatrypa sp., Desquamatia sp., Athyris subplana, Rugose corals Stringophyllum jipaoense, Phacellophyllumsinense,Argutastrea elegans, Sunophyllum regulare Mictophyllum shawoziense, Spinophyllum zhongguoense, Grypophyllum jiwozhaiensis, Temnophyllum sinense etc. Stromatoporoids: Trupetostroma dushanense, Actinostroma undulatum, Clathrocoilona sp., Sticotostroma sp., Stachyodes sp. etc. Tabulate corals: Thamnopora compacta, Cladopora fistula, Syringopora sp. Aulopora sp., etc.

Wide range of lithology reflects very different sedimentary environments, the Jipao and Jiwozhai Members represents an open marine shallow water environment of a carbonate shelf, while the Sonjiaqiao Member was deposited in a foreshore (delta and river mouth) setting.