Diachronism of river terrace formation? – Lessons to learn from luminescence dating results

Revealing an amazing diversity of forms, river systems have always to be interpreted as products of their specific landscapes. Extremely sensitive to external and internal forcing, they reflect the particular characteristics of climatological and geological conditions as well as the changes of these conditions. These changes are regularly preserved in depositional series whose varying sedimentary characteristics can be attributed either to palaeoclimatic variations or to tectonic activities and their corresponding changes in fluvial discharge and sediment load. What applies to fluvial sediments in general, is particularly true for river terraces. Regularly, they are regarded as valuable palaeoenvironmental and archaeological archives and their particular importance is well documented by a huge and still growing number of studies spanning a wide range of climatic and regional settings.

However, the information gained from fluvial terraces and their significance for palaeoenvironmental and present-day fluvial research strongly depend on an accurate and precise dating of the terrace formation. Numerical ages are of fundamental importance for the interpretation of sedimentological, morphological and stratigraphical findings. They are essential for assessing the influence of various driving forces and for providing insights into the mechanisms and dynamics of river adjustments over variable temporal scales.

In this contribution, we present luminescence ages of fluvial deposits originating from an Upper Pleistocene river terrace in a small valley located in the headwater of the Main River, Germany. For this study, several samples from various locations throughout the river longitudinal course have been analysed. Surprisingly, the determined luminescence ages for material from the lowermost part of the valley are significantly older than those from the middle section, which in turn are older than those from the valley’s upper reaches. Based on the evaluation of a high-resolution digital elevation model (DEM) and on intensive fieldwork, we can be sure that all samples originate from the very same morphological unit, a well-preserved late Pleistocene fluvial terrace.

Our results suggest a diachronic alignment of sedimentation ages for fluvial deposits, starting with old ages close to the mouth of a river and getting progressively younger for locations approaching the upper reaches. If these findings are confirmed in other fluvial systems and are not only the result of very specific local conditions, they will be of great relevance for geomorphological research in fluvial landscapes. As a result, the widespread approach of deriving age estimates for fluvial terraces from numerical results merely determined for a single location appears to be inadequate and should be subjected to a critical review.