The Dachstein paleosurface and the Augenstein Formation in the Northern Calcareous Alps – a mosaic stone in the geomorphological evolution of the Eastern Alps

Abstract

The central and eastern areas of the Northern Calcareous Alps (NCA) are characterized by remnants of the Dachstein paleosurface, which formed in Late Eocene (?) to Early Oligocene time and is preserved with limited modification on elevated karst plateaus. In Oligocene time, the Dachstein paleosurface subsided and was sealed by the Augenstein Formation, a terrestrial succession of conglomerates and sandstones, which are only preserved in small remnants on the plateaus, some in an autochthonous position. Thermochronological data suggest a maximum thickness of the Augenstein Formation of >1.3 km, possibly >2 km. The age of the Augenstein Formation is constrained by the overall geological situation as Early Oligocene to earliest Miocene. Fission track age data support an Early Oligocene age of the basal parts of the formation. The source area of the Augenstein Formation consisted predominantly of weakly metamorphic Paleozoic terrains (Greywacke Zone and equivalents) as well as the Late Carboniferous to Scythian siliciclastic base of the NCA to the south of the depositional area. To the west, the Augenstein Formation interfingered with the Tertiary deposits of the Inntal. Sedimentation of the Augenstein Formation was terminated in Early Miocene time in the course of the orogenic collapse of the Eastern Alps. The Augenstein sediments were eroded and redeposited in the foreland Molasse zone. From Pannonian times (~10 Ma) on, the NCA and the denuded Dachstein surface experienced uplift in several pulses. The Dachstein paleosurface has been preserved in areas, in which thick limestone sequences allowed subsurface erosion by cave formation and thus prevented major surface erosion.