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

The Ploučnice River system, located in the central Bohemian Massif, is draining an area 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 31 m above present floodplain). The middle terrace levels (22 to 16 m above present floodplain) indicate a fluvial style changing from a high- to medium-energy braided river. In the lower terrace levels (13 to 7 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. Terraces found at 36 m, 31 m and 16 m above present floodplain are dated with isochron burial dating whereas terraces 22 m, 13 m and 7 m above present floodplain are dated with OSL. Due to differences in age results between the two dating methods, we establish two different evolution models: The first is based on isochron burial and OSL dating and the second model is on the OSL dating results only. The time span represented by the river terraces remains unclear and varies from Eburonian to Eemian (1680 to 56 ka) or from Elsterian to Eemian (138 to 56 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 ka at 14 m above present floodplain and 138 ka at 19 m above present floodplain, we suppose 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 above present floodplain to be 248 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.