Very Large Cryoturbation Structures of Last Permafrost Maximum Age at the Foot of the Qilian Mountains (NE Tibet Plateau, China)

Abstract

Unusually large cryoturbation structures (4-4.5 m amplitude), developed in channel gravels and overbank fine-grained deposits of a river terrace on the NE Tibet Plateau, China, were formed by loadcasting as late Pleistocene-age permafrost degraded. It is suggested that the oversaturation and liquefaction of the thawed sediments could have been achieved only if the amount of ice in the upper 4-4.5 m of terrace sediments had been very high. The structures, dated at around 26-20 ka by OSL, point to the presence of a massive icy layer existing within a permafrost body during the Last Permafrost Maximum.

Very Large Cryoturbation Structures of Last Permafrost Maximum Age at the Foot of Qilian Mountains (NE Tibet Plateau, China): a Discussion

“…The gravel must have been deposited before 30 ± 2.5 ka, this being the earliest of three OSL ages from the overlying sandy silt (Table ). Similar results were obtained by Vandenberghe et al () from samples within the involutions. The ages of the silt in the lower part of the layer exhibit an orderly increase in age with depth, implying negligible mixing during subsequent events.…”

“…Unfortunately, our photographs do not totally match with the interpretation in Figure of Vandenberghe et al (). The ages obtained by both Vandenburghe et al and the present authors are particularly important in dating the sedimentary sequence and regional climatic events.…”

“…The section consists of a lower layer of river gravel more than 4 m thick (bottom not seen), overlain abruptly and conformably by a layer of sandy silt up to 1.4 m thick where not involved in the involutions. The upper undisturbed part of the surface of the gravel is parallel to the bed of the present river, implying that they were part of a former river bed deposit (Figure ), rather than local outwash from glaciers along the east side of the valley (Vandenberghe et al, ). The overlying sandy silt was interpreted as overbank deposits by those authors.…”

“…Various expeditions by European permafrost specialists have worked in the Hexi Corridor at relatively low elevations between the QTP and Russia. Vandenberghe et al () described what they saw on one of these visits to an outcrop of outwash gravels and silts at a road cut near Huangchengzi (39°49′35.3″N, 98°02′46.0″E, 1771 m a.m.s.l. ), Mengyuan County, Qinghai Province, China, on the northeastern margin of the Qilian Mountains.…”

Comment on ‘Very Large Cryoturbation Structures of Last Permafrost Maximum Age at the Foot of Qilian Mountains (NE Tibet Plateau, China): a Discussion’ by Stuart A. Harris, Huijun Jin and Ruixia He in PPP

“…In addition to the OSL ages that we have published (Wang et al , ; Vandenberghe et al , ), three new OSL ages, although without precise locations, are provided by Harris et al (). We agree with these authors that more ages are desirable to pinpoint the sequence of events that took place at the time of permafrost development and degradation in the study region.…”