Seismic investigation of sedimentary fill and bedrock geometry of impact crater lake Elgygytgyn, NE Siberia

Lake El'gygytgyn is an impact impact crater located in Chukotka, NE Siberia, with a 3.6 million years climate archive in its sediments. The lake has a diameter of about 12 km and is bowl-shaped with a maximum depth of about 170 m. Lake El'gygytgyn has been recognized as a key site in unraveling the paleoclimate history of the northern high latitudes and will be drilled in the near future by the International Continental Scientific Drilling Program.In a first step towards continental deep drilling, a seismic refraction and reflection pre-site survey was carried out during expeditions in 2000 and 2003. In combination with the echosounder (3.5 kHz) data, the seismic profiles allowed a first study on the characteristics of the lacustrine sediments and on the crater geometry.The lacustrine sediments can be subdivided into two major units (I and II). The upper unit I is well-stratified in the central parts of the lake and intercalated with mass movement deposits in the proximal parts of the lake. Unit II is acoustically more chaotic and massive.The lake's sedimentary history shows three stages associated with two shifts in the sedimentation regime: (i) sediments were dominated by mass movement deposits and the lake was likely concentric with the crater rim during an initial stage. Enhanced weathering in the western part of the crater probably due to a weaker bedrock or a higher initial relief led to a displacement of the lake towards the southeastern part of the crater; (ii) a bipolar sedimentation mode with pelagic sedimentation in the distal parts of the lake and mass movement deposits in its proximal areas was established without a significant further displacement of the shoreline; (iii) a gradual decrease in mass movement deposits in the proximal areas points at lower erosion rates in the surroundings and, thus, a more stable situation of the catchment relief. The central part with undisturbed pelagic sedimentation shows a N-S striking, elongated shape and suggests the onset of the present wind system.Mainly the western part of the crater is presently dominated by periglacial processes and produces large, partly subaquatic, deltas in front of the draining creeks. The eastern part of the crater is more stable and experiences lower erosion. Thus, not only changes in paleoclimate conditions were archived in the lake sediments, but also the local history of erosion and weathering due to an exceptionally high crater relief