| Abstract: | Lithogeochemical features of the Vendian mudstones and silty mudstones taken from Borehole Keltma 1 in the southern part of
the Vychegda trough of the Mezen syneclise are discussed. It is shown that fine-grained clastic rocks of the Ust-Pinega, Krasavino,
and Mezen formations have similar chemical compositions, suggesting their accumulation in sufficiently similar settings. The
main part of the studied samples has K2O/Al2O3 < 0.4. This fact, in combination with the absence of TM-FM and NPM-HM correlations, indicates a significant contribution
of recycled aluminosiliciclastics in their composition. At the same time, the absence of correlation between CIA and indicator
ratios of rock composition in the paleodrainage basins, such as Th/Cr and Th/Sc, indicates that CIA and some other lithochemical
indicators appropriately reflect the paleoclimatic conditions in source areas surrounding a basin. The CIA value in most of
the analyzed samples is no more than 70. Thus, the Keltma section is similar to Upper Vendian sequences of the Kvarkush-Kamennogorsk
anticlinorium and the Shkapovo-Shikhany depression. It has been established that felsic and intermediate magmatic rocks coupled
with a significant contribution of quartz-rich sediments served as the source of fine aluminosiliciclastics for the southern
Vychegda trough during the Vendian. High Ce/Cr values in the mudstones and silty mudstones suggest that the geochemically
primitive Archean protoliths were not involved in the washout. In the SiO2-K2O/Na2O diagram, the Vendian mudstones and silty mudstones are plotted in the field of sediments of active continental margins.
Typical low values of Mo/Mn and some other redox indices in these rocks indicate that oxidizing environment predominated in
bottom waters of the sedimentation basin during the entire Vendian. Analysis of variations of the lithochemical indicators
upward the Vendian sedimentary successions in borehole Keltma 1 made it possible to divide the section into three sequences
of different lithofacies and paleontological compositions. |
