Acta Musei Moraviae

Glauconitolite from Bystřice near Třinec (Outer Western Carpathians, Czech Republic)

Sedimentary rock with the psamitic texture extremely rich in glaucony is described from the Jablunkov Furrow, where the Subsilesian Unit is exposed in a tectonic half-window. The locality itself (GPS N 49° 37.324′ E 018° 42.370′) is a small outcrop at the left bank of the Kopytná Creek, revealing turbidites of the Frýdlant Formation. The strata were assigned to the middle Eocene based on previous foraminifer biostratigraphic study. Outcrop comprises of several clastic sediment beds with predominant psamites over massive polymict sandy conglomerates. Abundant green glaucony in typical for the whole sequence. According to the position of the erosion base of the beds and the gradation, the outcrop shows reverse (inverted) stratigraphy. Green glaucony grains are major component of both psamitic rocks (up to 60%) and psephitic rocks (up to 32%). Present bioclasts were originally carbonate, now partly silicified. Other clasts are composed of semiangular to oval quartz, feldspars, rare zircon, and coal. Pyrite and siderite are also present in the sediments. Glaucony grains form both oval clasts ca. 300 μm, max. 500 μm in size and xenomorphic aggregates between oval grains, which could be sometimes considered as a matrix. Glaucony grains are composed of densely stacked micaceous irregular to undulating plates thin only X00 nm. Other matrix is only locally present calcite and secondary quartz, in case of conglomerates also minor Fe-rich phyllosilicate, possibly of the chlorite group. PXRD analyses of glaucony revealed basal line (001) difraction peak at 10.042 Å collapsing to 9.845 Å after glycolation, typical for dioctahedral micas related to mixed-layer glauconite. Microchemical WDS analyses show that glaucony from the oval clast does not differ from the angular glaucony. Its simplified formula is (K_0.82Ca_0.03)_Σ0.85(Fe³⁺_0.93Mg_0.44Al_0.42Fe²⁺_0.19Zn0.01Cr0.01)_Σ2.00(Si_3.75Al_0.25)_Σ4.00O₁₀(OH)_1.98F_0.02. There is a less significant Al substitution into the octahedral sites (^IVAl = 0.25 apfu), significant share of Fe³⁺ in octahedral coordination (ca. 50%), and notably high content of K (of 0.85 apfu interlayer cations an average 96% is K⁺). Approximately 50% of the spots can be interpreted as glauconite and 50% as celadonite according to the latest mica group classification. High K₂O content (9.05 wt.%) and its combination with the total Fe₂O₃ classifies local glauconite as high mature or highly evolved. It is clear that the studied glauconite/celadonite is of shallow marine origin, redeposited from the continental shelf by the turbidite currents.