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

• Main Authors: Györgyi Juhasz, Pal Müller, Agnes Toth-Makk
• Format: Article
• Language: English
• Published: Croatian Geological Survey 2004-12-01
• Series: Geologia Croatica
• Subjects: Fluvial sedimentology; Depositional cycles; Sequence stratigraphy; Quaternary; Körös Basin; Pannonian Basin System; Hungary
• Online Access: http://www.geologia-croatica.hr/ojs/index.php/GC/article/view/206

<div>The thickness of the studied Quaternary alluvial complex, located</div><div>in the eastern part of the Pannonian Basin System, can exceed</div><div>500 m. Based on subsurface facies analysis the following large-scale</div><div>depositional elements were identified: channel-fill deposits, point bar</div><div>deposits, alluvial fan (sandy sheet-flood) deposits, floodplain and</div><div>floodbasin deposits, and thinner sandy&ndash;silty beds. They are classified</div><div>into four types of facies associations, showing a characteristic</div><div>stacking pattern on the logs. Facies zonation and basin-scale facies</div><div>mapping of the overall Quaternary sedimentary succession shows that</div><div>in several areas dominated by stacked, multistorey sandy channel fill</div><div>sediments, pre-existing superimposed channel belts can be presumed.</div><div>In the central and deepest part of the basin muddy floodbasin (distal</div><div>floodplain and wetland) sediments dominate. Between these the largest</div><div>area represents the floodplain where single channel fill sands are</div><div>interbedded in the alluvial plain muds. In the eastern part of the basin</div><div>the well-logs highlight the distal part of an alluvial fan where sandy</div><div>sheet-flood deposits alternate with floodplain sediments.</div><div>The recognized facies associations show a vertical pattern, i.e.</div><div>they form a 40&ndash;100 m thick fining-upward fluvial cycle. The most</div><div>characteristic and even ideal cycle can be observed in the channel</div><div>belts and in the proximal floodplain zone. Here the basal member</div><div>of the cycle is made up of multistorey channel fill beds cut into the</div><div>underlying floodplain deposits. This is overlain by an alternating</div><div>sandy&ndash;muddy succession of channel fill and floodplain deposits</div><div>forming the intermediate member. The upper member is composed</div><div>of silty&ndash;clayey floodplain deposits with occasional very thin, discrete</div><div>silty&ndash;sandy bodies.</div><div>These three members form a fining upward sedimentary cycle</div><div>interpreted as representing low-, increasing- and high-accumulation</div><div>space deposits, respectively. As the basal multistorey channel</div><div>fill sandstone facies association generally proved to lie above an</div><div>extensive erosional surface which can be correlated regionally in the</div><div>basin, allocyclic controls can be assumed. In some parts of the basin</div><div>the cycle is not complete as the incised channels can be single, so the</div><div>low-accumulation space deposits can be missing and the high accumulation&nbsp;space deposits, i.e. the aggrading floodplain sediments, can&nbsp;be truncated.</div><div>On a regional scale, six regionally extensive cycles were differentiated</div><div>above each other. Although these cycles were allocyclic</div><div>the question of whether they were tectonically or climatically driven</div><div>remains open. However, the fact that six of them have been identified,</div><div>suggests that they represent the large-scale 400 ka Milankovitch</div><div>cycles during the Quaternary. The tectonic overprint is apparent in the</div><div>thickness and internal architecture of the individual cycles.</div>