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Annamária Nádor, Geological Institute of Hungary (Hungary)
Ágnes Tóth-Makk, Geological Institute of Hungary (Hungary)
Edit Thamó-Bozsó, Geological Institute of Hungary (Hungary)
Edit Babinszki, Geological Institute of Hungary (Hungary)
Árpád Magyari, Geological Institute of Hungary (Hungary)
Zsolt Kercsmár, Geological Institute of Hungary (Hungary)
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Studies of long-term evolution of Quaternary fluvial systems in the Pannonian Basin and their responses to climate changes and tectonics significantly contribute to the fuller understanding of Late Neogene basin development of the Intra-Carpathian area. About 2 Ma ago a major change from the former extensional to compressional stress field led to basin inversion, which caused uplift of the marginal flanks and the western part of the Pannonian Basin, and increased subsidence of different sub-basins, which became areas of continuous fluvial sedimentation. The drainage pattern development of the Danube and Tisza Rivers and their tributaries was primarily controlled by differential subsidence rates at various parts of the basin, thus vertical motions were fundamentally responsible for the accumulation of alluvial strata in diverse thicknesses. The subsiding basins are excellent archives of long-term fluvial sequences which record changes in discharge regimes, sediment supply and sediment storage over time-scales of 1-100 ky, which are all manifested in different ways in the alluvial architecture.
Morphological responses of the studied fluvial systems are best reflected in changes in channel geometry. Clear and unambiguous recognition of plan-views are limited to Late Quaternary examples from the Tisza river and its tributaries. A clear correlation between fluvial activity and millennial-scale climate changes was detected in a basin margin setting, where different phases of braided and meandering fluvial styles were identified corresponding to stadials and interstadials during the Late Pleistocene, resulting in the formation of different geomorphic levels. This direct response was not demonstrated in a subsiding basin interior setting, where fluvial aggradation kept pace with subsidence irrespective of a meandering and/or braided fluvial activity, so different channel planforms could not be preserved as temporally and spatially separated geomorphic surfaces, such as on the basin margin.
Sedimentological and stratigraphic responses of rivers to allogenic controls can be defined in a number of ways: e.g. in the proportions and spatial distribution of different lithofacies and facies associations reflected in the basin scale fluvial architecture. Our model on fluvial responses to Milankovitch-scale climate changes suggests that an increased sediment flux towards the basin during interglacials was due to an increased discharge and transport capacity of the rivers, while a decreased sediment supply to the distal parts of the basin occurred during glacials. This means a delay between erosion and temporary storage of weathering products near to the source area and subsequent transportation and re-deposition of sediments into the basin interior.
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