The Lantian Formation is an Ediacaran aged geologic formation in Xiuning County, Anhui Province of South China. Deposited sometime between 635 – 551Ma and composed predominately of dolostones, the formation contains the Lantian Biota, a collection of large macrofossils dated to around 602±7Ma, which includes putative cnidarian-grade fossils and macro-algae.
This biota lived within a deep-water, but photic environment, inferred from the lack of certain sedimentary structures, as well as the presence of certain isotopes. This environment fairly consistently switched between being non-oyxgenated to being oxygenated, which not only permitted the quickened evolution and growth of fossil organisms during oxygenation events, but also preserved them during euxinic periods. The Lantian Biota was previously the oldest known Ediacaran macrofossil biota until the discovery of the Ancanping Assemblage in older rocks of the Doushantuo Formation.
Discovery and naming
Prior to its formal naming, the rocks of the formation would be found by Xing Yusheng, Bi Zi-guo and Xian-fang Wang in 1985, which also saw the discovery of the first macrofossils in the area. In 1988, Bi Zi-guo, alongside a number of other researchers, would publish a paper reporting more on the area and the fossils within. In 1989, Xing, alongside Gao and Liu, would publish a formal description of the whole formation and some of the fossils in The Stratigraphy of China - Vol.3, alongside the formal naming of the "Lantian Formation", taking its name from the nearby Lantian village.
Geology
The stratigraphy of Neoproterozoic rocks in the Piyuan region
The Lantian Formation can be found outcropping in the Anhui Province in South China, being overlain by the siliceous rocks of the Piyuancun Formation, whilst it is underlain by the Marinoan-aged glacial diamictites of the Leigongwu Formation. The formation itself is split up into four members, and is roughly ~150m (490ft) thick.
Members
The Lantian Formation is composed of four formal, but unnamed, members, which are as follows, in ascending stratigraphic order (lowest to highest):
Member I: This member is entirely composed of cap dolostone rock, ranging between 1.8–4m (5.9–13.1ft) in thickness, which conformably overlies the Marinoan-aged Leigongwu Formation below it.
Member II: This member is predominately composed of gray calcereous siltstone, and mudstone in a lower 21–24m (69–79ft) thick layer, all of which is inter-bedded with argillaceous, fine-grained sandstone, as well as finely laminated black shale, which becomes the dominate rock in the upper 40–69m (131–226ft) of the member, where here it is occsionally inter-bedded with argillaceous limestone. These upper shales are fossiliferous in nature, containing the Lantian Biota.
Member III: This member is primarily composed of a gray argillaceous dolostone, which is inter-bedded with black shale, in the lower 26–30m (85–98ft) thick layer, whilst in the upper 40–50m (130–160ft) thick layer, it is composed of thick ribbons of gray limestone intercalated with dolostone.
Member IV: This member is composed entirely of black to dark-gray silty mudstone and shale, up to 10–20m (33–66ft) in thickness, which are overlain by the Piyuancun Formation.
Geobiology
The Lantian Formation was amongst the first deep-water formations to be studied for its geobiology, which at the time had only been done on shallow-water formations. Data and samples were predominately gathered using a drill core close to Lantian Village, with some samples also coming from an outcrop of Member III near to the drill core site. Study of the samples yielded a range of isoptoes, including δ13C carbonate-carbon, carbonate-oxygen, and organic-carbon, δ34Spyrite and carbonate-associated calcium sulfide, as well as sedimentary δ15N.
The δ13C carbonate carbon isotopes from the lower layers present a notable large spatial gradient, with a magnitude of >10‰, which has been inferred to likely be the result of Carbon-13-depleted calcite cements within organic-rich but carbonate-poor sediments, which where deposited in a deep-water facies. This then becomes a notable negative δ13C carbonate carbon excursion in the upper layers, which has been previously described from other areas of South China as the "Ediacaran Negative excursion 3", which itself is correlated to the Shuram excursion. Meanwhile, the δ34S pyrite isotopes present a spatial pattern, and are more negative in deep-water facies, although the δ34S carbonate-associated calcium sulfide isotopes are considerably more scarce, thus precluding any proper determination of spatial patterns for all δ34S isotopes. The δ34S pyrite isotopes themselves are inferred to be evidence of a chemically stratified basin setting, with the environment itself primarily being an euxinic deep-water environment, meaning their was little to no oxygen and an abudance of sulfide. Here, pyrite would have precipitated better in the water column, presenting lower δ34S pyrite values in the deposited sediments as a result, whilst isotopes found in the non-euxinic shallower-water settings would be higher due to the pyrite instead forming within the sediments. As for the sedimentary δ15N isotopes, it has been noted that the recorded isotopes are strangely consistent, especially when compared to the variation seen in modern oceans. The isotopes themselves not highly positive, which is indicative of partial denitrification in suboxic environments, although any further study in regards to the sedimentary δ15N isotopes is precluded, due to its current sparse spatial and stratigraphic coverage.
Dating
Biostratigraphic correlation table of Neoproterozoic formations in South Anhui and Hubei, China, including the Lantian Formation
The Lantian Formation, and more specifically the Lantian Biota, was originally assumed to be late Ediacaran in age, being correlated with the Miaohe biota in the Doushantuo Formation at 551Ma, although a later study in 2011 by Yuan et al. would note that δ13C values in the lowest member where similar to that seen in the lower layers of the Doushantuo, which at this point was dated to 635Ma. Further up in Member III, there is a notably large negative δ13C excursion, similar to what is seen in Member III of the Doushantuo Formation in the Yangtze Gorges area. With this, Yuan et al. would see the Lantian Formation instead correlated to the lower members of the Doushantuo, giving it an increased temporal range of 635 - 551Ma.
