Glauconite of terrigenous-carbonate deposits from the lower Cambrian (Northern Siberia, Olenek uplift)
The detailed mineralogical peculiarities are reported for the first time of the glauconite of the Lower Cambrian (Tommotian stage) extracted from the terrigenous-carbonate deposits at the upper part of the Kessyusa series (the roof of the Mattaia Formation and the lower part of the Chuskuna Formation) as well as basal layers of the overlying limestone, too. Samples were taken from three sections of north-western slope of the Olenek uplift, northern Siberia; their stratigraphic position is given on the basis of modern data [Nagovitsin et al., 2015, and others]. Grains of layer silicates are represented by mica-smectite phyllosilicates with a relatively low content and with an elevated content of smectite layers (<10 and 10-20%, respectively), (the unit-cell parameter b = 9.06-9.12 Å). Micaceous minerals form a range from glauconite to Al-glauconite (КAl = VIAl/(VIFe3+ + VIAl) = 0.11-0.47 и 0.60, respectively; K2O content is 6.80-8.54%. Detailed litho-mineralogical peculiarities of glauconite-bearing sediments are given for the first time. Their primary deposits according to [Marusin, 2016] accumulated in the conditions of the shoreface and the offshore-transition zone of the Siberian epicontinental marine paleobasin. The origin (autogenic, allothigenic) of the studied grains, their secondary changes (rewashed in situ, phosphatization, pyritization, calcitization, ferruginization, etc.) at different stages of lithogenesis are discussed. It is shown that the obtained preliminary Rb-Sr dates (450-320 Ma) were ”rejuvenated”: they do not meet the age values previously adopted for the the Vendian-Lower Cambrian boundary - 541.0 Ma [Gradstein et al., 2012]. This may be due to various secondary alterations of glauconite grains during rewashing, transportation, and also at different stages of diagenesis in primary sediments and after re-deposition, as well as at the stages of catagenesis (deep diagenesis) and hypergenesis of glauconite-bearing rocks.