Control of the Late Neogene (Pannonian s.l.) sedimentation by basement deformation in the Zala Basin

Abstract

In the Late Miocene age the prograding margin of the shelf of Lake Pannon reached the Zala Basin, the latter is a deep
sub-basin in south-western Pannonian Basin and has an age of about 8 Ma. The composition of the Pannonian s.l. infill
of the Zala Basin is similar to the coeval successions of other sub-basins. The unique feature of the area is the strong
deformation of the Pannonian s.l. strata by a series of west–east trending synclines and anticlines. The latter form
structural traps and are significant for their hydrocarbon accumulations. In this paper the steps of the infill of the Zala
Basin are reconstructed in detail, based on the interpretation of approximately 1000 km of 2D seismic profiles. An
attempt is also made to detect whether the formation of the folds had any effect on the process of the infill.
Within the deposits of the slope connecting the shelf and the deep basin, nine individual lobes were distinguished and
mapped in the study area. The respective thicknesses of the lobes reach a few hundred metres, while their areal extent is
about 500 km2
. The oldest lobes reached the area from the north (i.e. from the direction of the Danube Basin) and,
subsequently, younger ones followed southwards. In some cases, a younger lobe is located 15–20 km east or west of the
previous one, probably as a result of autocyclic switching of the feeding deltaic system. However, the slopes uniformly
dip towards south-southeast even in these cases. The relief between the shelf and the slope toe indicates that the lobes
prograded into water depths of 200–300 m in the central part of the studied area, and about 400 m in its northern and
southern parts.
The Budafa anticline — now located in the central Zala Basin — has no effect on the process of progradation;
however, the area of the southern Belezna anticline is bypassed by the lobes of the slope. Following the passing away of
the shelf edge, the sedimentation rates of shallow lacustrine and alluvial deposits throughout the sub-basin are uniformly
much lower in the area of the present-day anticlines than they are above the present-day synclines. Hence the formation
of folds should have begun at the time when the shelf margin was crossing the southern part of Zala Basin (8–7.5 Ma).
During the following few hundred kiloyears, active folding resulted in considerable spatial differences in the rate of
basement subsidence (although uplifting and erosion from that time cannot be observed in any part of the area).