Decoding climatic and tectonic signals in the Middle Pleistocene Ludwigshafen formation (Upper Rhine Graben)

The Middle Pleistocene Ludwigshafen Fm. (LuFm) of the Upper Rhine Graben is an ideal target to study the interplay of tectonic processes and climatic factors impacting sediment deposition. It consists of a spatially variable fining-upward sequence, whose stratigraphic placement, timespan of deposition, and environmental context are still debated. We revisit previous research drillings, and present new sedimentological, geochronological, and palynological data to reconstruct its deposition in response to overarching factors. Common stacking patterns and facies transitions allow the subdivision into a diamictic lower, a coarse-grained middle, and a fine-grained upper LuFm. The depositional architecture in the graben is tectonically controlled, and the lower LuFm is interpreted to mark the onset of a period of accelerated relative subsidence and seismic activity. In contrast, middle and upper LuFm reflect the transition from glacial toward interglacial climatic conditions. Geochronological data based on different methodical approaches consistently suggest an age of 400–500 ka for this transition, corresponding to marine isotope stages (MIS) 12–11. The dating results also attest considerable age gaps at the formation boundaries. Pollen assemblages in the upper LuFm support the inferred transition to interglacial conditions. However, the assemblages do not fit the typical (Holsteinian) vegetation development during MIS 11 and, together with other biological indicators, rather point towards a position in MIS 13 (late Cromerian). The overall facies development supports this placement, as it coincides with rapid uplift in the downstream Rhenish Massif, accounting for some of the observed relative subsidence. Our data further stress the occurrence of several small- and large-scale hiati within and at the formation boundaries, respectively, and call into question the postulated broad continuity of the graben fill.