Depositional dynamics in the El'gygytgyn Crater margin: implications for the 3.6 Ma old sediment archive

Abstract. The combination of permafrost history and dynamics, lake level changes and the tectonical framework is considered to play a crucial role for sediment delivery to El'gygytgyn Crater Lake, NE Russian Arctic. The purpose of this study is to propose a depositional framework based on analyses of the core strata from the lake margin and historical reconstructions from various studies at the site. A sedimentological program has been conducted using frozen core samples from the 141.5 m long El'gygytgyn 5011-3 permafrost well. The drill site is located in sedimentary permafrost west of the lake that partly fills the El'gygytgyn Crater. The total core sequence is interpreted as strata building up a progradational alluvial fan delta. Four macroscopically distinct sedimentary units are identified. Unit 1 (141.5–117.0 m) is comprised of ice-cemented, matrix-supported sandy gravel and intercalated sandy layers. Sandy layers represent sediments which rained out as particles in the deeper part of the water column under highly energetic conditions. Unit 2 (117.0–24.25 m) is dominated by ice-cemented, matrix-supported sandy gravel with individual gravel layers. Most of the Unit 2 diamicton is understood to result from alluvial wash and subsequent gravitational sliding of coarse-grained (sandy gravel) material on the basin slope. Unit 3 (24.25–8.5 m) has ice-cemented, matrix-supported sandy gravel that is interrupted by sand beds. These sandy beds are associated with flooding events and represent near-shore sandy shoals. Unit 4 (8.5–0.0 m) is ice-cemented, matrix-supported sandy gravel with varying ice content, mostly higher than below. It consists of slope material and creek fill deposits. The uppermost metre is the active layer (i.e. the top layer of soil with seasonal freeze and thaw) into which modern soil organic matter has been incorporated. The nature of the progradational sediment transport taking place from the western and northern crater margins may be related to the complementary occurrence of frequent turbiditic layers in the central lake basin, as is known from the lake sediment record. Slope processes such as gravitational sliding and sheet flooding occur especially during spring melt and promote mass wasting into the basin. Tectonics are inferred to have initiated the fan accumulation in the first place and possibly the off-centre displacement of the crater lake.