Geochemistry of C, O, and Sr isotopes and chemostratigraphy of neoproterozoic rocks in the northern Yenisei Ridge

Abstract

Isotopic compositions of C, O, and Sr in carbonates, as well as Rb-Sr systems in the silicate mate rial from Upper Precambrian and Lower Cambrian rocks exposed by the Chapa River in the northern Yenisei Ridge, are studied. The Late Precambrian part of the section includes the following formations (from the bot tom to top): Lopatinskaya (hereafter, Lopatino), Vandadykskaya (hereafter, Vandadyk) or Kar'ernaya, Chivida, Suvorovskaya (hereafter, Suvorovo), Pod"emskaya (hereafter, Podyom), and Nemchanka. They are characterized by alternation of horizons with anomalously high and low δ 13 C values (such alternation is typ ical of the ~700-550 Ma interval). The lower, relatively thin (20 m), positive excursion (δ 13 C up to 4.3‰) was established in dolomites from the lower subformation of the Vandadyk (Kar'ernaya) Formation (hereaf ter, lower Vandadyk subformation). The upper positive excursion (δ 13 C = 2.2 ± 0.6‰) was recorded in the 3 km thick Nemchanka Formation enriched in terrigenous rocks. The lower negative excursion stands out as uniform, moderately low δ 13 C values (-2 ± 1‰) and significant thickness. It comprises the upper part of the Vandadyk Formation, as well as Chivida and Podyom formations. The upper negative excursion is related to a thin (~20 m) marker carbonate horizon of the upper Nemchanka subformation, in which δ 13 C values fall down to ⎯8.3‰. The lower part of the Lebyazhinskaya (hereafter, Lebyazhino) Formation, which overlies the Nemchanka Formation, shows a step by step increase in δ 13 C from -2.2 to 2.5‰ typical of the Vendian to Cambrian (Nemakit-Daldyn Horizon/Stage) transitional sequences. The absence of relationships between the carbon and oxygen isotope compositions and other parameters of postsedimentary alterations suggests that the excursions characterized above could form at the sedimentation stage and coincide in gen eral with δ 13 C fluctuations in seawater. The value of 87 Sr/ 86 Sr = 0.7076-0.7078 in limestones of the Podyom Formation points to their early Ediacaran age. Values of 87 Sr/ 86 Sr = 0.70841 and 0.70845 in dolomites of the lower Lebyazhino subformation correspond to the Early Cambrian. The Rb-Sr systems of the clay material from the Vandadyk and Chivida formations are approximated by a straight line, parameters of which corre spond to the age of 695 ± 20 Ma ( 87 Sr/ 86 Sr 0 = 0.7200 ± 0.0013) and probably characterize the epigenetic stage of older sedimentary rocks, which were subjected to very rapid exhumation and "polar" sulfuric acid weath ering in the course of glacioeustatic regression.

Birth and evolution of a Cryogenian basin: Glaciation, rifting and sedimentation in the Vorogovka Basin, Siberia

“…The Podyom Formation in the north‐eastern Yenisei Ridge has glacial diamictites at the base (Sovetov et al ., ; Sovetov, ) and contains cap dolomites (Pokrovsky et al ., ) similar to the lowest Ediacaran (Sovetov & Komlev, ; Sovetov, ) cap dolomites in the Marnya and Goloustnaya formations from the Sayan Mountain foothills and Baikal areas, respectively. The Podyom Formation lies under the Nemchany continental red molasse and has three members in its upper section (conglomerate sandstone, dolomite sandstone and sandstone) which previously were joined in the Uglovoi Formation.…”

“…The amagmatic diamictite suite found in the Teya‐Chapa Basin is genetically related instead with the overlying deep‐water siliciclastic mass flows and turbidites (fans). The Chivida tillites in this section were assigned to the Sturtian glaciation, with age constraints of 690 ± 27 Ma obtained from Rb/Sr ratios in the clay fraction of overlying siltstones and from chemostratigraphy using 87 Sr/ 86 Sr ratios (Pokrovsky et al ., ). It is noteworthy that this relationship compares closely with the situation in the Death Valley area of California, USA.…”

Resolving Terreneuvian stratigraphy in subtidal–intertidal carbonates: palaeontological and chemostratigraphical evidence from the Turukhansk Uplift, Siberian Platform

“…Moreover, the Terreneuvian strata of the Olenek Uplift (Kessyusa Group) exhibit a diverse assemblage of Fortunian and Cambrian Stage 2 trace fossils . However, despite extensive study of the western facies region (Butakov et al 1975;Vinogradov et al 1994;Bartley et al 1998;Khomentovsky et al 1998Khomentovsky et al , 2004Pelechaty 1998;Pokrovsky et al 2012;Kochnev et al 2018), the sections there demonstrate depauperate diversity of small skeletal taxa and an ambiguous chemostratigraphical record. Possible explanations include A B C Fig.…”

“…, ; Pelechaty ; Pokrovsky et al . ; Kochnev et al . ), the sections there demonstrate depauperate diversity of small skeletal taxa and an ambiguous chemostratigraphical record.…”