A later study done in 2022 by Yang et al. using Re–Os dating in the middle layers of Member II, recovered a date of 602±7Ma, giving an absolute date for the Lantian Biota. From this, they were able to infer that the youngest fossils of the Lantian Biota was older than 580Ma, based on the known dates of the Shuram excursion in Member III, which is 574.0 – 567.3Ma. This would also see the youngest fossils of the Lantian Formation become contemporary with the oldest microfossils of the Weng'an biota in the Doushantuo Formation. There are no known date constraints at the base of the Lantian Formation, with it only being correlated to 635Ma, as such the researchers tentatively put forward a nominal age of 615Ma for the oldest fossils, partially inferred from the absolute date of 602±7Ma, noting an uncertainty ranging within a few million years.
Taphonomy
The fossils of the Lantian Formation are predominately preserved as carbonaceous films, similar to a Burgess Shale type preservation style, and are also randomly oriented on the bedding planes they are found on. The lack of breakages and folding in fossil material suggests that the fossils were not transported, and thus are preserved in-situ. A study done in 2014 found that the preserved material contained densely-packed framboidal pyrites, leaving clusters of cavities within the fossil specimens. It was noted that the pyritization of fossil material likely occurred some time after fossilization of the organisms. The pyrites of fossils in the Lantian Formation are considerably larger than the pyrites seen in the surrounding matrix, which was inferred as the pyrites formed under differing redox conditions.
Paleoenvironment
The environment of the Lantian Formation at the time of deposition was likely to have been below the storm wave base, inferred from the lack of cross-stratification, wave- and current-influenced structures, as well as no grading being found within the laminae of Member II. Alongside this, the aforementioned in-situ preservation also likely means that the environment was well within the photic zone, although was still relatively deep, being somewhere between 50–200m (160–660ft) deep overall, and was notably quiet due to the lack of high density currents. The presence of macroalgae also suggests that unlike modern anoxic environments within the photic zone today, the environment of the Lantian had some form of free oxygen below the storm wave base, which was likely locally available during oxic episodes.
The aforemetioned study done on various isotopes in the Lantian Formation further supported this, suggesting that the usually common euxinic environments where replaced by brief oxygenation events, permitting macro-algae and putative metazoans to evolve and thrive. They would have then been killed off and preserved when these events ended, with the environment returning to its normal euxinic state.
Paleobiota
A slab of Flabellophyton fossils, inferred to likely be a single reproduction cohort
The Lantian Formation is home to the Lantian Biota, which was previously the oldest known macroscopic biota before the discovery of the lower Doushantuo Formation macrofossils. This biota is predominately composed of macro-algal forms such as Flabellophyton and Doushantuophyton, although contains a sparse record of metazoan fossils, such as the palaeopascichnid organism Palaeopascichnus, and putative cnidarian-like organisms such as Qianchuania and Piyuania. Although recent studies that do not agree with the placement of the putative cnidarian-grade fossils, noting that this is predominately based on the gross morphology of the fossil material in question, and are likely macro-algae. Although it has also been noted that if confirmed, then they could be of use as a maximum calibration for crown Metazoa.
The biodiversity of differing forms shows that after the Marinoan glaciation, eukaryotes developed fairly rapidly and increased in diversity before the classical Ediacaran biota, likely further influenced by the locally intermittent oxygen, permitting larger, epibenthic macro-algae communities to evolve and become more present. This would be further supported with the discovery of the older Ancanping assemblage from the Doushantuo Formation, and although it contains a greater diversity, the fossils there are considerably smaller than what is seen in the Lantian Formation, fitting in with the temporal trend of increasing sizes and morphologies after the Cryogenian period into the later Ediacaran biotas, such as the Lantian Biota.
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123456Guan, Chengguo; Zhou, Chuanming; Wang, Wei; Wan, Bin; Yuan, Xunlai; Chen, Zhe (May 2014). "Fluctuation of shelf basin redox conditions in the early Ediacaran: Evidence from Lantian Formation black shales in South China". Precambrian Research. 245: 1–12. doi:10.1016/j.precamres.2014.01.003.
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12Wang, Wei; Guan, Chengguo; Zhou, Chuanming; Wan, Bin; Tang, Qing; Chen, Xiang; Chen, Zhe; Yuan, Xunlai (1 March 2014). "EXCEPTIONAL PRESERVATION OF MACROFOSSILS FROM THE EDIACARAN LANTIAN AND MIAOHE BIOTAS, SOUTH CHINA". Palaios. 29 (3): 129–136. doi:10.2110/palo.2013.085. ISSN0883-1351.
123Yuan, XunLai; Chen, Zhe; Xiao, ShuHai; Wan, Bin; Guan, ChengGuo; Wang, Wei; Zhou, ChuanMing; Hua, Hong (March 2013). "The Lantian biota: A new window onto the origin and early evolution of multicellular organisms". Chinese Science Bulletin. 58 (7): 701–707. doi:10.1007/s11434-012-5483-6.
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