Alluvial fan dynamics in the El'gygytgyn Crater: implications for the 3.6 Ma old sediment archive

Abstract

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 structurally and texturally 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 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 meter is the active layer 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 that takes place especially during spring melt are thought to 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

Past climate changes and permafrost depth at the Lake El'gygytgyn site: implications from data and thermal modeling

“…However, these can largely be excluded due to several reasons: on the one hand thermal properties have been determined by different approaches (indirect as well as measurements), on the other hand permafrost has been present for at least some ten thousand years. The warmer period during the Allerød (13.3-13.1 kyr BP), when the lake level was some 5 m higher than modern levels and flooded the coring site at that time (Schwamborn et al, 2008(Schwamborn et al, , 2012, was studied by different model runs and showed that there is no influence on the current temperature field anymore.…”

“…Regarding the thermal properties of the prevailing sediments and rocks in frozen condition, we performed thermal conductivity measurements on one core sample retrieved from borehole (2) This sample consists of sandy gravel, and is representative for much of the material from the permafrost core (Schwamborn et al, 2012) associated with the model's unit 2 in Fig. 2.…”

Past climate changes and permafrost depth at the Lake El'gygytgyn site: implications from data and thermal modelling

“…This sample consists of sandy gravel, and is representative for much of the material from the permafrost core (Schwamborn et al, 2012) associated with the model's unit 2 in Fig. 2.…”