Postglacial sediment storage and rockwall retreat in a semi-closed inner-Alpine sedimentary basin (Gradenmoos, Hohe Tauern, Austria)

Abstract

This study quantifies postglacial sediment storage in the Gradenmoos basin – a glacially over-deepened, semi-closed sedimentary basin (4.5 km²) in the central Gradenbach catchment (32 km2, Schober Mountains, Carinthia, Austrian Alps) – and reconstructs rates of postglacial rockwall retreat and mechanical denudation. The topographic setting of the basin facilitates the reconstruction of small-scale postglacial landscape evolution since clastic sediment removal can be neglected due to lake existence over 7,500 years after Younger Dryas deglaciation (Egesen). Furthermore, source areas are clearly delineated and intermediate sediment storage is low due to short source-sink distances and steep slope gradients. Sediment storage volumes are quantified using high-resolution surface (terrestrial laser-scanning, geomorphological mapping) and subsurface information (geophysical prospection, core drilling). These data are an input for GIS-based bedrock interpolation and 3-D modelling of sediment storage. The timing of sedimentation and rockwall retreat after deglaciation and the amount of basal till deposits underneath present-day landforms are estimated using core drilling, stratigraphic analyses, palynological observation, and AMS ¹⁴C dating. Total (postglacial) sediment storage within and surrounding the basin amounts to 19.7 (18.3) x 106 m³ whereas hillslope storage overbalance basin fill deposits by a factor of five. Rockwall retreat rates of less than 520 mm/ka, however, indicate comparatively low values despite of steep slope gradients and coarse and blocky weathering conditions in the area. The study presents a new and almost complete small-scale sediment budget approach, provides postglacial rates of rockwall retreat for the eastern Alps, and copes with several uncertainties disregarded in previous studies. For the first time, several uncertainties are quantitatively addressed, such as pre-Holocene basal till underneath present-day landforms, re-deposited sediment volumes, and the absolute timing of the period available for postglacial sedimentation and denudation.