Evolution of the Middle Westphalian river valley drainage system in central Bohemia (Czech Republic) and its palaeogeographic implication

Abstract

The basal unit of the Kladno–Rakovník Basin (KRB) in central and western Bohemia, the Radnice Member (Bolsovian), is interpreted here as the fill of a combined incised and tectonically formed system of river valleys having a palaeotopography of the depocentre of up to 200 m. Restoration of the palaeotopography on basement surface is used to reconstruct the palaeo-drainage system of this unit consisting of streams of four or five orders. The main valleys form conjugated system of narrow straight and more then 15 km long tectonically controlled NW–SE and NNE–SSW striking valleys entered by incised valleys. These erosional valleys consist of several orders and display a dendritic pattern. The fill of this valley system, the Radnice Member, consists of two tectonically controlled sequences separated by a basin-wide erosional surface with a relief of at least 20 m. Both units differ in their architecture as a consequence of base-level changes expressed by the A/S ratio (Accommodation/Sediment supply). Tectonics and sediment supply are considered as the most important controls on deposition whereas palaeotopography controlled the distribution of sediments within a valley system. Restorated palaeotopography indicates the existence of two orographically separated drainage systems in the KRB. The eastern part of the Kladno–Rakovník Basin was drained to the NW, possibly to the North Variscan Foredeep, whilst its western part was probably drained through a central tectonic valley to the south–southwest to the Radnice and Plzeň basins and then further SSW, perhaps to the Naab Basin in SE Bavaria. The KRB was probably located about 1000 m above the Carboniferous sea level.