Lithological characteristic of the riverbed alluvium of the sixth terrace of the Dnister–Stryvihor in the Dubrivka section (Forecarpathians)

The main lithological characteristics of riverbed facies of alluvium of the sixth terrace above the floodplain of the Dnister-Stryvihor Rivers (Loyeva level) near Sambir were analysed. The granulometric and petrographic composition of the gravelly-pebble material, its roundness, form and the orientation of grains were characterised. Lithological analyses helped to determine that investigated layer of alluvium are poorly sorted. The three main components of alluvial strata (pebbles, gravel and sand-loamy filler) are present in approximately comparable proportions. Boulders are least common in the composition the alluvium. Coarse material is preferably well and medium rounded. Poorly and very well rounded fragments happen considerably rarely. The preferred direction of transport of fragment materials was Northwest–Southeast. Its direction is approximate to the current of modern Stryvihor. The fragments of the Carpathian rocks (sandstones, siltstones and silicates) dominate in a petrographic composition of coarse-grained alluvial deposits. That means that the Carpathians were the main source of supply of clastic material during the formation of investigated strata of alluvium. The rocks of local origin, which is involved from the bed of alluvium during the cutting of paleoStryvihor in the thickness of pre-Quaternary rocks of the Carpathian Foredeep, were mixed with the fragments of the Carpathian rocks in small amounts. The results of lithological studies show that paleo-Stryvihor accumulated the alluvial strata of Dubrivka section after that time when it had cut into the socle of Torhanovychi terrace up to 8–10 m before the deposition of alluvium. In other words, the preserved fragment of Loyeva level between the villages Torhanovychi and Dubrivka that is on the Dnister-Stryvihor interfluves consists of two terraces of different ages. One of them is a hypsometric higher and older Torhanovychi terrace, which was formed by paleo-Dnister River, and the other one is a hypsometric lower and respectively younger Dubrivka terrace, which was formed by paleo-Stryvihor River. Key words: alluvium, granulometric composition, petrographic composition, roundness of grains, sandstones, siltstones, silicates, gaizes, current of palaeochannel, Loyeva level.