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Ustʹ Pinega Formation
Geological formation in northwestern Russia From Wikipedia, the free encyclopedia
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64°9′0.000″N 41°55′59.988″E The Ustʹ Pinega Formation is an Upper Ediacaran geological formation in northwestern Russia. It spans from around 580 Ma to 550 Ma and contains fossils of the Ediacaran biota throughout its sequence.
The formation was first established and described from the core of the Ust-Pinega borehole drilled in the settlement of Ust-Pinega, Arkhangelsk Region of Russia, where its thickness is 284 m. Its natural outcrops within the same region are known from the Onega Peninsula and the Winter Coast of the White Sea, as well as from small, isolated outcrops in the basin of the Onega River and on the northern slope of the Vetreny Poyas (Windy Belt) Ridge.[2]
The formation rests with erosion on Proterozoic terrigenous deposits or Archean crystalline rocks. It is subdivided, from bottom to top, into the following Subformations (initially defined as "Beds" by A.F. Stankovsky, 1985): Tamitsa, Lyamtsa, Arkhangelsk, Verkhovka, Syuzma, Vaizitsa, and Zimnegory. It is unconformably overlain by deposits of the Ediacaran Mezen Formation, as well as by Paleozoic or Quaternary sediments.
In 2003, D.V. Grazhdankin proposed an alternative subdivision scheme for these deposits, based on the original scheme by A.F. Stankovskiy. This scheme includes the Lyamtsa (= Lyamtsa + Arkhangelsk Beds), the Verkhovka (= Verkhovka + Syuzma Beds), and the Zimnegory (= Vaizitsa + Zimnegory Beds) Formations. In this revision, the use of the Ust' Pinega Formation was abandoned in favor of these three formations[3].
Currently, the Ust' Pinega Formation and its subformations remain the official stratigraphic scheme used on state geological maps of Russia.
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Dating
The Lyamtsa, Verkhovka and Vaizitsa Subformations contain volcanic ashes. Zircon U-Pb dating dating of volcanic tuffs from the Verkhovka and Vaizitsa beds has yielded ages of 557.28 ± 0.14 Ma and 552.96 ± 0.19 Ma, respectively[4][5]. Furthermore, an Rb–Sr age of 565 ± 9 Ma was obtained for clays of the Arkhangelsk Subformation[1]. The formation is unconformably overlain by the deposits of the Mezen Formation, from the lower part of which a U-Pb age of 550.2 ± 4.6 Ma has been obtained for tuffites[6].
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Paleobiota
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The Ustʹ Pinega Formation is home to many rare and common Ediacaran fauna, from the well known motile forms such as Dickinsonia and Kimberella,[7] to the rarer, more elusive forms like Ventogyrus and Zolotytsia. All forms within this formation are preserved in layers of ash beds, which are not only good at preserving the fine exterior details of organisms, but also their internals, like Burykhia.[8]
Color key
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Notes Uncertain or tentative taxa are in small text; |
Proarticulata
Petalonamae
Trilobozoan
Cnidarian
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Chordata
Annelida
incertae sedis
Flora
Ichnogenera
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See also
References
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