Aragonite and calcite cementation in “boulder-controlled” meteoric environments on the Fern Pass rockslide (Austria): implications for radiometric age dating of catastrophic mass movements

Abstract

On the Fern Pass rockslide (Eastern Alps, Austria), projecting boulders collected surface runoff and delayed percolation of water into the rockslide mass, leading to decimetre-scale, fluctuating, phreatic/vadose diagenetic systems along their contact. In these systems, aragonite and calcite precipitation were nourished mainly by dissolution of carbonate-rock flour. Cement precipitation was limited to southern- and eastern-exposed “runoff haloes” of boulders and mainly resulted in cemented breccias. Aragonite precipitation was related to dissolved Mg2+ and/or to high CaCO3 supersaturation in evaporative-concentrated pore waters. Early aragonite cement yielded a 234U/230Th age of 4,150 ± 100 years. Relative to other radiometric ages (36Cl, 14C; by other authors) for the rockslide event, the U–Th age of the aragonite is the most precise proxy of depositional age. Carbonate cements are present in other rockslide and rockfall deposits also. U–Th dating of such cements is thus a comparatively rapid and inexpensive method of minimum-age dating catastrophic mass movements.