U-Pb zircon ID-TIMS geochronology of the Ediacaran White Sea assemblage from Onega peninsula, NW Russia

<p>Establishing an absolute timeline for the evolution of Ediacaran (latest Neoproterozoic) biota is an outstanding challenge for the geochronology community. Ediacaran biota (&#8220;Vendian&#8221; in Russian literature; 580-539 Ma) is traditionally divided into three assemblages: Avalon, White Sea and Nama, considered as a partially overlapping evolutionary and temporal successions. The geochronological record of the White Sea assemblage is hampered by the lack of datable volcanic beds, with very few exceptions like in Onega peninsula and along the Winter coast in northwestern Russia. There, we find examples of the most complete and diversified White Sea assemblage of the entire East European Patform (EEP).</p> <p>We report here results from a field study and U-Pb geochronology on the Lyamtsa and Verkhovka formations from Onega peninsula (Lyamtsa and Agma rivers) and Zimnie Gori formation along the Winter coast. The studied sedimentary successions consist mainly of fine-grained sandstone, mudstone and claystone, reflecting shallow marine conditions, influenced by a large river delta. The base of the White Sea Ediacaran deposits starts with sandstones and conglomerates, overlain by the brown clay with volcanic tuff (only in drillcore) in the base of the Lyamtsa formation, which contain&#160; first simple horizontal trace fossils, worm tubes <em>Calyprtina</em> and cyanobacterial colonies <em>Beltanelloides,</em> predating the onset of the rich Ediacaran fossil assemblage. In the middle part of the Lyamtsa formation (accessible in outcrops) the first <em>Dickinsonia</em>, <em>Parvancorina</em> and <em>Aspidella</em> fossils are found, as well as the oldest trace fossils of arthropods <em>Cruzian&#1072;</em>, vertical burrows <em>Altichnus</em> and traces of polychaete-like worms, previously known only from the uppermost Edicaran and Cambrian. The full range of diverse Ediacaran fossils (e.g., various dickinsoniids, trilobozoans, <em>Kimberella</em>, <em>Charnia</em>, <em>Rangea</em>, <em>Pteridinium</em>, etc.) is reached further up in the section (Verkhovka, Zimnie Gori and Erga formations).</p> <p>We have sampled two ash layers from the base of Verkhovka formation (Agma river) and one from the base of Zimnie Gori formation (along the Winter coast). Our high precision U-Pb zircon CA-ID-TIMS age determinations yielded weighted mean ²⁰⁶Pb/²³⁸U dates of 555.7 &#177; 0.6 Ma and 555.1 &#177; 0.7 Ma for the two ash layers of the Verkhovka formation and 552.6 &#177; 0.6 Ma for the base of Zimnie Gori formation. Applying age depth model calculations we can estimate that the onset of the Ediacaran White Sea assemblage in Lyamtsa formation must be older than 561 Ma. These results, combined with the age of the top of Zimnie Gori formation (550 &#177; 5 Ma, Llianos et al., 2005) have important implications for: i) estimate of the evolutionary trend of Ediacaran White Sea assemblage or at least for some Ediacaran organisms, which are showing philogenetic evolution (for examle <em>Dickinsonia</em>); ii) better correlations between distribution of Ediacaran macrofossils in the sedimentary sequences of the Eastern and Northeastern part of the EEP.</p> <p>&#160;</p> <p>&#160;</p> <p>REFERENCES</p> <p>&#160;</p> <p>Llanos, M.P.I., Tait J.A., Popov, V. & A. Abalmassova (2005) Palaeomagnetic data from Ediacaran (Vendian) sediments of the Arkhangelsk region, NW Russia: An alternative apparent polar wander path of Baltica for the Late Proterozoic&#8211;Early Palaeozoic. <em>Earth and Planetary Science Letters 240</em><em>, </em><em>732&#8211;747</em><em>.</em></p>