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Interplay between tectonics and compaction in a rift-margin, lacustrine delta system: Miocene of the Eger Graben, Czech Republic



The Early Miocene Bílina 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 Bílina 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 Bílina 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 Bílina 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 Bílina 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.