Quaternary evolution of the Ploučnice River system (Bohemian Massif) based on fluvial deposits dated with optically stimulated luminescence and in situ produced cosmogenic nuclides

Abstract

The Ploučnice River system, located in the central Bohemian Massif, is draining an area almost not covered by continental ice sheets but instead archiving the fluvial deposits. The fluvial style changes from a high-energy braided to a long-bend meandering river in the upper terrace levels (36 to 29 m above present floodplain). The middle terrace levels (22 to 14 m above present floodplain) indicate a fluvial style changing from a high- to medium-energy braided river. In the lower terrace levels (13 to 5 m above present floodplain), the terrace deposits indicate high-energy braided to long-bend meandering river environments. To provide greater details on the timing of fluvial terrace formation, this study applied 26Al and 10Be isochron burial and optically stimulated luminescence (OSL) dating methods to terraces of the Ploučnice River system. Terrace fluvial infills are dated with isochron burial dating in relative altitude 34, 29, and 14 m above present floodplain (apf); whereas terrace accumulations in relative altitude 19, 12, and 6 m apf are dated with OSL. Because of differences in age results between the two dating methods, we propose two different evolution models: the first is based on isochron burial and OSL dating, and the second model is on OSL dating results only. The time span represented by the river terraces remains unclear and varies from Eburonian to Weichselian (1680 to 56 ka) or from Saalian to Weichselian (138 to 56 ka) with the estimation of the highest terrace (248 ka) respectively. The former river evolution model is based on tectonic activity at least since 1000 ka. Morphotectonic analysis recognized new lineaments of which the general direction corresponds with the main direction of the Ohře fault zone (NE- to ENE-striking) and Lužice fault zone (NW-striking). Based on dated terrace ages of 1153 ± 123 ka at 14 m apf and 138 ± 21 ka at 19 m above present floodplain, we propose a normal fault being active from at least 1153 ka. The second river evolution model assumes possible remobilization of clasts analyzed by isochron burial dating before their final deposition. From three OSL ages we calculated a mean incision rate and estimated an age of upper terrace levels at 34 m apf to be 248 ± 38 ka (Saalian age). As remobilization of clasts in high-energy fluvial and glaciofluvial environments is very likely, age determination is challenging. Nevertheless, we interpret the terrace record in the Ploučnice River system as a product of Quaternary climatic changes influenced by tectonic processes.