engleski

Muscovite micro textures as a clue for interpretation of sediment transport

Each investigation of a loess section begins with field work and the first question: Is it loess sensu stricto or redeposited loess material? Often it is very difficult to distinguish, but the whole approach to research depends on the answer. Our research was focused on wear micro textures of muscovite and quartz grains and their usability in the sediment transport interpretation. In previously published papers on mineral grains transport, researchers are almost exclusively focused on abrasion and impact micro textures on mechanically and chemically resistant quartz grains. There is a very extensive work on this topic, especially in the field of aeolian sediment transport. Based on quartz grain research, it is possible to distinguish micro textures formed as a consequence of water, wind or ice transport media. Still, there is a problem in the case of short or non-intensive sediment transport, when quartz grains are not stressed enough to develop characteristic micro textures. This is particularly important in the case of multiple grain transport or resedimentation. The last transport mechanism in most cases cannot overprint micro textures developed on quartz grains during the previous transport stage. This is especially case for the aeolian reworking of fluvial deposited material, but also for short fluvial resedimentation of aeolian sediments. In our research, muscovite grains came out as an excellent material for micro texture study. Muscovite has very high chemical resistance in common atmospheric conditions (almost like quartz), but low mechanical hardness (Mohs 2.5 to 3.0). Layered structure and perfect basal cleavage result in muscovite characteristic platy appearance. In our study, material from known localities (the Zmajevac loess section), with previously interpreted sedimentological conditions, is examined ; the micro textures were compared with the interpretation based on quartz grains and sediment textures (Figure 1). In the second stage of our research, muscovite from the source rocks (pegmatite, granite and mica schist) is collected and used as a reference material. Furthermore, stream sediments representing mineralogical composition of catchment area (occupied by above mentioned muscovite bearing rocks) are sampled on the several downstream locations. Collected material is compared with the reference material from the source rocks to establish relation between transport distance, water energy and developed micro textures on both, muscovite and quartz grains. Additionally, a numerous experiments in water and wind transport media were conducted in a well-controlled laboratory? environment. An experimental setup was constructed in order to simulate fluvial and aeolian grain transport. During each experiment flow intensity of water or air stream was varied and measured. Experiments duration was between 2 hours and 7 days. Our results showed that muscovite wear micro textures can be successfully used in sediment transport interpretation. Quartz micro textures indicate transport mechanisms that grains were exposed to and often it is hard to distinguish order, duration and energy needed for their formation. Thus, we cannot exclude possibility that formation of a micro texture on quartz grain is related to physical weathering of source rock or transport just after weathering, nor we could unequivocally conclude about the last sedimentation process. However, muscovite micro textures reflect the last sediment transport media better than quartz grains.

muscovite ; quartz ; microtextures ; alluvial transport ; aeolian transport

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