Depositional record of an avulsive fluvial system controlled by peat compaction (Neogene, Most Basin, Czech Republic)



This paper illustrates the response of a fluvial depositional system to the interplay between peat compaction and clastic sediment supply, at a range of spatial and temporal scales, as documented by extensive exposures in an open-cast mine in the Most Basin, part of the Oligo-Miocene Ohře Rift (Eger Graben) basin system in the Czech Republic. The Most Basin is characterized by the occurrence of a number of phenomena resulting from syn- and post-depositional interactions between clastic sedimentary systems and the underlying accumulation of organic material that was the precursor of the main lignite seam of up to 45 m thickness. The studied strata are interpreted as deposits of an avulsive, mixed-load fluvial system. The large-scale depositional architecture documents an existence of at least five stratal packages up to 1500 m wide and up to several tens of metres thick, representing a record of long-term evolution of a clastic floodplain bordered by accumulating peat. Within each of the packages, several small-scale channel-belts were documented. Individual packages are separated by carbonaceous mudstones indicating a period of reduced clastic input and interpreted as due to avulsion of the fluvial channels out of the floodplain limit. Two main, mutually linked, processes controlled the evolution of the studied fluvial system: (i) syndepositional compaction of the underlying peat and (ii) avulsions of the channels away from the original floodplain, resulting in formation of a new floodplain. The processes which caused the channels of the Hrabák fluvial system to reach the avulsion threshold were: (i) decrease of rate of creation of accommodation leading to increased sinuosity and thus to a decreased channel slope, and (ii) cross-floodplain tilting of the channel belt caused by differential compaction of underlying organic-rich substratum.