The Dengying Formation is a large geologic formation found in South China, and was deposited during the late Ediacaran between 551 – 539Ma on a shallow marine carbonate platform. Due to the great area the formation covers, it not only contains a wide range of members for each province it outcrops in, but also contains differing modes of taphonomic preservation, from the common Ediacaran-type preservation, to the rarer Burgess Shale-type preservation.
Recent studies from some of the middle members of the formation have also yielded rare transitional fossiliferous rocks, namely between the Ediacaran biota and the Cambrian biota, providing insight as to where some Cambrian lineages may have started and diversified from, and filling in many empty gaps between the Ediacaran and Cambrian.
Geology
Due to the large area that the Dengying Formation covers, the geology and members within the formation differ from province to province. These members are listed below in ascending stratigraphic order (lowest to highest), under the respective provinces they are found in:
Hamajing Member: This member is characterized by light grey, medium- to thick-bedded dolomite, intercalated with thin layers of chert.
Shibantan Member: This member is interpreted to represent a subtidal environment and is composed of dark grey, thin-bedded bituminouslimestone. It is the most fossiliferous member of the Dengying Formation in Hubei and has yielded several trace fossils, such as Lamonte, as well as possible lobopodian trackways.
Baimatuo Member: This member is primarily composed of light grey, thick-bedded dolomite.
Traditionally, the Tianzhushan Member was considered to be the uppermost unit of the Dengying Formation in Hubei. However, its small shelly fossils and Micrhystridium-like acritarchs are a shared characteristic with the Cambrian aged Yanjiahe Formation.
Donlongtan Member: This member is primarily composed of argilaceous dolomite rocks.
Jiucheng Member: This member is dominated by various siltstone layers, with layers of sandstone throughout. This member is also fossiliferous in nature, containing the Jiangchuan Biota, with known forms such as Alienum and Lobodiscus being present with in member, as well as a number of yet to be described fossils which bear striking resemblances to many Cambrian animals.
Baiyanshao Member: This member is predominately composed of dolomitic siltstone, which is argilaceous in nature.
Shaanxi Province Members
The formation in the Shaanxi Province is composed of three members, and is underlain by the Doushantuo Formation, whilst it is overlain by the Cambrain aged Kuanchuanpu and Huoshiwan Formations.
Algal Dolomite: This member is predominately composed of light-gray peritidal dolostones, with paleo-karst structures being common throughout the member.
Gaojiashan Member: This member is further sub-divided into three units. Unit 1 of the member is primarily composed of silstone, which it then inter-bedded with thin dolomite. This then transitions in Unit 2, which is predominately composed of dolomitic siltstones, with layers of calcareous mudstone and limestone found throughout. Then, at the top, Unit 3 differs greatly in being composed of conglomerate and sandstone, with layers of carbonate cement. This is also the fossiliferous member of the province, containing organisms such as Cloudina.
Beiwan Member: This member is composed of grayish-white intra-clastic dolostone with occasional layers of chert near the base of the member, as well as locally occurring bitumen. It also contains an extensive microfossil assemblage.
Much like the Yanjiahe Formation in Hubei, the Kuanchuanpu Formation is also occasionally placed as a member of the Dengying Formation in Shaanxi, however it is likely separate for the same reasons.
Sichuan Province Members
The formation within the Sichuan Province is found outcropping in the Sichuan Basin in Southwestern China, as is primarily most well known for its gas reservoirs that can be found throughout the local area, such as the Weiyuan Gas Field. It is underlain once more by the Doushantuo Formation, whilst it is overlain by the Cambrian aged Qiongzhusi Formation.
First Member: This member is predominately composed of gray dolomite, which is micritic in nature. There are also occasional algal dolomites throughout, with gypsum and mudstone inter-bedded within.
Second Member: The algal dolomite notably increases in this member, which is grayish-white in colour. There are also layers of clotted dolomite and algal mat-stromatolite dolostone.
Third Member: The algal dolomite decreases in this member, which is primarily composed of argillaceous dolomite, inter-bedded with mudstones and shales.
