Sediment storage quantification in the Black Forest highlights tectonic influence on typically wide and shallow valleys

Annette Sophie Bösmeier, Jan Blöthe

Quantifying sedimentary volumes in mountain valleys can not only enhance our understanding of Quaternary valley evolution and river dynamics but also yield critical insights into hydrogeological characteristics. In contrast to the thoroughly investigated Upper Rhine Graben, little coherent information is available on the subsurface structure of adjacent Black Forest valleys. This study therefore aims at estimating the thickness, spatial distribution, and volumes of alluvial material in valleys of the southwestern Black Forest. We utilized an extensive borehole database, high-resolution digital topographic data, and information from geological maps to integrate two complementary approaches. First, local valley cross sections were compiled to investigate subsurface bedrock morphology, allowing for a rough approximation of valley fill volumes. Second, catchment-specific linear and random forest regression based on morphometric and hydrologic variables were utilized to estimate sediment depths in valleys.
Our results reveal a considerable spatial heterogeneity regarding shape, symmetry, ruggedness, and thickness of valley floor deposits. The composite valley cross sections with valley floor widths between 16 m and 3 km and average sediment depths ranging from 2 m to 36.3 m include v-shaped geometries prevailing in narrow headwater valleys and main valleys mostly showing a surprisingly flat erosion surface and a shallow (on average < 15 m) sediment cover. Yet, towards the Upper Rhine Graben (URG), some valley sections widen and are rather box- or trough-shaped, comprising up to 100 m thick sediments. Overall, the valley orientation as well as sediment thickness and valley shape in the main Black Forest catchments appears to be largely structurally controlled.
For our study area of about 2100 km2 including nine main catchments with sizes between 13 km^2 and 1034 km^2, estimated median values of valley fill volumes of the entire area range between 1.2 km^3 and 2.8 km^3. Specifically the disproportionately high sediment volumes of two of the larger catchments, Dreisam and Schutter, are striking. Both areas exhibit a particular structural imprint, the one being located within a deep-seated, large-scale Late Paleozoic deformation zone, the other one crossed by the Cenozoic Main Border Fault along the URG. These crustal discontinuities may be connected to an enhanced incision, which further underscores the importance of tectonic boundary conditions on the valley infill. In comparison with alpine settings, the sediment storage within the predominantly wide and shallow valleys is lower.