Paraglacial adjustment of alluvial fans to the last deglaciation in the Snežnik Mountain, Dinaric karst (Slovenia)

Abstract

Glaciokarst depressions are major glacigenic depocenters in the Dinaric mountain karst areas and often store important information about the timing and nature of glacial processes and paraglacial sediment reworking. This study focuses on Praprotna draga, which is one of the largest glaciokarst depressions in the Snežnik Mountain (Dinaric karst), with an area of ~3.4 km 2 and a maximum depth of 140 m. The western slopes of the depression are characterized by undulated moraine morphology and alluvial fans are filling its entire floor. We present the results on the thickness, origin and age of the sediment infill using a complementary geomorphological, sedimentological, geophysical and dating approach. Distribution of moraines point to two glacial advances that were associated with two main alluvial fan aggradation phases recognised using the electrical resistivity tomography measurements. The youngest alluvial deposits were sampled for cosmogenic 36 Cl analysis using amalgamated carbonate pebbles. The depth profile of 36 Cl concentrations suggests an age of 12.3 ± 1.7 ka when assuming a likely denudation rate of 20 mm ka-1. Since the existence of the Younger Dryas glaciers in the study area is climatically difficult to explain, we tentatively propose that the youngest alluvial deposition in Praprotna draga took place after the glacier retreat during the paraglacial period. Our findings suggest that the time window of paraglacial adjustment in the Snežnik Mountain was brief and likely conditioned by quick recolonization with vegetation and inefficient surface runoff on deglaciated karst terrain.

Simulation of a former ice field with PISM – Snežnik study case

“…The first were delineated based on end moraines and outwash fans, while the latter were drawn over areas that only show suggestive evidence for glaciation. The geochronological data (Marjanac et al, 2001;Žebre et al, 2019), although still scarce, points to a maximum ice extent during the last glacial maximum (LGM), that is 30-17 ka BP (Lambeck1 et al, 2014).…”