THE ENIGMATIC AUGENSTEIN LANDSCAPE OF THE EUROPEAN ALPS. THE KEY TO DECIPHERING THE LANDSCAPE EVOLUTION OF THE WORLDS MOST STUDIED MOUNTAIN RANGE?

There is an active discussion about the landscape evolution of the European Alps that centers on a peculiar observation: The mean slope decreases above about 1500 m altitude. Two models have been proposed to explain this. Firstly, it has been argued that this may be caused by a “glacial buzz saw“ that planed the range at high elevations during the glacial periods in the last two million years. Secondly, it has been argued that this may be due to very recent surface uplift and the resulting fluvial immaturity.

In the Eastern Alps a peculiar sediment deposit offers a key for solving this debate: The Augenstein deposits. These are silicic, fluvial gravels that occur as rare remnants on karstified planation surfaces of the limestone ranges across the Eastern Alps at up to 3000 m. Knowledge of the initial deposition age could provide a constraint on the onset of surface uplift. However, the deposition age has – regrettably – only been indirectly inferred to be deposited during the Oligocene.

In this contribution we summarize our current and past efforts using morphometric analyses, cosmogenic isotope data and U-Pb dating to extract information on surface uplift history of the Eastern Alps from these deposits. In particular, these efforts include: (a) ²⁶Al and ¹⁰Be derived burial age dating of Augenstein gravels in karst caves indicate surface uplift of at least 600 m in the last 4 million years. (b) ¹⁰Be-derived erosion rates of catchments outside the glaciated parts of the Eastern Alps range between 39 and 238 m/Ma and are consistent with several hundreds of meters of base level drop within the last 4 Ma with a clear difference between erosion rates in old “relict” landscapes and young “incised” parts of the landscape. (c) U-Pb dating of calcite crusts cementing Augenstein gravel inside cave passages yielding an age of appr. 682 ka. While unrelated to the deposition of the gravel, this age provides an important constraint on the surface elevation during the MIS17 interglacial. (d) Cosmogenic ²¹Ne concentrations in Augenstein quartz pebbles indicating re-exposure of 115 ka and 262 ka. The difference between these ages is used to constrain aspects of the transport history of the sediments in the Oligocene.