Post-last glacial alluvial fan and talus slope associations (Northern Calcareous Alps, Austria): A proxy for Late Pleistocene to Holocene climate change

Abstract

Near Innsbruck city (Austria, Eastern Alps), following the Last Glacial Maximum (LGM), an alluvial fan-to-talus slope succession was supplied from a carbonate-rock cliff more than 1000 m in height. 234U/ 230Th ages of 9.5 to 9.37 isotope kyrs of diagenetic cements in the alluvial-fan succession suggest that the fan/talus deposit accumulated mainly during late-glacial to, perhaps, early Holocene times. The deepest-exposed interval of the fan succession contains cracked lithoclasts probably fractured by overburden from late-glacial ice; this interval is topped by an intra-sequence unconformity. Following final glacial retreat, and rapid aggradation of the alluvial fan and talus slope, the geomorphic regime changed to erosion, as recorded by fanhead trenching and cutting of fluvial terraces, abandonment and vegetating of scree slopes, and excavation of 'talus flatirons'. The changeover from the accumulation of fan and talus to abandonment and dissection probably took place during the terminal late-glacial interval to perhaps the early Holocene. This erosional regime persists until present. A record of rapid late-glacial to early Holocene accumulation of an alluvial fan/talus deposit followed by: (i) abandonment and vegetation growth, combined with (ii) cutting of intra-sequence unconformities of limited lateral extent, is typical of Alpine mountain-flank deposystems situated at comparatively low altitudes. This record consists of (a) an autocyclic component, that is, progressive lowering of sediment input due to onlap and burial of freshly-deglaciated mountain flanks supplying alluvial fans and talus slopes, and (b) an allocyclic component, that is, deglacial climatic warming and upward rise of an altitudinal range with a maximum number of freeze-thaw cycles ('talus window'), also leading to progressive vegetation-induced hillslope stabilization and lowering of scree production.