Fourth Member: The algal dolomite becomes a light gray to grayish-white algal dolostone, which is laminated in nature, alongside clotted dolomite and micritic dolostone, with siliceous nodules found throughout the member.
Jiangxi Province Outcrop
The Dengying Formation can also be found outcropping in the northern areas of the Jiangxi Province as a light-gray, thick-bedded stromatolite dolostone, and is underlain by the upper phosphorite and dolomicrosparite dominant layers of the Doushantuo Formation, whilst it is overlain by the black shales of the Cambrian aged Hetang Formation. Due to the limited exposed outcrops in Jiangxi, proper sequencing of the local stratigraphy has not been fully studied.
Dating
The Dengying Formation goes right up to the boundary of the Ediacaran and Cambrian periods, currently dated to 538.8±0.6Ma, whilst the age of the base of the formation varies depending on the province. The middle Gaojiashan Member from the Shaanxi Province has had U-Pb dating done on several detrital zircon crystals, which recovered a maximum deposition date of 548±8ma. Meanwhile, the middle of the Third Member has been dated to 547.3±4.8ma. The base of the Dengying Formation in the Yangtze Gorges Hubei Province has been firmly dated to 551.1±0.7ma.
Taphonomy
The Dengying Formation is home to at least three different styles of preservation, which includes the classic Ediacaran-type preservation seen in the Shibantan Biota where only a negative or positive print "death mask" of the organism is made, Pyritization as seen in the Gaojiashan Biota where any organic parts of the organisms are replaced with pyrite, and the rare Burgess Shale-type preservation seen in the Tongshan and Jiangchuan Biotas, where the bodies are preserved as carbonaceous films, including some examples of Phosphatization throughout the Jiangchuan Biota and Beiwan microfossil assemblage.
It has been noted that the Burgess Shale-type preservation may be rare due to a taphonomic bias of requiring exceptional conditions such as low oxygen, clay sediments and authigenic mineralisation of soft tissue to long-living minerals, as such making it difficult to find and Cambrian-like biotas in the Ediacaran.
Paleobiota
The Dengying Formation is unique from most other Ediacaran formations, in that the top of the formation goes right up to the Ediacaran - Cambrian boundary, resulting in not only a wide array of Ediacaran organisms such as Paracharnia and Dickinsonia, but also rare examples of Cambrian-like organisms, such as Alienum. Alongside this there is also a notable number of ichnogenera in the form of burrows, which also become more common closer to the boundary. The formation is also well known for its abundant algal organisms, from Longfengshania to the enigmatic Vendotaenia.
Recent studies have also found rare examples of Burgess Shale-type Preservation, in what is known as the Tongshan Lagerstätte and the Jiangchuan Biota from the Jiucheng Member. These preserves a wide range of fossil forms, from the first rangeomorph fronds with Burgess Shale-type preservation in the Tongshan, to the first reports of more recognizable Cambrian-like organisms, bolstering evidence that bilaterian animals where diversifying around this time, back up by the increase in the well known various burrows, movement traces, etc.
The discovery of White Sea assemblage fossils such as Dickinsonia, and more recently Kimberella, in the lower Shibantan Member provide evidence that the fauna of the White Sea assemblage survived into the Nama assemblage, and permit researchers to see how the White Sea-Nama extinction affected White Sea organisms. This also brings into question the validity and definitions of the three assemblages, with calls to provide further clarity on said assemblages.'
Tubular organism. Previously reported as a junior synonym of Nenoxites and Gaojiashania, although recent studies regard it as a body fossil of a distinct genus.
Frondose organism, 3 distinct lobe-like regions, consisting of a thick stalk and two petaloids, which are made up of tightly packed branches.
Fan-shaped rangeomorphs
???
Tongshan Lagerstätte (Wanjia section)
Two frondose organisms with a similar appearance to Bradgatia, with one of the specimens, ESEN 0018, preserving with a possible holdfast.
Undescribed rangeomorph "ESEN 0019"
???
