The Alpar canyon system in the Pannonian Basin, Hungary – its morphology, infill and development

Giant incised canyons were recently recognized in Late-Miocene post-rift sediments in the central part of the Pannonian Basin. Though not connected to the world seas, Lake Pannon shows significant signs of relative lake level variations controlled by tectonics and climate changes. The incision surface of the Alpar canyon system is connected to SB Pa-4 (6.8 Ma sensu Vakarcs, 1997), earlier reported to represent a significant relative base-level fall in the basin, however, debated recently. Incised several hundred meters in the preexisting substrate, the individual canyon valleys of the Alpar canyon system are enormous in size and display a multi-story nature. They loose topographic expression headwards and basinward. Widths of individual valleys range from 5 to 10 km, with smaller tributaries. In the study area several adjacent canyon valleys can be seen on seismic profiles. The valley depth is greatest near their confluence, where a major trunk valley (600–700 m deep) was formed by eroding most of the Upper Miocene succession. The canyons are filled with clay marls. They are incised into an extremely thick aggrading deltaic complex and are overlain by fluvial sediments, suggesting a major transgression in between. The Late Miocene Alpar canyon system developed on the southern margin of the Mid-Hungarian Mobile Belt, the latter is characterized by NE-SW oriented fold axis and NE-SW oriented left lateral strike-slip faults. The canyon system coincides with a large releasing bend and/or extensional duplex of the Paks–Szolnok strike-slip system being active as sinistral during the Late Miocene. Presumably, the formation of the deep canyons was generated by the close interaction of several factors and events in space and time, among them tectonic uplift forced relative base-level fall, the reactivation and bending/duplexing of a strike-slip fault system located near the coeval zone of the lake shoreline and shelf edge, and the possible change of sediment supply carried by overfed rivers.