LOESSFEST'09 | Aug. 31st – Sept. 3rd, 2009 |Novi Sad-Serbia
Grain-Size Analysis of Plio–Pleistocene Aeolian Dust Deposits (Loess and Red Clay) in the Carpathian Basin
Varga, G.1
1University of Pécs, Instituteof Geography, Departmentof Geology, Pécs, Hungary
The loess–palaeosol sequences are widely considered as the most important terrestrial archive of Late Cenozoic climate changes. Especially in the Carpathian Basin where after the desiccation of the Lake Pannon aeolian processes played dominant role in the sedimentation. Wind-blown loess and loess-like deposits cover more than half of this area, generally underlain by red clay. The stratigraphic relationship between these two formations is still controversial in Hungary, while in China the analytical data have been demonstrated that the Pliocene red clay is also wind-blown in origin, and the loess formation was caused by the higher sedimentation rate and the weaker pedogenic processes.
Several hundreds of samples from Hungarian loess–palaeosol and red clay successions were colected for grain-size analysis. The granulometric parameters have been commonly used as environmental indicators in sedimentary investigations. The grainsize distribution curves of most aeolian sediments are polymodal, and illustrate different transport and/or depositional processes, and can be interpreted as a mixture. The grain-size populations can be partitioned from each other with different mathematic techniques, like parametric curve-fitting (PCF) or end-member modeling algorithm (EMMA). The difference between the results of these two methods arises from their approach. In this paper, due to the many single data, the PCF technique has been used. According to this method the bimodal particle size distributions (PSDs) can be interpreted as the sum of two overlapping Weibull functions.
The detailed granulometric analyses of the red clays show similarity in terms of their bimodal grain-size distribution patterns with loess deposits, and very similar sediment populations could be partitioned from the samples. The coarse grain population (16–63 micron) has been generally transported by surface winds in short suspension episodes. The source of this coarse fraction was largely local material. While the secondary maximums (2–8 micron) could be caused by a second, distant source area; and/or post-depositional weathering processes; and/or particle aggregation. The role of these secondary factors is still controversial.
The PSD’s of the loess and palaeosol samples could be easily distinguished from each other, while the difference in the direction of the older deposits is continually decreasing, indicating smaller amplitude variations in the climatic and depositional system. These changes in the sedimentary cycles could be the result of the secular variations in the Earth’s orbital motion (precession, obliquity, eccentricity). The thick loess–palaeosols sequences of the last 1 Myr represent the pronounced warm–cold 100 kyr cycles. While the dominating 41 kyr cycles, during the Early Pleistocene provided unsuitable conditions for the prolonged, undisturbed loess-formation. The frequent variation of the thin palaeosols and the carbonate nodule beds in the sections record these short-term oscillations. The many carbonate nodule horizons and the colour shifts in the red clay deposits also reflect these low-amplitude variations.