Landscape evolution of the Sölk Valleys and adjacent regions from the last Interglacial to today (Niedere Tauern range, Austria)

The Gröbminger Mitterberg, an isolated flat hill located within the Enns Valley, consists of fluvial, deltaic and lake bottom sediments on top of bedrock, which are covered by subglacial till. In comparison, the sedimentary succession in the Sölk Valleys, which drain into the Enns Valley, is more divers. The highest peaks reaching 2680 m a.s.l. and cirques are dominated by talus, relict rock glaciers and two groups of moraine ridges. Latero-frontal moraine ridges located higher than 1900 m a.s.l. are remarkable. Frequently, two or three ridges are located close to each other and have a morphologically fresh shape. Further downvalley, single laterao-frontal moraine ridges occur, which are often flattened and less prominent. They appear in different altitudes according to their catchment area. However, they do not reach the main valley floor. The slopes of the main valleys and secondary valleys are often covered by subglacial till and reworked slope deposit,s which are dominated by a silty-sandy matrix and angular to subrounded clasts. Additionally, many slopes have been affected by mass movements. At the valley mouth of secondary valleys, ice marginal sediments occur consisting of very rounded pebbles in a sandy matrix and in some areas, cross bedding can be observed. Slightly above the valley floor of the main valleys, gently sloping terrace bodies interfingering with truncated alluvial fans and slope sediments described above occur. These deposits are diamicts, which consist of sandy or silty matrix with rounded and angular clasts.

An interpretation of these findings suggests the following landscape evolution:

The sedimentological record of Gröbminger Mitterberg suggests aggradation of the Enns Valley floor to at least 850 m a.s.l. (200 m higher than today) prior to the Last Glacial Maximum (LGM). During the LGM, the area was covered by the Enns Glacier with tributary glaciers from the Sölk Valleys. The ice surface reached 1800 m a.s.l. in the northernmost part (in the Enns Valley), roughly 2100 m a.s.l. in the southernmost part at a transfluence pass (Sölkpass) and even higher altitudes in cirques. During that time, large areas were covered by basal till. With the breakdown of the ice mass and ice surface lowering at the onset of the phase of ice-decay, trunk glaciers and cirque glaciers got separated resulting in the formation of ice-marginal lakes. On the already ice-free slopes, reworking of the previously deposited sediment and mixing with talus started. Further, climate warming proceeded and ice retreat resulted in mass movements and rock falls. As soon as the valley floor was ice-free, aggradation started by large river systems accumulating sediment in the valley floor. This was followed by two separate cold stages, the Gschnitz Stadial (Heinrich Event 1, ~16-17 ka) and the Egesen Stadial (Younger Dryas, ~12-13 ka), where cirque glaciers developed in equilibrium with climate oscillations (up to three stabilisation phases recognised during Egesen Stadial). In the Holocene, climate warming led to river incision in the main valleys and resulted in today´s landscape.