Tongshan Lagerstätte (Wanjia section)
Frondose organism, with a similar appearance to Charniodiscus, consisting of a thick stalk and two petaloids, which are made up of tightly packed branches, also preserves a notable holdfast.
Undescribed rangeomorph "ESEN 0020"
???
Tongshan Lagerstätte (Wanjia section)
Elongated frondose organism, with a similar appearance to Paracharnia, consisting of a thin stalk and two petaloids, which are made up of tightly packed branches, alongside a possible holdfast at its base.
Undescribed frond "ESEN 0021"
???
Tongshan Lagerstätte (Wanjia section)
Typical frondose organism, possibly with three petaloids in total, and a holdfast structure and probable stem are also present.
Undescribed tetraradial organism "YNGIP 90305"
???
Middle Jiucheng Member (Shanglijiao section)
Tetra-radial organism, noted to bear similarities to Tribrachidium and Lobodiscus in the shape of its lobes, whilst its tetra-radial body plan is similar to Conomedusites.
Undescribed Haootia-like organisms
???
Middle Jiucheng Member
Haootia-like organisms, which are cup-like in appearance with terminal arms, and is fibrous in nature. The largest specimen is 15mm (0.6in) in length and 20mm (0.8in) in width, making them smaller than Haootia, and may represent a new genus alongside Haootia and Mamsetia.
A singular fossil that bears similarities to known ctenophore fossils, getting up to 18mm (0.7in) in length and 10mm (0.4in) in width, and has been compared to Ctenorhabdotus.
Undescribed Mackenzia-like organism "YNGIP 90304"
???
Middle Jiucheng Member
A singular fossil that bears similarities to Mackenzia, with the fossil being compared to known mackenziid fossil from the Chengjiang Biota, although it is noted they are much smaller, only attaining a length of 18mm (0.7in) in length and 3mm (0.1in) in width.
Undescribed worm-like organisms
???
Middle Jiucheng Member
The most numerous bilaterian organism in the, consisting of a large holdfast disc and an elongated vermiform body with a terminal mouth, with the largest example getting up to 61mm (2.4in) in length and 2.7mm (0.1in) at the widest, whilst the holdfast is up to 15mm (0.6in) in diameter. It has been noted that the previously described Cycliomedusa, also from this member, bears a striking resemblance to the holdfasts of these fossils, and may likely be said holdfasts preserved in another layer, with the body in another.
Undescribed sausage-shaped organism "YNGIP 90311"
???
Middle Jiucheng Member
A single sausage shaped organism with a circular terminal opening inferred to be a mouth, and a long darker lined down the centre, interpreted as a gut.
Undescribed Herpetogaster-like organisms
???
Middle Jiucheng Member
A number of fossils that bear similarities to Herpetogaster, with a coiled body set on top of a stem, with tentacle-like structures on wider head-region, although are notably simpler than that seen on Herpetogaster.
Undescribed Margaretia-like organisms
???
Middle Jiucheng Member
Elongated fossils with ovoid holes running along the full length of the body in rows, bearing similarities to Margaretia, a tube-like organism used as a dwelling for hemichordate worms. The largest complete fossil reaches up to 30mm (1.2in) in length and 4mm (0.2in) in width.
Cloudinids
???
Beiwan Member (Yuanbaoshan section)
Undescribed tubular organisms.
Problematic spherical fossil
???
Beiwan Member (Yuanbaoshan section)
Undescribed spherical fossil of unknown placement.
Conical tube with round apex
???
Beiwan Member (Lijiagou section)
Tubular organisms of unknown placement.
Conical tube with transverse annulations
???
Beiwan Member (Lijiagou section)
Tubular organisms of unknown placement.
Conical tube with transverse ridges
???
Beiwan Member (Lijiagou section)
Tubular organisms of unknown placement.
Zhenba columns
???
Tongshan Lagerstätte (Wanjia section)
Possible affinities with coralline algae.
Doushantuophyton-like form
???
Tongshan Lagerstätte (Wanjia section)
Doushantuophyton-like form, possibly macroalgae. Bears similarties to Doushantuophyton.
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