Geochemical and mineralogical characteristics of alluvial Quaternary sediments on the slopes of Medvednica Mt. (Croatia)

During research for the Basic Geological Map of Republic of Croatia 1:50000 and Croatian Science Foundation Project No. 4425 - SAPIQ, alluvial Quaternary sediments on the slopes of Medvednica Mt. which belong to the informal lithostratigraphic unit Bistra Formation (AVANIĆ et al., 2006 ; GRIZELJ et al., 2017), were studied using several different approaches. Geochemical analysis included determination of major and trace element compositions using inductively coupled plasma emission spectroscopy (ICP-ES) and inductively coupled plasma mass spectrometer (ICP-MS). Mineralogical–petrological analysis included: microscope analysis of unconsolidated sedimentary rocks, heavy minerals analysis and X-ray powder diffraction (XRPD). Palynological analyses were used for paleoenvironment reconstruction. The interpretation of the depositional environment and the provenance of the sediments are based on the obtained data. The informal Bistra Formation lies unconformably on the Pannonian informal Formations Medvedski Breg or Andraševec. It consists of gravely, sandy, silty and clayey poorly sorted sediments which were deposited in an alluvial environment in the area of sheet flows, crevasse splays, longitudinal bars and flood plains. It is overlaid by loess-like sediments and fluvial Holocene sediments. The chronostratigraphic position of the informal Bistra Formation is still under debate due to a lack of absolute dating results, but according to palynological data and superposition it is assumed to be of Pleistocene age. The main mineral components of clayey sediments are quartz, feldspar, swelling clays (vermiculite and/or smectite), illite/muscovite and in a lesser quantity kaolinite and chlorite. The composition of gravels and the presents of epidote group minerals in the heavy mineral fraction of silty sand sediments indicate a derivation of detritus from different types of schist rocks occurring on Medvednica Mt. On the other hand, quartz grains and chert fragments in the light mineral fraction of the silty sand sediments, together with elemental ratios critical of provenance (LREE/HREE, Eu/Eu*, La/Sc, Th/Sc, Th/Co, Th/Cr and La/Co) and the source rock discriminant diagram of ROSER & KORSH (1988) imply that the source were different types of rocks, but predominantly acidic (silicic) rocks. Keywords: Bistra Formation, Quarternary, Medvednica Mt., Alluvial sediments, provenance AVANIĆ, R., BAKRAČ, K., GRIZELJ, A., WACHA, L., ŠIMIĆ-STANKOVIĆ, M., HEĆIMOVIĆ, Lj., TIBLJAŠ, D. & KRUK, B. (2006): Ivošević Gaj ceramic clay deposit in the vicinity of Vojnić. In: VLAHOVIĆ, I., TIBLJAŠ, D. & DURN, G. (eds.): 3rd Mid-European Clay Conference. 18-23. September, 2006, Opatija, Field Trip Guidebook, 39-47. GRIZELJ, A., BAKRAČ, K., HORVAT, M., AVANIĆ, R. & HEĆIMOVIĆ, I. (2017): Occurrence of vivianite in alluvial Quaternary sediments in the area of Sesvete (Zagreb, Croatia). Geol Croat, 70, 41-52.