The Nantuo Formation is a Cryogenian aged geologic formation in Zigui County, Hubei Province of South China. The formation contains the Songluo Biota, a collection of large macro-algae fossils dated to around 650Ma during a notably short break in the Marinoan glaciation. The Songluo Biota is currently the only known macrofossil biota from the Cryogenian.
Geology
The stratigraphy of Neoprotozoic rocks in the Miaohe region
Due to the large area that the Nantuo Formation covers, the geology and any noted members within the formation differ from town to town within the Hubei Province. These different outcrops are listed below, under the respective regions or towns they are found in or near to:
Miaohe Outcrop
The Nantuo Formation can be found outcropping along the western edge of the Huangling Anticline in Hubei Province in South China near to the village of Miaohe, where it is overlain by the Ediacaran-aged shale rocks of the Doushantuo Formation, whilst it is underlain by the quartzsandstones of the Liantuo Formation, although the contact point between the Nantuo and Liantuo Formations remains unknown as they are not exposed. The Nantuo Formation here is predominately composed of greenish tillite, roughly 100m (330ft) in thickness. Within the tillite can be found a varying range of rock fragments, from rounded to angular, all composed of schist and granite.
Wangjiawan Outcrop
The formation can also be found outcropping near the village of Wangjiawan, where it is overlain by the sandstones rocks of the Lunasi Formation, whilst is it potentially unconformably underlain by the conglomerate rocks of the Chengjiang Formation, although the contact point is not easy to locate. The formation at Wangjoawan is considerably thinner, only attaining a thickness of roughly 27m (89ft). The lower 10m (33ft) are still composed of the greenish tillite, whilst the upper 17m (56ft) are instead composed of red shales.
Shennongjia Outcrop
The formation is also known to outcrop in the Shennongjia area, although this outcrop is noted to have not been well studied as others until recently. Here, the formation is again overlain by the Doushantuo Formation, although this time it is underlain by the Datangpo Formation at the town of Dajiuhu, unconformably underlain by the Shennongjia and Macaoyuan Groupss at the town of Songbai and the Macaoyuan region respectively, whilst in the Three Gorges region it is unconformably underlain by the Liantuo Formation. The formation here attains its greatest thickness, coming in at around 300m (980ft), and here it is predominately composed of dark orange, massive glacial diamictites beds up to 150m (490ft) thick, dolostone dominated clast conglomerates, lonestone-bearing sandstones, yellowish-green siltstone, and black shales which are fossiliferous in nature.
Nantuo Outcrop
The Nantuo Formation can also be found outcropping near to Nantuo village, where it is overlain and underlain by the same formations seen at Miaohe, but shows a similar thickness to the Wangjiwan outcrop, coming in at 26m (85ft). Here, there can also be found three distinctive members, the basal Gucheng Tillite, the middle Datangpo Member which is composed of dark siltstones, shales and limestones, and then upper Nantuo Tillite found throughout all known outcrops.
Dating
Biostratigraphic correlation table of Neoproterozoic formations in South Anhui and Hubei, China, including the Nantuo Formation
Prior to proper dating, the Nantuo Formation had been correlated with the Leigongwu Formation in Anhui Province. Since then, more refined dates have been yielded using radiometric dating, which recovered an age range of 650 - 635Ma for the Nantuo Formation, which thus places the entire formation within the Marinoan glaciation that occurred at the end of the Cryogenian.
Taphonomy
The fossils of the Nantuo Formation are preserved as carbonaceous films, similar to a Burgess Shale type preservation style.
Paleoenvironment
The overall environments of the Nantuo Formation at the time of deposition were likely to have been ice-proximal glaciomarine to subglacial, with some layers showing a distal glaciomarine environment brought on by icebergs and turbidity currents, all of which has been inferred by the presence of the massive diamictites, conglomerates, and sandstones of the Shennongjia region. Meanwhile, the Songluo Biota was notably deposited with no signs of glacial activity, inferred from the shales and siltstones, with the area likely being deposited on a distal shelf area away from the margins of the large ice sheets of the late Cryogenian.
Paleobiota
The Nantuo Formation is home to the Songluo Biota, which, whilst it is less diverse than fossil assemblages that came before and after, is notable in that it is currently the only macro-algae assemblage known from the Cryogenian. It shows that even though macro-algae likely suffered a substantial extinction worldwide during the major glacial periods of the Cryogenian, they managed to persist even at less diverse numbers, further helped by short but vitally importance appearances of an open-marine environment during the Marinoan Glaciation. The biota is found within the Shennongjia outcrops of the formation, and is composed of relatively simple macro-algae forms, such as the filamentous Vendotaenids.
Macroalgae, with a similar appearance to Wenghuiphyton
References
1234567Hu, Jun; Li, Chao; Tong, Jinnan; Ye, Qin; Tian, Li; An, Zhihui; Dodd, Matthew S.; Algeo, Thomas J. (December 2020). "Glacial origin of the Cryogenian Nantuo Formation in eastern Shennongjia area (South China): Implications for macroalgal survival". Precambrian Research. 351 (105969). doi:10.1016/j.precamres.2020.105969. ISSN0301-9268.
