Fluvial terrace formation in the northern Upper Rhine Graben during the last 20 000 years as a result of allogenic controls and autogenic evolution

Abstract

The northern Upper Rhine Graben hosts a well-preserved Late Weichselian and Holocene fluvial terrace sequence. Terraces differ in elevation, morphology, and overbank sediment characteristics. The purpose of this study was to determine the relative importance of allogenic controlling factors versus autogenic evolution on the successive formation of these terraces. For a representative valley segment (the Gernsheim region), results from previous research were integrated with newly obtained borehole data and digitized elevation maps to construct palaeogeographic maps and cross sections. Coarse-grained channel deposits below terrace surfaces were dated using Optically Stimulated Luminescence, and fine-grained abandoned channel fill deposits were dated using pollen stratigraphy and radiocarbon analysis. Initiation of terrace formation was caused by climatic change in the Late Pleniglacial (after ~20 ka), but fluvial response was complex and slow and continued locally until the middle Boreal (~9 ka). Early to Middle Holocene (~6 ka) changes in fluvial style and associated overbank lithofacies are not necessarily controlled by climatic change as was previously proposed. Instead, autogenic processes combined with river reach-specific factors explain the observed terrace development. Continuous incision, autogenic evolution, and high preservation potential provide an alternative explanation for the presence of a terrace sequence in this subsiding area.