International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Mineralogy and geochemistry of Upper Miocene pelitic sediments of the Zagorje Basin (Croatia): implication for evolution of the Pannonian Basin

Pages: 263 - 276


Abstract: According to X-ray powder diffraction analyses of Upper Miocene pelitic sediments from the Zagorje Basin all of the samples contain same mineral species, but in significantly different quantities. Calcite is a dominant component in most of the samples (31–74 wt. %), clay minerals (18–50 wt. %), quartz (5–21 wt. %) and feldspars (1–5 wt. %) are less abundant, while dolomite and pyrite are present only in a few samples. Among clay minerals, in <2 mm insoluble residue fraction, smectite, illite, chlorite and kaolinite were determined. The dominant constituents in 0.09–0.16 mm fraction, as determined by optical microscopy, are quartz, feldspars, rock fragments (chert, quartzite and schists) and micas. Chlorite, limonite, pyrite, garnet, tourmaline, zircon, epidote and staurolite are present as heavy minerals. Pelitic sediments within older (Upper Pannonian) investigated sediments are, in accordance with the mineral composition of insoluble residue and CaCO3 content, classified as marls, while those in the younger (Lower Pontian) sediments are silty marls. The observed gradual decrease in carbonate content, and simultaneous increase of clayey-silty component, going from older to younger deposits, is the result of the gradual increase of terrigenous influence. Mineral composition of marls together with elemental ratios critical of provenance (SiO2/Al2O3, K2O/Na2O, Eu/Eu*, La/Sc, Th/Sc, Th/Co, Th/Cr and La/Co) and source rock discrimination diagrams (Fe2O3-K2O-Al2O3 and La-Th-Sc), point out that source rocks were from the Upper Crust and remarkably felsic in nature. The chemical composition of the sediments and modal composition of silt-size fraction indicate Alpine provenance of the clastic material.

Keywords: Upper Miocene, Zagorje Basin, chemical composition, clay minerals, marl, X-ray diffraction

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