Middle to Late Pleistocene landscape evolution and glacial dynamics in the Eastern Alps: the Gröbminger Mitterberg record, Austria

Abstract

Glacial erosion during the Last Glacial Maximum (LGM) has removed much evidence of earlier glaciations and interglacials in the European Alps. At Gröbminger Mitterberg (GM), beneath a blanket of LGM till, a distinctive sediment archive preserves deposits predating the LGM. GM is a flat-topped hill in the Enns Valley, Styria (Austria), rising ∼200 m above the valley floor between Mesozoic carbonates (north) and crystalline basement units (south). Crystalline bedrock (phyllite, greenschist) is overlain by subglacial, deltaic, fluvial and glaciolacustrine sediments, with soft-sediment deformation (clastic dykes) and capped by LGM till. Outcrop, borehole, and electrical resistivity tomography data reveal a bedrock surface incised by a channel. The Middle–Late Pleistocene succession begins with the Lower Till, attributed to the Rissian glaciation (MIS 6), which reshaped a fluvially moulded bedrock surface by intense subglacial erosion, including meltwater activity. Above lies the Mitterberg Unit. The up to 100 m thick Dorf (Allo-)Member comprises delta foresets with slumps, rare dropstones and dead-ice contact structures, correlated to Termination II (MIS 6) by luminescence ages of 140±20 ka. The 10 m thick Zirting (Allo-)Member records fluvial deposition. The upper Frankenbichl (Allo-)Member consists mainly of interbedded lacustrine and deltaic sediments, showing overall aggradation. Luminescence ages of 47±8 ka suggest a lake formed in MIS 3 by a prograding alluvial fan under cold, ice-free conditions. The lacustrine succession records alternating delta progradation and dam-burst-caused regression. Uppermost Frankenbichl glaciolacustrine deposits reflect glacier-proximal settings marking the onset of Würmian Pleniglacial (MIS2) ice build-up. The LGM till unconformably overlies the Mitterberg Unit and forms drumlins. Overall, these reconstructions link climate change, erosion and sedimentation in the Enns Valley, providing a framework for modelling Alpine glaciations and intervening greenhouse phases.