Comprehensive temporal and spatial analysis of Early Pleistocene drainage patterns on the Swiss Alpine foreland

Abstract

Deckenschotter are glaciofluvial gravels that cap isolated bedrock plateaus and are largely disconnected from today's local drainage. They were deposited when glaciers from the Alps were first extensive enough to reach the northern Swiss foreland, thus providing a unique record of the foreland landscape and its evolution during the earliest Quaternary glaciations. To decipher this record, we employ two robust methodologies: cosmogenic ²⁶Al/¹⁰Be burial dating and GIS-based topographic analysis. ²⁶Al/¹⁰Be burial ages from both new and published sites are calculated using a consistent procedure with the P-PINI code. Detailed swath projected and local 360° profiles were generated with GIS data in an R-toolset developed specifically for this study. Integrating results from both methodologies with outcrop sedimentological data, we interpret three main periods of Deckenschotter deposition: 1.3–1.2, 1.1–1.0 and ~0.8 Ma. The interpreted age ranges indicate glaciers must have reached the forelands in response to intensifying climatic cooling across the Mid-Pleistocene Transition (1.25–0.75 Ma). Deckenschotter outcrops provide a disjointed image of past topography from which we piece together glacial meltwater pathways in each time interval. Between the glacial phases, stepwise incision of 50–100 m occurred as depicted in the projection profiles, with some spatial variability in magnitude of incision. Incision was driven by decreasing sediment supply during glacial terminations, set against a backdrop of minor foreland uplift. While the path of the Aare River has changed little since the Early Pleistocene, the Rhine River has radically altered its path. Initially a tributary of the Danube River with northward flow, glacial modification to topography led to its re-routing to the west into the lower base-level Aare River-Upper Rhine Graben system. Based on our analysis, we estimate this event occurred after ~0.8 Ma.