The Relic Landscapes of the Grazer Bergland: Revisiting the Piedmonttreppen Debate

Abstract The Grazer Bergland is a mountainous region at the eastern end of the Alps that escaped glacial erosion in the Pleistocene and thus preserves low-relief landforms that are relics of the earlier uplift history. These relic landforms may reflect a Piedmonttreppe that formed during a series of stages of a wide-reaching Pliocene uplift event that interacts with the landscape evolution, but this model is not uniquely accepted for the region. In order to test this model and for a future better correlation of the paleosurfaces with those mapped in other regions, it is important to benchmark these relic landforms. We do so by presenting a geomorphic map of the Bergland region over some 600 km2. We describe the well-known levels Stadelberg/Zahrerberg- (at 540 – 700 m a.s.l.), Kalkleiten/Hochstraden- (at 700 – 850 m a.s.l.), Trahütten- (at 950 – 1100 m a.s.l.), Hubenhalt- (at 1200 m a.s.l.), as well as Wolschenegg- and Kor- (at 1200-1720 m a.s.l.) levels and correlate their distribution in space and time. Fluvial channels between segments of the relic surfaces have knickpoints that correlate with the planation surfaces, which is in strong support of the Piedmonttreppe model. Our analysis results in a model that interprets the course of the Mur river to be the product of a river piracy event near Peggau at the time of the planation of the Trahütten level (about 4 Ma), diverting the paleo-Mur from an eastwards course along the Mürz valley in direction Vienna, towards Graz. Thereafter, the Mur remained antecedent with respect to the uplift of the surrounding massifs resulting in massive base level drop for many tributaries, like the Mixnitzbach or Rötschbach. The resulting knickpoints have since migrated upstream to cause successive minor river capture events, for example the Rötschbach capture at Kesselfall. We also show that the presence of lower levels in the Passail Basin is best interpreted in terms of the more efficient erosion of basin sediments that filled a Miocene half graben north of the Schöckl ridge and analyze the two major drainages of the region, the Raab and the Weizbach.