Effects of spatially variable accommodation rate on channel belt distribution in an alluvial sequence: Authigenic 10Be/9Be-based Bayesian age-depth models applied to the upper Miocene Volkovce Fm. (northern Pannonian Basin System, Slovakia)

Abstract

Relationships between spatially variable accommodation rate and the distribution of channel belt deposits are important issues in alluvial stratigraphy. This segment of sedimentology still records important progress using experimental models and field examples. This study focuses on the alluvial stratigraphy of the Volkovce Fm. which was deposited between ~9.0–6.0 Ma in the northern Pannonian Basin System, and it provides an excellent natural laboratory to study this topic. The spatial variability of channel belt distribution was determined in 809 borehole logs drilled to 200–300 m depths while the facies associations were defined on the outcrops. A significant variance in channel belt content was observed, since it reaches 10–40% of the stratigraphic column in the southwestern part of the study area and only 0–15% in the northeastern portion. Moreover, the transition between the areas of different channel belt distribution is oriented perpendicular to the paleo-drainage network axis. Geochronology was then established by applying the authigenic 10Be/9Be dating method to three borehole cores. The Bayesian age-depth models used herein proved a decrease in average accommodation rates from ~200 m/Myr to ~50 m/Myr towards the area with <15% of channel belts. The results indicate that the four times higher subsidence caused dominant deposition of channel belts in the area of maximum subsidence and avoided the basin-scale avulsion to relocate the channels in the area of lower accommodation rate. This was most likely caused by topographic confinement. The deposition of poorly-drained floodplain and swamp facies appeared in the area of lower accommodation rate, where absence of channels caused low sediment supply. The study highlights that authigenic 10Be/9Be dating Bayesian age-depth models can provide a strong geochronological framework for the late Cenozoic alluvial depositional record necessary for interpretation of the influence of allogenic factors on stratigraphic architecture.