MILANKOVITCH-SCALE CYCLICITY IN FLUVIAL SEDIMENTS OVER THE LAST 2.6 MA, PANNONIAN BASIN, HUNGARY: CAUSES AND CONTROLS

Unlike terrace sections with large unconformities, thick cyclic fluvial sediments deposited in large alluvial basins under ongoing subsidence may provide fairly complete records over multiple glacial-interglacial cycles, and thus may be used to unravel the relationship between cyclic sedimentation and climate changes. Despite these promising possibilities, little consideration has been given to their use as long continental paleoclimate proxies.

An almost 500 m thick sequence of cyclic fluvial sediments in the eastern part of the Pannonian Basin in Central Europe has been preserved due to continuous basin subsidence. Magnetostratigraphy revealed that the series span the last 2.6 Ma. Comparison of systematic variations in magnetic susceptibility with changes in grain size distribution characterized by Mz values in key boreholes along an E-W transect, parallel to the main transport direction, revealed a cyclicity in sedimentation. The periodicity of the cycles was estimated by dividing the stratigraphic timespan analysed by the number of MS maxima, which gave a cyclicity of 100 kyr for the borehole successions younger than 1 Ma, and a 40 kyr cyclicity down to the bottom of the Matuyama. By comparing the Mz and MS records with the ODP 677 oxygen isotope proxy, using polarity reversals as fixed points, it was shown that despite the two vastly different environments, the sedimentary cycles of the Hungarian boreholes clearly show a similarity to the marine record, revealing there was an orbitally forced control on the fluvial sedimentation.

The depositional model for the Pleistocene fluvial sediments 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 storage of weathering products near to the source area and subsequent transportation and re-deposition of sediments in the basin interior setting.

The research was supported by the Hungarian National Scientific Research Fund (OTKA T-32956).