Reconstructing the Enns valley in the course of the ice ages based on findings on Gr&#246;bminger Mitterberg (Austria)
Authors: Griesmeier Gerit E.U. , Reitner Jürgen M. , Le Heron Daniel P. , Lüthgens Christopher , Firla Gustav 
Published: March 2024

Within the Alps, the erosive effects of glaciers during the Last Glacial Maximum (LGM) means that evidence for earlier glaciations is rare. At Gr&#246;bminger Mitterberg (GM), traces of the history prior to the LGM are conserved below a layer of basal till of the LGM. The GM itself is a flat-topped hill located in the Enns valley in Styria (Austria), rising to an elevation of ca. 200 m above the Enns valley floor. It is situated between Mesozoic carbonates in the north and crystalline basement units in the south. The GM comprises crystalline basement covered by fluvial and deltaic sediments, overlain by a subglacial till. Based on the distribution of the sediments, borehole data and geoelectric data, an ancient river channel across GM can be reconstructed.&#160;The lithological spectrum of the fluvial and deltaic sediments at GM shows that the distribution of material from the south and the north is around 70 : 30 % throughout the GM, which is the same as that of the modern Enns river. This suggests that all sediments at GM and the channel across it were greatly impacted by the Enns river. The Enns valley in the area of GM can now be reconstructed as follows:Some time before the Riss Glaciation (MIS 6), the Enns river meandered in a valley, situated at an elevation ca. 100 m higher than the present-day river. Large alluvial fans flowing northward into the Enns valley forced the Enns river to flow across Mitterberg in a channel, which was probably already partly created during earlier glaciations. The first crystalline pebbles reached the north of GM. During the phase of ice decay of the Riss Glaciation, ice marginal lakes developed at the margin of GM, where deltaic sediments developed. After the Riss Glaciation, the Enns river found itself in a similar situation like today and the Enns valley aggraded until it reached the top of GM shortly before the last glaciation. Large alluvial fans further east dammed a lake, which covered GM and was quickly filled with sediments. This part of the chronology is also supported by optically stimulated luminescence data using single grains of potassium-rich feldspar. They will be presented at the conference. The braided Enns valley was not only much wider than today, but also transported crystalline pebbles to the northern part of GM. In the course of the LGM, most of the previously deposited sediments were preserved and covered by basal till.&#160;The evolution of the Enns valley emphasises the close coupling between climate, erosion and sedimentation processes. Today, the Enns river incises again and sediments at GM are going to be eroded, but parts remain in their position. These current changes have probably repeatedly occurred through time and we can never be sure, how much time is really preserved on GM. Nevertheless, the proposed reconstruction in the Enns valley can also give hints on the history of other alpine valleys and may be helpful for future models of alpine wide glaciation and greenhouse phases.