A giant canyon system incised into the Late Neogene (Pannonian s.l.) post-rift sediments?

Abstract

Giant canyons incised into the Pannonian s.l. post-rift sediments have been recognized in the central
part of the Hungárián Plain along several seismic lines. The canyon system has a surprisingly large size
for a lacustrine setting. The incision surface is connected to sequence boundary (SB) SB Pa-4 (6.8 Ma by
VAKARCS 1997), similarly to other incised valleys in the region. This SB surface was considered to have
been associated with a significant base-level fali in somé areas of the basin due to tectonics (CSATÓ 1993,
VAKARCS et al. 1994, SACCHI et al. 1999, JUHÁSZ et al. 2006,2007). Accordingly, the canyon developement is
presumably alsó connected to these structural changes. The study area documents the deepest part of
the canyon system, it is incised several hundred metres in the preexisting substrate, into an extremely
thick aggrading deltaic complex. The canyon fills are overlain by fluvial sediments. The respective width
sof the individual canyon valleys rangé from 5 to 10 km but 1 to 2 km wide tributaries can alsó be seen.
At least two or even more canyons can be seen next to each other. This could be either the representation
of a meandering feature, or this is the area of a major trunk valley where the confluences can be seen on
the seismic profiles. The depth of the valley is greatest at and around this confluence area (it can reach
600-700 m) and in the deepest part it has almost entirely eroded the Late Miocéné (Pannonian s.l.)
sedimentary succession. The canyons are fiiled mostly with clayey deposits, and in a few rare instances
intercalated with somé silty-sandy succession of gravity sediments. In a definite time interval would
have served as feeder channels for the turbidites of the Makó-trough.
Conventional and modern models and examples of deep-water canyons were all worked out for
deep-marine settings, and these cannot be readily applied to the relatively small, restricted basin of Laké
Balaton. What could have caused such large-scale phenomena in a laké? The major valleys of the
canyons have a multiphase history based on their dimensions and shape in connection with the fact that
they are embedded in a lacustrine succession. On the other hand their size is several times larger than
the height of active feeder channels of large deep sea fans.
According to structural analysis, the canyon system occurs at a large releasing bend and/or extensional
duplex of the Paks-Szolnok strike-slip system, which was active as sinistral during the Late Miocéné.
Therefore, the formation of the deep canyons was presumably generated by the interaction of a relatíve
base-level fali, the reactivation and bending/duplexing of a strike-slip system precisely in the same area,
and the large sediment supply carried by overfed rivers.