Sandstone diagenesis, salt-induced weathering and landslide formation: the case of Slano Blato Landslide, Slovenia

The Slano Blato Landslide (Vipava Valley, SW Slovenia) is an active landslide complex known for more than 200 years. Geomorphology of the area is primarily determined by a fold-and-thrust structure, composed of a series of... [ view full abstract ]

The Slano Blato Landslide (Vipava Valley, SW Slovenia) is an active landslide complex known for more than 200 years. Geomorphology of the area is primarily determined by a fold-and-thrust structure, composed of a series of nappes of Mesozoic carbonates thrust over Palaeogene flysch domains. The region is characterised by escarpment profiles with steep upper slopes in carbonate rocks, underlain by weaker shales, marls and sandstones, forming lower slopes with a very high landslide susceptibility. The Slano Blato complex differs from other landslides in the area by a mud-dominated composition of the transported material and the occurrence of salt efflorescences (predominantly sodium sulphates of the thenardite-mirabilite system) on the landslide surface (hence the name Slano Blato meaning 'salty mud'). A hypothesis of a localized source of sodium and sulphate ions from the flysch clastics has been invoked to explain the salt precipitation and, consequently, accelerated weathering.
Compared to the chemical composition of spring waters in a broader area, generally characterised by karst water discharge, groundwater and seepage water from the landslide area show anomalously high levels of Na+ and increased SO4-2. Both sodium and sulphate probably originate from confined aquifers in metre-thick sandstone beds in the Slano Blato catchment area.
Flysch sediments are very rich in pyrite, which appears dispersed and accumulated along the shale-sandstone contacts. Oxidation of pyrite most probably supplies SO4-2 ions in the groundwater. The origin of Na+ is less straightforward. To find out if the sandstones are rich in plagioclases or other minerals containing Na+ we performed a quantitative petrographic analysis by point-counting. Results indicate that the sandstone is an arkose (Qm57.3 F34.9 Lt7.8) with high amount of plagioclase (P/K index, 3.6).
Microprobe analyses confirmed the presence of abundant Ca-Na plagioclases and albite grains. Additionally, K-feldspars and Ca-Na plagioclases show features of partial albitization under cathodoluminescence. Chemically pure nonluminescent albite grains and cements indicate an albitization process, which probably occurred during diagenesis. Thus, the flysch arkoses are potential Na sources. Furthermore, plagioclase grains show weathering features and in some cases are replaced by calcite. Such replacement process can be a mechanism of releasing Na, contributing to formation of salts, enhanced weathering and atypical local production of unstable muddy slope sediment.