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Pre-sedimentary palaeo-relief and compaction: controls on peat deposition and clastic sedimentation in the Radnice Member, Kladno Basin, Bohemia

Abstract Pre-sedimentary palaeo-topography was a major control on facies distribution during the deposition of the basal unit (the Radnice Member) in the Kladno Basin. The undisturbed accumulation of peat took place in calm valleys protected by surrounding ridges from the effect of fluvial dynamics, whereas in open depressions only thin or split coals occurred. Facies distribution and the thickness of deposits depend on the palaeo-relief which influenced compaction rates and peat bog development without a major role of local synsedimentary tectonism within the study area. Different positions of both the maximum thicknesses of the clastic interseam intervals and lines of splitting of the coals have been documented. The thickest coals (up to 7m) originated near the base of the Carboniferous deposits in the axes of depressions, either locked or protected by the adjacent ridges. They are underlain mostly by fine-grained sediments, responsible for an even compaction rate compensated by peat accumulation. Stratigraphically higher coals have maximum thicknesses of only 2.5 m at the depression margins or in shallow depressions. Towards the axes of the main depressions they are either split or thinner due to the rapid compaction rate of a thick sequence of fine-grained sediments and organic matter. Moreover, the drainage pattern changed as a result of the burial of relief. Owing to compaction, however, the river systems occupied routes along the palaeo-valley axes. No mineable coals occur in the area underlain by a thick sequence of coarse-grained sediments as a result of weak compaction.