Upper Cenozoic fluvial history in the Bohemian Massif

Abstract Two periods of fluvial development are identified in the Bohemian Massif (BM) uplands during the Upper Cenozoic. The first, Lower Miocene period is characterised by large unterraced gravel bodies, up to 100 m thick, filling overdeepened valleys or forming nowadays highly fragmented spreads upon plateaux and interfluves. For the second, Upper Pliocene–Holocene period a diverse style of deposition is typical. The sedimentation is discontinuous and all types of continental sediments show repeated breaks in deposition. This type of deposition is believed to be driven by the very essence of this time span, i.e. climatically controlled alternation of erosion and accumulation phases. Because the uplands of the BM remained ice-free, and higher relief ensured repeated down-cutting whenever the environment permitted, the river valleys preserve complete records of the entire younger period. The progressive series of incisions, each of which is followed by an aggradation phase, resulted in the formation of stairway-like terrace sequences starting some 120–150 m above the rivers. The onset of a new style of fluvial deposition, together with the origin of a new drainage pattern, is dated approximately to the Gauss/Matuyama palaeomagnetic reversal. This corresponds in Central Europe with the start of deposition of loess and formation of complex, “Quaternary type” sequences of slope sediments in uplands and can be taken to indicate a new geological era. These changes may be, therefore, taken as support for the lowering of the Quaternary/Tertiary Erathem boundary.