Glauconite in the Lower Cambrian Terrigenous–Carbonate Rocks, Olenek Uplift, North Siberia

This work considers for the first time mineralogical features of the Lower Cambrian (Tommotian) glauconite collected from terrigenous–carbonate rocks of the Kessyusa Group (upper part of the Mattaia Formation and lower part of the Chuskuna Formation), as well as from basal beds of the overlying limestones of the Erkeket Formation. Samples were taken from three sections on the northwestern slope of the Olenek Uplift, North Siberia. Their stratigraphic assignment is given on the basis of recent data (Nagovitsin et al., 2015 and others). Grains of the layer silicates are made up of the mixed layer mica–smectite phases with relatively low (<10%) and higher (10–20%) contents of expandable layers (unit cell parameter b = 9.06–9.12 A). Micaceous minerals form a series from glauconite to Al-glauconite (Al index KAl = VIAl/(VIFe3+ + VIAl) is 0.11–0.47 and 0.60, respectively), with the K2O content varying from 6.80 to 8.54%. Detailed lithological-mineralogical characteristics is given for the first time for the glauconite-bearing rocks, the primary sediments of which were accumulated in the prefrontal beach zone and transitional beach–shelf zone in the Siberian epicontinental marine paleobasin (Marusin, 2016). Origin of the studied grains (authigenic, allothigenic) is discussed and their secondary alterations at different stages of lithogenesis are considered (rewashing, phosphatization, pyritization, calcitization, ferrugination, and others). It is shown that the obtained preliminary Rb–Sr dates (450–320 Ma) are “rejuvenated” and do not correspond to the age value of 541.0 Ma accepted for the Vendian–Lower Cambrian boundary (Gradstein et al., 2012). This can be related to diverse secondary alterations of the glauconite grains during the rewashing, transportation, and different stages of diagenesis of primary sediments after their redeposition, as well as at stages of catagenesis and hypergenesis of the glauconite-bearing rocks.