Alluvial Architecture and Fluvial Cycles in Quaternary Deposits in a Continental Interior Basin, E Hungary

Abstract

The thickness of the studied Quaternary alluvial complex, locatedin the eastern part of the Pannonian Basin System, can exceed500 m. Based on subsurface facies analysis the following large-scaledepositional elements were identified: channel-fill deposits, point bardeposits, alluvial fan (sandy sheet-flood) deposits, floodplain andfloodbasin deposits, and thinner sandy–silty beds. They are classifiedinto four types of facies associations, showing a characteristicstacking pattern on the logs. Facies zonation and basin-scale faciesmapping of the overall Quaternary sedimentary succession shows thatin several areas dominated by stacked, multistorey sandy channel fillsediments, pre-existing superimposed channel belts can be presumed.In the central and deepest part of the basin muddy floodbasin (distalfloodplain and wetland) sediments dominate. Between these the largestarea represents the floodplain where single channel fill sands areinterbedded in the alluvial plain muds. In the eastern part of the basinthe well-logs highlight the distal part of an alluvial fan where sandysheet-flood deposits alternate with floodplain sediments.The recognized facies associations show a vertical pattern, i.e.they form a 40–100 m thick fining-upward fluvial cycle. The mostcharacteristic and even ideal cycle can be observed in the channelbelts and in the proximal floodplain zone. Here the basal memberof the cycle is made up of multistorey channel fill beds cut into theunderlying floodplain deposits. This is overlain by an alternatingsandy–muddy succession of channel fill and floodplain depositsforming the intermediate member. The upper member is composedof silty–clayey floodplain deposits with occasional very thin, discretesilty–sandy bodies.These three members form a fining upward sedimentary cycleinterpreted as representing low-, increasing- and high-accumulationspace deposits, respectively. As the basal multistorey channelfill sandstone facies association generally proved to lie above anextensive erosional surface which can be correlated regionally in thebasin, allocyclic controls can be assumed. In some parts of the basinthe cycle is not complete as the incised channels can be single, so thelow-accumulation space deposits can be missing and the high accumulation space deposits, i.e. the aggrading floodplain sediments, can be truncated.On a regional scale, six regionally extensive cycles were differentiatedabove each other. Although these cycles were allocyclicthe question of whether they were tectonically or climatically drivenremains open. However, the fact that six of them have been identified,suggests that they represent the large-scale 400 ka Milankovitchcycles during the Quaternary. The tectonic overprint is apparent in thethickness and internal architecture of the individual cycles.