There are many fossil sites, particularly in the Mia Le (D1ml), Na Quan (D1-D2enq), Toc Tat (D1-C3ttt), Bac Son (C-Pbs), Dong Dang (P3dd), Song Hien (T1sh), and Na Duong (E3-N1nd) formations.
The Mia Le formation have diverse rock composition of clayish, silty clayish shale, siltstone, sandstone interbedded with some siliceous rocks, argillaceous limestone, siliceous limestone, and a few limestone lenses located in narrow strips scattered in almost every district in Cao Bang Geopark. The formation has an abundance of fossils, the most common of which are brachiopods, anthozoa and bivalves.
There are dozens of fossil sites including:
– Brachiopod and Anthozoan sites in Hanh Phuc commune, Quang Uyen district (coordinates X=648129; Y=2499178; Z=295m asl);
– Brachiopod and Anthozoan sites in Lang Mon commune, Nguyen Binh district (coordinates X=616924;
Y=2506353; Z=302m asl);
– Brachiopod, Anthozoan and Bivalve sites in An Lac commune (coordinates X=665174; Y=2511480; Z=449m asl) and Cai Bo commune (coordinates X=659376; Y=2511183; Z=445m asl). Brachiopod sites in Minh Long commune (coordinates X=680376; Y=2522841; Z=350m asl) and Thang Loi commune (coordinates X=674781; Y=2517108; Z=452m asl). All these sites are in Ha Lang district;
– Brachiopod sites in Duc Quang commune (coordinates X=667350; Y=2520338; Z=591m asl), Trung Khanh district. Because the formation often outcrops in narrow strips along with the overlapping Na Quan formation, fossils typical of the latter can also be found at the same sites.
The Na Quan formation mainly consists of argillaceous and siliceous limestone, scattered in strips together with rocks of the Mia Le formation. The most common and conspicuous fossils are stromatoporoids, followed by anthozoa, fusulinacean and bivalves as follows:
– Stromatoporoid site in Hanh Phuc commune, Quang Uyen district (coordinates X=649348; Y=2498589; Z=266m asl), almost at the same position as the afore-mentioned Brachiopod and Anthozoan sites;
– Stromatoporoid site in Duc Quang commune, Trung Khanh district (coordinates X=662890; Y=2521870; Z=601m asl), almost at the same position as the afore-mentioned Brachiopod site;
– Stromatoporoid site (coordinates X=652204; Y=2487055; Z=328m asl) and Anthozoa site (coordinates X=652312; Y=2486316; Z=382m) in Thuy Hung commune, Thach An district;
– Stromatoporoid site in Danh Sy commune, Thach An district (coordinates X=665971; Y=2482567; Z=277m asl);
– Fusulinacean site in Phi Hai commune, Quang Uyen district (coordinates X=644934; Y=2513275; Z=503m asl);
– Bivalve site in Luu Ngoc commune, Tra Linh district (coordinates X=631882; Y=2522933; Z=642m asl).
The major composition of Toc Tat formation is the red banded limestone interbedded with thin to moderately-bedded limestone, found in many places in Cao Bang Geopark and often located with the Lung Nam formation at the base of Bac Son formation. There is an Ammonitida site in the kink fold range in Lung Luong, Chi Vien commune, Ha Quang district (coordinates X=609861; Y=2536500; Z=607m asl).
The Bac Son formation has large outcrops within Cao Bang Geopark mainly composed of limestone, dolomitized limestone with abundant fossils including foraminifer, anthozoa and crinoid. Crinoid and anthozoa are found in almost every limestone outcrop of Bac Son formation, for example, Phuc Sen forging village, Quang Uyen district (coordinates X=645882; Y=2509448; Z=533m asl).
The Dong Dang formation (P3dd) includes cornstone, bauxite, claystone and limestone in small outcrops, scattered in Ha Quang, Tra Linh, Hoa An and Trung Khanh districts and having both an upper and lower unconformity relationship with the Bac Son formation (C-Pbs) and Song Hien formation (T1sh). This is considered a very significant formation, especially at its upper boundary – P/T boundary – indicating one of the major biotic extinction events in the past. The formation contains a wide range of fossils including calcareous algae, foraminifer, rugose, brachiopod, bivalves, gastropod, ammonitida, trilobite and bryozoan etc., typically in Noi Thon commune, Ha Quang district (coordinates X=622206; Y=2535902; Z=910m asl).
The Song Hien formation (T1sh) mainly consists of grit, gravel, sandstone, clay shale, and siltstone interbedded with porphyritic rhyolite, dacite rhyolite, dacite rhyolite porphyry and their tuff, siliceous shale, calcareous claystone lenses, limestone, marblized limestone and dolomitized limestone, mainly distributed west of the Cao Bang-Tien Yen fault, in Ha Quang, Hoa An, Tra Linh and Nguyen Binh districts. Typical fossils are foraminifers and pelecypods, while ammonites can sometimes be found in, for example, Binh Long commune, Hoa An district (coordinate X=615589; Y=2514874; Z=206m asl).