Geografie 1967, 72, 99-114

Report on the Study of Cryogene Phenomena in Yakutia (USSR)

Jaromír Demek

Geografický ústav ČSAV, Mendelovo náměstí č. 1, Brno, Czechia

The author presents in his report a brief survey on the results of his study of cryogene phenomena in Yakutia during his stay from August 4, to September 2, 1966. In Yakutia the natural conditions are characteristic and resemble those in Europe towards the end of the last glaciation. Permafrost occurs on the whole territory of Yakutia and it reaches the thickness of as much as 1500 m (P. I. Melnikov, 1966). These natural conditions are the cause of an extensive development of cryogene processes and forms. In Yakutia, the study of cryogene phenomena and forms is carried out in the very well equipped Institut merzlotovedeniya sibirskogo otdeleniya Akademii nauk SSSR. The author studied the cryogene phenomena in three different areas: the central part of Yakutia, the Aldanskoye Nagorye in the southern part and on the Kular Ridge in the northern part of Yakutia. In the lowland part of central Yakutia the author dealt especially with the study of thermokarst phenomena. Central Yakutia is from the geological point of view a part of the Siberian Platform. The area is built of Mesozoic rocks covered with thick layers of continental Tertiary and Quaternary deposits. Thermokarst phenomena occur predominantly on the middle terraces of the Lena River where thick loam layers of Middle- and Upper Pleistocene age containing large masses of secondary ground ice accumulated due to neotectonic subsidence movements. Through thermokarst processes depressions of various shape and size develop termed by the Yakutian expression "alas". In the enclosure the various stages of the development of the alas according to P. A. Solovyev are represented. The alasy are of considerable importance for Yakutia's economy. A further form, very common in central Yakutia, are the pingos called in this area "bulgunyakhy". During his voyage on the Lena and Aldan Rivers the author investigated the ice veins in fresh exposures on underwashed blanks. In the enclosure the representative exposure through the Tertiary and Quaternary deposits on Mamontovaya Gora Mt. (cf. profile No.4) is shown. In Yakutia's southern part, in the Aldanskoye Nagorye, the author paid attention especially to the study of cryoplanation terraces. The Aldanskoye Nagorye is a large ancient mountain range forming a part of the Siberian Platform and built of crystalline fundament covered with Cambrian and Jurassic deposits. Permafrost of variable thickness is developed in the mountain range. The cryoplanation terraces are very common here. The study of the exposures revealed that the cryoplanation terraces are destruction surfaces covered with but a thin layer of slope material. The cryoplanation terraces develop due to the retreat of the steeper parts having in the mountain range more often the form of frost-riven scarps than that of frost riven cliffs. Several stages can be noticed in the development of the terraces corresponding to the stages established in Czechoslovakia. The cryoplanation terraces develop in the mountain range even at present. But it may be supposed that there was a period in the past when the cryoplanation developed at a considerably higher rate. In the northern part of Yakutia the author studied cryogene forms in the area of the Kular Ridge especially in its northern spur called Ulakhan Sis. In the whole territory permafrost of a thickness of 500-600 m is developped. The author studied on the one hand the cryogene phenomena on slope deposits in the Burguat River Valley and on the other hand the cryoplanation terraces on Turku Hill (elevation 278 m a. s. l.). The Burguat River is the right tributary of the Kyuchchuguy Kyuegyulyur River and its flow is almost rectilinear, from ESE towards WNW. The valley is in its cross profile markedly assymetric. The slope facing north is steeper and sandstones and shales of Permian and Triassic age outcrop on it. The slope with southern exposition is gentler and covered with slope and fluvial deposits of a thickness of as much as 40 m. In slope deposits both, ice veins and segregated ground ice can be found. The ice veins form a polygonal network (enclosure No. 7). Due to their thawing pyramidal forms, called "baydzharakhy", develop on the surface. Solifluction occurs but in places where the vegetation cover of the tundra was disturbed. The solifluction is always accompanied by sheet wash. The cryoplanation terraces are very common on the Kular Ridge. The author investigated them in detail on Turku Hill where they are developed in sandstones and shales. In contradistinction to the Aldanskoye Nagorye the vegetation is very scarce on the cryoplanation terraces and the processes observed on the terraces are more active than those in the southern part of Yakutia. But one can suppose that even in this area the cryogene processes in Pleistocene passed more intensely than they do in present climatic conditions.