Abstract:
The Pannonian basin contains the thickest continuous continental Pleistocene sedimentary record in Europe. The sedimentary cycles of this 400-500 m thick fluvial sequence were studied in two cored boreholes (Devavanya D-1. and Veszto V-1.) in the Koros basin, at the southeastern part of the Pannonian basin. Different parameters (average grain size, magnetic susceptibility (MS), mineralogical maturity, palaeontological data including pollen and gastropods) showed a cyclical distribution in the studied boreholes. The few m scale lithological variations of the average grain size proxy represent the autocycles of the fluvial sequence, while its smoothed curve corresponds to the climate controlled allocycles. Pollen and gastropod data, as well as mineralogical maturity, showed that the high values of magnetic susceptibility correspond to warm climate periods, while the low susceptibility values refer to cold climate periods. It was discovered that a 40 000 year cyclicity characterises the older Pleistocene strata (older than 1 Ma), while the cyclicity is about 100 000 years in the younger Pleistocene, similar to the (super 16) O/ (super 18) O oxygen isotope variations of the marine ODP 677 site. Despite a striking difference between the continental fluvial sequence of the Pannonian basin, and the marine oxygen isotope variations in the ODP 677 borehole, the good correlation between the number of the cycles, and the variance of the proxies suggest that the sedimentary cycles in the D-1. and V-1. boreholes correspond to orbitally forced 5th order Milankovic cycles. The full correlation of the MS and the oxygen isotope proxy of the ODP 677 site further supports the idea that magnetic susceptibility is a good paleoclimate indicator in fluvial environments. Comparing the grain size distribution and the climate changes, it was discovered that sandy parts characterise warm climate periods, while fine-grained parts correspond to cold climate intervals. The studied boreholes represent the central part of the basin, where sedimentation was subordinate due to low average discharge during the cold climate periods. Although seasonal floods with high peak discharge temporarily deposited a large amount of coarse-grained debris in alluvial fans at the basin margin, only fine-grained dispersed sediments from glacial outwash reached the distal parts of the basin. However, during the warm climate periods, due to the higher average discharge and transport capacity, the rivers incised in the hinterland areas, and deposited their sandy sediments in the distal parts of the basin. The full correlation of the magnetic susceptibility and the marine oxygen isotope proxies shows that sedimentation was continuous, at least on the scale of Milankovic cycles.