The Mogilev Formation, sometimes spelt as Mohyliv or Mohylivska, is a late Ediacaran aged geologic formation in Moldova and Ukraine. Deposited sometime between 575 – 556.62±0.26Ma, the Yarishev Formation is predominately composed of sandstone rocks. The formation is also fossiliferous in nature, containing various large macrofossils, and an abundance of microfossils.
History of research
Prior to the 1960s, knowledge on the formations and fossils within the wider East European Platform were relatively unknown, save for the formal descriptions of the Mogilev and Ushitsa Formations in 1939 by L.F. Lungersgauzen, which were then regarded by Lungersgauzen as Cambrian and Ordovician respectively. With the onset of the 1960s, various studies were undertaken in order to compile a State geological map, which would see previously described formations such as the Mogilev Formation, and new formations, correlated to other Ediacaran formations in the wider region.
Geology
The Mogilev Formation can be found outcropping near to the village of Mereșeuca in Ocnița District, Moldova, as well as by the Dniester River in Ukraine, where it is overlain by the argillites of the Yarishev Formation, whilst it is unconformably underlain by the basalts of the Hrushka Formation and a Paleoproterozoic aged crystalline basement, and is a part of the wider Mogilev-Podolisk Group. The formation itself is primarily composed of sandstones, and is split into three formally named members. The overall thickness of the formation ranges between 17.3–21.0m (56.8–68.9ft).
Members
The Mogilev Formation is split into three formal members, sometimes referred to as beds. These members are as follows, in ascending stratigraphic order (lowest to highest):
Olchedaev Member: This member, sometimes referred to as the "Olchedaev Beds", is roughly 4.4–6.0m (14.4–19.7ft) thick, and is predominately composed of coarse-grained gravels, as well as very-fine- to very-coarse-grained, cross-bedded sandstones. There can also be found occasional intercalations of argillites and siltstones. Going from the base to the top of the member, the sizes of the grain decrease, and the colour of the rocks change from a grayish-pink to light-gray.
Lomozov Member: This member, sometimes referred to as the "Lomozov/Lomosiv Beds", attains a thickness between 2.9–8.4m (9.5–27.6ft), and is primarily composed of dark blue-gray argillites, which is inter-bedded with thin layers of gray sandstones and siltstones.
Yampol Member: This member, sometimes referred to as the "Yampol/Yampil Beds", ranges between 8.9–12.1m (29–40ft) in thickness, and is composed of white to light-gray, very-fine- to medium-grained clayey sandstones, in which coarse-grained sandstones and gravels sit on top.
The Lyadova Member was previously, and sometimes still is, regarded as being a part of the Mogilev Formation, although in more recent studies this has flipped back and forth to also being attributed to the overlying Yarishev Formation,
Dating
The Mogilev Formation has been dated using U-Pb dating on zircon samples collected above in the Yarishev Formation, as well as the top of the formation itself. The dates yielded from these samples were 556.62±0.26Ma and 557.12±0.41Ma respectively, seeing the date of deposition for the top of the Mogilev Formation being between 557 - 556Ma. The formation has also been correlated to the Kholonevychi Formation, which had its base dated using the U-Pb dating method on zircon samples, which yielded a date of 575±7Ma.
Paleoenvironment
The environment at the time of the Mogilev Formations deposition was likely that of a cold-water, shallow marine one, set within a basin with a calm, hydrodynamic regime. This has been inferred from the notable absence of carbonate sediments. This environment was likely maintained by the presence of local basement high-islands, as well as submerged shales, all of which would have brought around conditions fit for colonisation of the seafloor by early metazoans.
Paleobiota
The Mogilev Formation is home to a wide variety of Ediacaran macrofossils, from discoidal forms such as Cyclomedusa and Nimbia, to proarticulates like Dickinsonia and Podolimirus. There can also be found a range of cyanobacterial forms and microfossils, such as Volyniella and Stictosphaeridium respectively. Most fossils are restricted to the Lomozov Member, although some can be found higher up in the Yampol Member.
Ascidian tunicate. Fossils were first found from this formation prior to their formal description, where they were noted to bear similarities to Ausia and Gibbavasis.
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