Tephra From Lacustrine Sediments of the Magadan Region

Tephra of two generations is found in the sediments of numerous lakes located on the territory from the coast of the Sea of Okhotsk in the south to the basin of the Indigirka River in the north. Tephra of different ages are distinguished in their distribution area, petromagnetic, geochemical, mineralogical, and thermomagnetic characteristics, which makes it possible to diagnose them in sections. The accumulation of lower tephra is associated with the eruption that formed the Kuril Lake caldera in Kamchatka about 7 600 years ago. It is found in all lakes where sedimentation took place at that time. The nature of tephra occurrence was controlled by the distance from the source, morphological and hydrodynamic parameters of the lakes, and post sedimentation processes. Tephra has a rhyolitic composition with a silica content of 75.6–75.9%, weakly magnetic. Magnetic particles are predominantly pseudodomain. It is dominated by titanomagnetites with average titanium contents in the range of 3.6–6.5 wt. %. The distribution of upper tephra is limited to the area of the Northern Okhotsk region. It is highly magnetic, dominated by low-titanium titanomagnetites (Ti = 2.6 – 3.2 wt. %). The size of magnetic particles is pseudodomain-multi-domain (closer to multi-domain). The magnetic susceptibility of the tephra interlayers, which is an order of magnitude higher than the magnetic susceptibility of the lake sediments, is its main distinguishing feature. Tephra has a rhyodacite composition with a silica content of 70.2–70.5%. It is about 3 000 years old. Tephra layers in lake sediments are important chronological and correlation markers of the Holocene.