ИСТИНА

Lovozerskiye Tundry is a relatively low (up to 1120 m) mountainous massif about 28 km in diameter, located in center of the Kola Peninsula. It is formed by the Devonian pluton of nepheline syenites partly exposed by denudation. Plateaus with relative heights up to 500 m are deeply incised by glacial troughs and cirques, erosional valleys and tectonic lineaments. Most of the valleys are regularly affected by debris flows and slushflows. The latter represent a specific subtype of debris flows essentially composed by water-saturated snow masses with limited (<12%) content of clastic material occurring in rockfall or avalanche troughs and low-order stream channels [Hestnes, 1998; Bozhinsky et al., 2001]. Widespread occurrence of distinctive large relic landforms and deposits indicates much higher magnitude debris flows in the past, most likely during the last deglaciation, particularly those associated with moraine-dammed lakes outbursts. The Sengisjok valley (about 7 km long) is one of the largest dissecting the Lovozerskiye Mountains western slope. It runs from the Sengisjavr Lake in the glacial cirque bottom and after several sharp bends discharges into Lake Umbozero. The modern V-shaped valley deeply cuts into the older wide valley bottom of complex origin partly infilled by glacial, glaciofluvial, glaciolacustrine and debris flow deposits. Within it, there are two terrace-like units, lower of which (up to 20-25 m above the river floor) is undoubtedly formed by repeated high-magnitude debris flows. Its unsorted coarse clastic material (boulders up to 3 m in diameter) with dense loamy sand matrix cemented by nepheline gels, up to 15-20 m thick, overlies the laminated glacial lake sediment. The latter rise up to more than 3 m above the bedrock base and consist of varved silt series alternated with layers composed of allochthonous sharp-edged broken clasts (1-3 cm) of the same varved silt. The whole unit is slightly folded and broken by microfaults probably caused by hydraulic shock or overloading by rapidly superimposed accumulation from extreme glacial debris flow. The present-day Sengisjavr Lake has a low moraine threshold controlling its water level. Several larger water bodies existed in the upper part of the Sengisjok valley since its deglaciation as evident from several separated lacustrine sediment layers exposed along its bottom at different elevations. Apart from the mentioned glaciolacustrine unit, there are laminated silt to fine sand layers 1-2 m thick exposed in the upper reaches. Those are either clamped between debris flow bodies or overlay them forming fragments (5-8 m high) of lake terraces leaning to one another. Such sequences can be tentatively correlated with several moraine-dammed lake outbursts during the earlier colder stages of Holocene when smaller glacier still survived in the headwaters cirque. It can also be possibly linked to extreme debris flow discharges and large-scale deposition on the Sengisjok relic fan (area >4 km2) at the western piedmont. Relatively thin younger lake silts and sands (10-50 cm) separate flash flood units underlain by debris bodies with distinctive humic layers (sampled for 14C dating). The former probably represent smaller-scale events during the later warmer period of Holocene, and the latter – much smaller and shorter-lived waterbodies formed locally where stream was dammed by slushflow deposits or slope failures. The modern Sengisjok valley morphology reflects later dominant incision trend, mainly associated with continuing debris flow and slushflow activity, though at much smaller scales than in the past. In the lower reaches, several well-preserved organic layers buried by debris flows at 1-2 m above the stream were also sampled for 14C dating. During the 2017 field campaign initial stages and consequences of a complex event starting as slushflow and continuing as granular debris flow were observed in the valley on July 3 and described in details. Repeated morphological description and geomorphic mapping of the valley bottom allow suggesting such events occur approximately once per 10 years (at least twice since 2009-2010 field investigations). The study was funded by RFBR project №17-05-00630 and GM АААА-А16-11632810089-5.