Lithochemistry of Upper Vendian–Lower Cambrian Clayey Rocks in the Central Part of the Moscow Syneclise: General Features of Formation

• Published in: Geochemistry international 2022, Vol.60 (1), p.16-32
• Main Authors: Podkovyrov, V. N., Maslov, A. V., Kotova, L. N.
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Analysis; Banks (Finance); Cambrian; Catchment area; Catchment areas; Chemical analysis; Chemical composition; Chlorite; Clay; Climate change; Climatic conditions; Earth and Environmental Science; Earth Sciences; Formations; Fossils; Geochemistry; Illite; Illites; Kaolinite; Mudstone; Paleoclimate; Phosphorus pentoxide; Platforms (geology); Potassium oxides; Ratios; Rocks; Rocks, Sedimentary; Silica; Silicification; Silicon dioxide; Smectite; Smectites; Titanium dioxide; Volcanic rocks
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702922010104

Approximately ~120 bulk chemical analyses of the Upper Vendian hereinafter Valdai Group and Lower Cambrian (Lezh and Galich formations) clayey rocks from the central part of the Moscow syneclise (PRECSED data bank, IPGG RAS, St. Petersburg) were examined to reveal the main features of their formation (composition of source rocks, paleoclimate, etc.). It is shown that mudstones and mudstone-like clays of the above-mentioned lithostratigraphic units belong mainly to smectite clays with kaolinite, illite, and chlorite–smectite–illite admixture. The data set has a very homogeneous chemical composition. Their K 2 O/Na 2 O and SiO 2 /Al 2 O 3 ratios are typical of clayey rocks and indicate the absence of alteration by K‑metasomatism and silicification. It was found that the studied rocks include both petrogenic (mudstones of the Ust-Pinega Formation) and lithogenic clayey rocks, which bear a significant contribution of sedimentary material evolved through more than one sedimentary cycle (mudstones and mudstone-like clays of the Pletenevka, Lyubim and Galich formations). The sources of fine aluminosiliciclastic for the Upper Vendian and Lower Cambrian clayey rocks from the axial zone of the Moscow syneclise were the rocks of the crystalline basement of the East European Platform and underlying sedimentary and metasedimentary rocks eroded under relatively warm climatic conditions. Changes in climatic conditions on the catchment areas surrounding the Moscow syneclise basin during the Late Vendian–Early Cambrian were not established. An analysis of the average values of paleoproductivity indicators for clayey rocks of different formations, such as the P 2 O 5 content and EF P2O5 , suggests that the sedimentation areas that existed in the Upper Vendian–Lower Cambrian in the central and northern regions of the East European Platform were not characterized by noticeable bioproductivity. The average values of MgO, Na 2 O, (Na 2 O + K 2 O), as well as TiO 2 /Al 2 O 3 and K 2 O/Al 2 O 3 indicator ratios suggest that the mudstones and mudstone-like clays of the Upper Vendian, as well as the Lezh and Galich formations practically do not contain “camouflaged” pyroclastics.