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The Oligo‐Miocene Most Basin is the largest preserved sedimentary basin within the Eger Graben, the easternmost part of the European Cenozoic Rift System (ECRIS). The basin is interpreted as a part of an incipient rift system that underwent two distinct phases of extension. The first phase, characterised by NNE–SSW‐ to N–S‐oriented horizontal extension between the end of Eocene and early Miocene, was oblique to the rift axis and caused evolution of a fault system characterised by en‐echelon‐arranged E–W (ENE–WSW) faults. These faults defined a number of small, shallow initial depocentres of very small subsidence rates that gradually merged during the growth and linkage of the normal fault segments. The youngest part of the basin fill indicates accelerated subsidence caused probably by the concentration of displacement at several major bounding faults. Major post‐depositional faulting and forced folding were related to a change in the extension vector to an orthogonal position with respect to the rift axis and overprinting of the E–W faults by an NE–SW normal fault system. The origin of the palaeostress field of the earlier, oblique, extensional phase remains controversial and can be attributed either to the effects of the Alpine lithospheric root or (perhaps more likely because of the dominant volcanism at the onset of Eger Graben formation) to doming due to thermal perturbation of the lithosphere. The later, orthogonal, extensional phase is explained by stretching along the crest of a growing regional‐scale anticlinal feature, which supports the recent hypothesis of lithospheric folding in the Alpine–Carpathian foreland.

The Early Miocene Bilina Palaeodelta consists of fluvio-deltaic and lacustrine clastics deposited along the south-eastern margin of the extensional Most Basin, part of the Eger Graben in north Bohemia (Czech Republic). The Bilina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault. Exposures up to ca 4·5 km long in the Bilina open-cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small-scale and basin-scale stratal geometries to be evaluated. The Bilina Palaeodelta deposits include sand-dominated, fluvial channel fills and heterolithic sheets interpreted as delta plain strata, sand-dominated mouth-bar wedges and heterolithic sheets of prodeltaic deposits, passing distally into lacustrine clays. The depositional environment is interpreted as a fluvial-dominated, mixed-load, lacustrine delta with a high degree of grain-size segregation at the feeder-channel mouths. On the largest temporal and spatial scales, variable tectonic subsidence controlled the overall advance and retreat of the delta system. The medium-term transgressive-regressive history was probably driven by episodes of increased subsidence rate. However, at this temporal scale, the architecture of the deltaic sequences (deltaic lobes and correlative lacustrine deposits) was strongly affected by: (i) compaction of underlying peat and clay which drove lateral offset stacking of medium-term sequences; and (ii) growth of a fault-propagation fold close to the active Bilina Fault. At the smallest scale, the geometries of individual mouth bars and groups of mouth bars (short-term sequences) reflect the interaction among sediment loading, compaction and growth faulting that produced high-frequency relative lake-level fluctuations and created local accommodation at the delta front.

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 Hrabak 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.