U-Pb (SHRIMP II) age of zircons from ash beds of the chernokamen formation, Vendian Sylvitsa group (Central Urals)

Abstract

The Serebryanka and Sylvitsa groups on the western slope of the Urals characterize the Vendian section of the eastern folded framework of the East European Platform [1]. They are conspicuous against the background of the coeval platformal sequences by their relatively complete sedimentary record, sufficiently good outcrops, and opportunities for detailed sedimentological study both along and across the regional paleoslope. The tillite units serve as key elements of the stratigraphic framework of the Serebryanka Group. However, their number and continuity along the strike remain a matter of debate [1, 2]. The Sylvitsa Group comprises the following formations recognized on a regional scale (from bottom to top): Staropechny Formation (with a thin tillite unit at the base), Perevalok, Chernokamen, and Ust’-Sylvitsa formations. Their specific structural features serve as criteria for largescale correlation with Upper Vendian platformal sequences [1‐3]. The Ediacaran-type soft-bodied fossil biota is abundant in the Chernokamen Formation. The great density of fossil populations, the high taxonomic diversity of this biota, and the presence of endemic (for the Southeast White Sea region) taxons give grounds to suggest biogeographic links between the Central Ural and White Sea segments of the Late Vendian paleobasin [4]. The fossil biota from sedimentary rocks of the Chernokamen Formation comprises Cyclomedusa davidi Sprigg, Dickinsonia sp., Dickinsonia tenuis Glaessner et Wade, Ediacaria flindersi Sprigg, Eoporpita medusa Wade, Inaria khatyspytia (Vodanjuk), Irridinitus multiradiatus Fedonkin, Medusinites mawsoni (Sprigg), Nemiana simplex Palij, Paliella patelliformis Fedonkin, Palaeopascichnus delicatus Palij, Protodipleurosoma wardi Sprigg, Vaizitsinia sophia Sokolov et Fedonkin, and Yorgia sp. These data make it possible to correlate the Chernokamen Formation of the Central Urals with the reference section of the southeastern White Sea region. The Upper Vendian of the White Sea region includes a very characteristic taxon Yorgia , which appears for the first time in the Zimnegorsk Formation. However, insufficiently reliable stratigraphic and facies affiliations of the Sylvitsa Group and the absence of isotopic datings remained serious obstacles for such correlation until recently. In 2002, we found thin volcanic ash beds in the upper part of the Staropechny Formation. These beds are associated with the chocolate brown thin-bedded mudstones that crop out on the left bank of the Sylvitsa River downstream of the Kernos Creek. In 2003, volcanic ash beds, which are also closely related to the chocolate brown mudstones, were revealed in the Staropechny and Perevalok formations and in the lower subformation of the Chernokamen Formation in the Vilukha ravine on the left and right banks of the Us’va River. In 2004, ash beds were traced for 4.5 km in the lower subformation of the Chernokamen Formation along the Mezhevaya Utka River. In all the above cases, ash beds were readily discernible as a poorly cemented light gray (with pinkish, cream, and greenish hues) rock among the host chocolate brown mudstones. Members of chocolate brown mudstones with volcanic ash beds are important for Upper Vendian stratigraphy of the East European Platform. The tuffaceous rocks serve as key units for subdivision and correlation of outcrops and borehole sections [2, 3, 5, 6]. The most complete Upper Vendian platformal sections of the Mezen syneclise and the southeastern White Sea region