Magnitude and frequency of debris and slush flows in the Khibiny mountain valleys, Kola Peninsula, NW Russia

Abstract. Available results for five studied valleys of the Khibiny Mountains, Kola Peninsula, suggest that slush flows and, possibly for some valleys, typical debris flows with lower frequency, are a leading mechanism for downstream sediment delivery and valley floor topographical formation. Typical fluvial topography in slush flow-affected basins is extremely suppressed or nonexistent, since under such conditions, stream channels are unable to rework slush flow deposits. The recovery phase of fluvial topography can serve as an indicator of the magnitude and time passed since the last extreme event. A combination of grain size analysis, radionuclide fingerprinting with the 232Th content in the finer-grained sediment matrix (size <10 mm) and 14C dating, were applied to reveal the age and common structure of debris and slush flow environments and to investigate the main factors in their lithodynamics. Those helped to estimate transportation distances and capacities of the flows and the amount of fluvial reworking of its deposits with time. Application of radiocarbon dating to determine absolute ages (about 30 dates) of stabilization periods for the colluvial cones, mountain fans and valley bottoms and integration with other available chronological data provided a basis for distinguishing several stages of decreased activity of debris and slush flows and extreme slope failures through the second half of the Holocene. Field mapping and remote sensing data interpretation revealed spatial distribution patterns of debris and slush flows. Geomorphic analysis of large relic landforms in valley bottoms confirms, in general, the case for a significant reduction of debris flow magnitude since the last deglaciation and distinct shift to slush flow processes with much lower clastic content. A reliable chronology of the early events is yet to be obtained representing a challenging problem for future research.