The Yarishev Formation, sometimes spelt as Yaryshiv or Yarishyvska, is a late Ediacaran aged geologic formation in Moldova and Ukraine. Deposited sometime between 556.8 – 555.4±2.9Ma, the Yarishev Formation is predominately composed of argillite rocks. The formation is also fossiliferous in nature, containing various large discoidal macrofossils, and an abundance of microfossils.
Geology
The Yarishev 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 unconformably overlain by the argillites of the Nagoriany Formation, whilst it is underlain by the sandstones of the Mogilev Formation, and is a part of the wider Mogilev-Podolisk Group. The formation itself is primarily composed of argillites, and is split into three formally named members. The overall thickness of the formation ranges between 40.6–100.7m (133–330ft).
Members
The Yarishev Formation is split into three formal members, sometimes referred to as beds. These members are as follows, in ascending stratigraphic order (lowest to highest):
Lyadova Member: This member, sometimes referred to as the "Lyadova Beds", is roughly 8.1–21.7m (27–71ft) thick, and is primarily composed of green and brown argillites, which is intercalated with layers of siltstone and sandstone in the upper sections. There can also be found rare intercalations of bentoniticclays and tuffitic argillites. This member was previously considered to be a part of the underlying Mogilev Formation.
Bernashev Member: This member, sometimes refered to as the "Bernashivka/Bernashevka Beds", attains a thickness of 22m (72ft), and is composed of clayey, medium- to fine-grained sandstones, which are inter-bedded with siltstone layers in its lower section. Towards its middle, the sandstones give way to thinly-bedded, dark-gray to green mudstones, with occasional thin layers of sandstone laminae. At the top, the mudstones become coarse- to very-coarse-grained arkosic sandstone, which is occasionally gravelly. Throughout the member there can be found beds and interlayers of bentonitic clays.
Bronnitza Member: This member, sometimes referred to as the "Bronnitsa Beds", ranges between 9.0–30.0m (29.5–98.4ft) in thickness, and is predominately composed of distinctive tuffeceous inter-beds. Used as a regional marker horizon, these inter-beds are comprised of massive, red to light-brown tuffitic argillites, which sometimes posess rare intercalated layers of red brown and brown argillites and siltstone. Towards the top of the member, argillites become the dominate rock.
Zinkov Member: This member, sometimes referred to as the "Zinkov Beds", ranges between 15.3–27.0m (50.2–88.6ft) in thickness, and is composed of greenish-gray, gray and dark-gray argillites, which are intercalated with layers of light-gray sandstones towards the upper sections of the member. One distinctive feature of this member are the increasing sizes and coarseness of the grains from bottom to top.
Dating
The Yarishev Formation has been dated using U-Pb dating on zircon samples collected from the Lyadova and Bernashev members. The dates recovered from these samples yielded dates of 556.8±0.18Ma and 555.4±2.9Ma respectively.
Paleoenvironment
The environment at the time of the Nagoriany Formations deposition was likely that of a low-energy, inner-shelf environment, similar to formations found from the Nama Group. This has been primarily inferred from the presence of burrows found within the formation.
Paleobiota
The Yarishev Formation contains a sparse record of Ediacaran macrofossils, from discoidal forms such as Cyclomedusa and Tirasiana, to frondose fossils like Charniodiscus, the agglutinated fossils of Palaeopascichnus, and the jaw-like fragments of Redkinia. Meanwhile, throughout the formation, there can be found an increasingly abundant record of filamteneous microfossils, which are commonly referred to as cyanobacteria, with examples such as Lyadovia and Volyniella, the former of which was described from this formation.
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