A Pleistocene meander loop near Fonyód: remnants of channels on ultra-high resolution seismic images, Lake Balaton

Abstract

A curved erosional surface with a radius of 1.5 km and a width of more than 600 metres can be identified on the
ultra-high resolution seismic sections acquired on Lake Balaton in the vicinity of Fonyód. The corresponding sediments
above the surface are 1–2 m thick. The three-dimensional shape, as well as the oblique reflections parallel with the trend
of incision reveal the remnant of a meandering channel. This channel fill unit is situated unconformably above the tilted
and folded Pannonian strata, and is overlain disconformably by the Holocene mud of Lake Balaton. The channel-fill
deposit is younger than the deformation of the Pannonian, and was most likely formed somewhat before the wave
activity associated with the development of the lake. Therefore the supposed age of the sedimentary body is Late
Pleistocene.
The curvature of the meander seems to follow the Fonyód Hill on the shore of Lake Balaton, suggesting that the
topography and the lithology of the hill determined the course of the Pleistocene river. Moreover, this meandering
channel corresponds in terms of size, location and curvature with meanders situated outside the study area, below the peat
of the Nagyberek, as mapped by MIKE (1976) from a series of densely-spaced shallow cores obtained near Fonyód. MIKE
regarded these fluvial sediments as remnants of the „early Pleistocene Danube” which used to flow across this part of the
country, although recent palaeohydrographic concepts dispute this idea. Nevertheless, a river of significantly high
discharge must have left these channel belt deposits as it crossed the valley south of the Balaton Highland, routing towards
the subsiding Danube Valley during the Late Pleistocene.