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SIGNS OF PALEOSOL FORMATION IN BASHKIRIAN CARBONATE ROCKS OF VOLGA-URAL REGION
Abstract
Fragments of ferruginous brecciated limestones were studied in Bashkirian geological cross-sections in central part of Volga-Ural sedimentary basin. It is established that ferruginization of carbonate rocks is not controlled by facies. Mudstones, stromatolite boundstones and foraminiferal packstones have traces of diagenetic ferruginization. All carbonates are characterized by leaching traces in the form of sub vertical and inclined fractures and big pores filled by fine-grained calcite. Clasts of ferruginization carbonate breccias cemented by mixed carbonate-clay material. Clasts cemented by mixed carbonate-clay material. X-ray diffraction analysis showed that kaolinite and / or mixed-layer illite-montmorillonite mineral phases predominate in the composition of the clay component. Impurities there are grains of quartz, plagioclase, microcline, amphibole, pyroxene, clinoptilolite, mordenite and muscovite. Such mineral association is not characteristic of offshore shelf Bashkirian deposits. Usually, clay minerals in rocks are represented only by chlorite and illite. Amphiboles, pyroxenes and zeolites are absent. The study of ferruginous fragments of carbonate rocks showed that the calcite crystals contain structural defects in the form of isomorphous impurities of the ions Mn2+, SO2-, Corg, and PO2, by electron paramagnetic resonance (EPR). The group of paramagnetic defects indicates depositional environments as normal marine with the predominance of reducing conditions near sea bottom. Simultaneously, on EPR spectra are fixed broad intensive absorption lines characteristic for finely dispersed goethite phases that formed in subaerial conditions. Lithological and mineralogical composition of ferruginous breccia show that they are marking horizons of subaerial exposure. Structure and mineral composition of the rocks indicate them as fragments of paleosoils. Mineral associations of breccia cement show aeolian origin and primary material of Bashkirian paleosoils was volcanic ash. During meteoric diagenesis of humid paleoclimate, pyroclastic material was transformed to kaolinite and illite-smectite. This material together with fragments of other minerals, penetrated to fractures during dissolution of the carbonate subsoil horizon. Colloidal particles of goethite and humus penetrated into the underlying soil cover together with the infiltration waters. Fragments of different facial type of limestones acquired a reddish and black color, as a result. The subsequent transgression of sea washed away the soil layer and saved dense subsoil carbonate horizon composed by ferruginization carbonate breccias.
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