Mapping bathymetry and thickness of lacustrine deposits of Lake Balaton (Hungary), using lake seismic profiles

Abstract

Previous research of the geological evolution of Lake Balaton (Hungary) has been based on sporadic borehole data,
evaluation of the topography of the surrounding regions, and water seismic data characterized by low horizontal
resolution. The lake bed itself has been surveyed four times at regular intervals since 1895, but the latest complete survey
was carried out in 1975 using the echosounding and optical geodetic methods of the time.
In the framework of the cooperative efforts of the Eötvös Loránd University and Geomega Ltd, high resolution lakeseismic surveys were carried out on the Eastern Basin of the lake between 1997 and 2007. These provided precise and
valuable data on the Pannonian strata overlain by Holocene sediments. The ultra-high resolution dataset made possible the
determination of the water depth and the thickness of lacustrine sediments on a decimetre scale. A new digital elevation
model of the lake bed was created for the Eastern embayment of the lake, together with the mapping of the Holocene
sediment thickness and the erosion surface underneath. The calculated digital elevation model of the lake bed was
compared to a model interpolated from the 1975 survey and thus the sedimentation rate could be calculated for the last 30
years. Although these surveys carry several possible sources of error, the calculated average deposition rate is in the same
order of magnitude as the results acquired by previous authors using other methods. The present study also demonstrates
that there are considerable differences in the sedimentation pattern and the differences show similarities with the
topography of the discordance surface underlying the Holocene lacustrine sediment. The discordance surface separating
the Pannonian and other pre-Holocene strata from the overlying Holocene lacustrine sediment shows a row of elevated
features along the axis of the lake; these features have low slope angles towards the south and are relatively steep on their
northern side. The line of this northern slope breaks in several places. There are no elements on the discordance surface
surveyed which are similar to the meridional valleys of the areas to the south of the lake. The 30-year-scale lake bed
changes, and the discordance surface of the lacustrine sediments and Pannonian strata also exhibit similarities with the
characteristic directions of the surrounding topography. Thus they can partly be correlated with the previously
documented neotectonic movements.