Reconstruction of a Pleistocene meandering river in East Hungary by VHR seismic images, and its climatic implications

The upper 20-30 m of the Pleistocene alluvial sequence on the Tisza River, Hungary is investigated by 200 km of VHR single-channel seismic sections. Frequent internal configurations, such as (1) bundles of inclined reflections, (2) trough-shaped converging, and (3) horizontal, parallel reflections are interpreted, respectively, as lateral accretion of point bars, abandoned channel fills and floodplain deposits. The series of 5-8 m-thick inclined strata sets occurring at the same depth throughout the area suggest the presence of a meandering river dated to about 43-47 ka in the Late Pleistocene according to OSL data. Newly recorded quasi-3D seismic profiles reveal that each 1-3 km-long inclined series correlates with an individual point bar continuously migrating in the same direction. Small deviations in the direction of migration are caused by natural wandering of the meander bend. Morphometric parameters of point bars are used to calculate ancient channel parameters (width and depth of 200-500 m and 5-8 m, respectively). Values imply a channel-forming discharge of Q = 400-900 m3/s. These results imply a greater amount of runoff and supply in the basin during MIS3 compared to the present day. Long-lasting meandering indicates the role of local factors and a different fluvial environment compared to the commonly anastomosing systems in NW Europe. Our data support the thesis of the three-axis drainage system for the basin. New evidence is provided for the antecedent existence of a temporally active third trunk river in addition to the ancestors of Danube and Tisza Rivers, contrary to the present-day two-axis drainage pattern.

► VHR seismic and shallow drilling revealed the architecture of a meandering river. ► Ages based on OSL dating are in range of 43-47 ka in the Middle Pleniglacial. ► 3D morphometric analysis of 1-3 km-long lateral accretion series of point bars. ► Discharge calculations indicate more humid climate. ► Results provide evidence for the 3axis-hydrographic model of the Carpathian Basin.