engleski

Diverse provenance of Pannonian Basin loess indicated by quartz grain SEM images

Loess is terrestrial clastic sediment, composed dominantly of silt-sized particles formed by the accumulation of wind-blown dust. It is usually inter-bedded with soil horizons forming loess-paleosol successions (LPS). Two loess profiles were chosen for the purpose of this investigation. First one is Zmajevac Pleistocene loess-paleosol sequence (LPS) in the easternmost Croatian region of Baranja. It is exposed along the southern slope of Bansko hill on the western bank of the Danube River. The investigated 17.5m thick section displays 4 paleosols embedded in loess. Second one is located in Daranovci, on the northern slopes of Požeška Mt., in Western Slavonia region and it is 150km westward from Zmajevac LPS. It is 25.5m thick section comprised of loose silt and thin conglomerate lenses within. The complex process of producing and transporting silt/sand particles can be best explained with SEM images, which display shape of quartz grains and their surface microtextures. The study of surfaces under high magnification provides insight into the mechanical fractures of quartz grains and explaines different types of origin and transportation processes. On most grains from Zmajevac LPS conchoidal fractures and V-shaped percussion marks are visible. Conchoidal fractures were detected in over 40% of the grains in the samples. A small number of grains have conchoidal fractures that are nearly the size of the length of the longer axis of the grain, while the majority of grains have conchoidal fractures with 1/3 or 1/4 of axis length. V-shaped percussion marks are visible in 15% of the grains. They are usually clustered on smooth, flat surfaces of grains, although in a small number of grains single V-shaped percussion marks are present. SEM images of quartz grains confirmed complex multi-phase transport mechanisms preceding final deposition. The proposal for division into five phases for Zmajevac profile has been made as follows: (1) Grinding, abrasion, thaw-freezing process, (2) fluvial transport, (3) sedimentation in plains, (4) dried sediment deflation (5) aeolian silt sedimentation. The second loess profile in Daranovci dispalys different provenance of quartz grains. Conchoidal fractures that are visible in 40% of grains are one of the most distinct features observed on quartz grains from loess deposits. They can be an effect of glacial and aeolian transport of silt and sand. Another process which can produce these kind of features on quartz grains is salt weathering. Salt weathering can produce up to 12% of silt and sand quartz relative to the whole rock mass from which they originated. Conchoidal fractures observed in samples from Daranovci are not so deep and distinct as in samples from Zmajevac LPS, which leads us to conclusion that salt weathering, and not glacial grinding is dominant process which induced their formation. A majority of grains on their surface also have fracture faces. These features can be formed in almost any kind of geological processes. Feauters observed on SEM images from Daranovci samples imply that quartz grains originated by salt weathering during arid period of early Miocene.

loess; quartz; SEM images; Pannonian Basin

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