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MINERALOGICAL CHARACTERISTIC OF PELITIC ALLUVIAL PLEISTOCENE SEDIMENTS IN THE AREA OF SESVETE (ZAGREB, CROATIA)

During the mapping for the Basic Geological Map of the Republic of Croatia 1:50.000 two boreholes (Badel-1 of 71.50 m and Badel-2 of 84.40 m) were studied in the area of Sesvete near Zagreb. Sediments from boreholes belong to the Pleistocene, informal Bistra Formation which is composed of silts, clays, sands and gravels. Within these sediments three different facies were interpreted [1]: 1) gravels and sands typical for alluvial environments 2) structureless beds of silts and clays deposited in oxbow lakes, marshes and floodplains 3) chaotic sediments deposited in the upper part of the alluvial fan. Mineralogical analysis of pelitic sediments from structurless beds facies of oxbow lakes, marshes and floodplains included X-ray powder diffraction (XRPD) analyses on random mounts of bulk samples and oriented mounts of the <0.063 µm fraction. Oriented mounts of the <0.063 mm fraction were record after the following treatments: a) air drying, b) saturation with K+ and Mg2+, c) ethylene-glycol solvation, d) DMSO solvation, e) glycerol solvation, f) heating to 400°C and 550°C. The main mineral components of these sediments are quartz, swelling clays and illite/muscovite. Among the swelling clays smectite and vermiculite are determinate in various ratios. Calcite, feldspar, kaolinite are present in a lesser quantity, while chlorite is present only in some samples. According to [1] studied sediments from boreholes are a product of intensive weathering of different types of rocks from the Medvednica Mt. with the prevalence of green-schists. Smectite present in pelitic sediments probably originated from Miocen marls. Absence of chlorite and presence of vermiculite are in accordance to intensive chemical weathering during the humid climate period in the Pleistocene. This study was supported by Croatian Science Foundation Project No. 4425. [1] 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). Geologia Croatica 70, (in press), doi: 10.4154/gc.2017.01

Clay minerals, Pleistocene, alluvial sediments, Zagreb

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