Evolution of an Alpine over-deepened valley (Upper Enns Valley, Austria) within the context of geomorphic and tectonic expressions

Abstract

In the study area, the Eastern Alps have been under surface uplift since Early Miocene times. Furthermore, Quaternary processes as uplift and cyclic glaciations likely interfere with neotectonic activity as the Upper Enns Valley is bounded to the Salzach-Enns strikeslip fault. The interplay between differential uplift and denudation and glacial carving results in a present-day diversity of morphology. An asymmetric drainage pattern with short northern tributaries to the Upper Enns Valley and long southern ones suggests N-ward tilting of the entire region. Provenance analysis of the Upper Pleistocene Ramsau Conglomerate on the northern valley slope indicates primarily a derivation from the basement terrain of the Niedere Tauern located to the south of the Enns Valley. The transport from the southern side of the valley and the presence of a terrace at 1,100 m allows the assumption that the Upper Enns Valley was filled up to that elevation during Late Pleistocene times. The suggested continuity of the southern planation surfaces and the top of the Ramsau Conglomerate (ca. 1,100 m a.s. l.) is interpreted to represent a relic former valley bottom. An intercalated coal seam in the conglomerate dated back to 53.3 kyr. by 14C method (uncalibrated) is a crucial element to reconstruct the evolution of the valley. Geomorphological and sedimentological investigations coupled with a fluvial landscape along a major strike-slip fault allowed developing the morphogenetic history of the Upper Enns Valley (Austria).