Permafrost

Reconstruction of paleocryogenic strata through
micromorphological indications
A. Kurchatova 1 , E.Slagoda 2
(1.Subarctic Center of Tumen State Oil and Gas University; 2.Institute of Earth’s Cryosphere SB RAS)
Abstract: Relicts of cryogenic strata are known in Central Asia on the territory of Mongolia and
northern China. In order to reconstruct permafrost formation and degradation the data of the
changes of cryostructure and maximum thickness during climate fluctuations and sedimentation
dynamics are necessary.
The information about cryogenic events is contained in buried permafrost and thaw strata of
the intermountain depressions. According to Baikal paleoclimatic scale the seasonal and
perennially frozen ground of the Cenozoic downwarps and intermountain depressions of the
southern Siberia could be formed from the Late Pliocene (2.8 Ma ago).
Evidences of the deep ground freezing (up to 118 m) have been received on the example of
downwarps of the southern Siberian platform. Cryolithozone consisted of two layers: the upper
syngenetic strata from the surface and the lower epigenetic strata of the Miocene and Paleogene.
On the exposures of the Quaternary deposits the direct cryoindications are distinguished such as
polygonal wedge structures, cryogenic and postcryogenic textures, ctyoturbations, traces of the
frost heave and solifluction ground flows, seasonal and perennial frost heave mounds testifying
about repeated degradation of permafrost and partial thawing from the surface.
By studying the core samples it is possible to distinguish only the fragments of these
formations less changed by modern exogenic processes. The researches have proved that the
ancient cryogenic processes caused by phase transitions of water/ice, have left the influence
traces of the different scale: seen deformations of sedimentation and postcryogenic structures.
Micromorphological analyses of the thin soil sections have shown that cryogenic processes
also caused the irreversible changes of the microstructure. The deformed relicts of the syngenetic
microstructure similar to an Ice Complex are kept in the thawed and subsided in situ sincryogenic
strata. They are formed by different combinations of sedimentary, soil and cryogenic features of
the microstructure. These attributes allow to distinguish the horizons of syngenetic and seasonal
freezing in a paleocryogenic strata. Micromorphological evidences of syngenesis are absent in
the lower strata. Presence of postcryogenic textures and deformations together with good
safety of sedimentary microstructure testify only about epigenetic freezing.
Thus the former syncriogenic and seasonal freezing horizons and also lower epicryogenic
strata can be distinguished in the sections of the layered sandy-clayish sediments using
cryoindications. The bottom border of the maximal epigenetic freezing can be established in the
closed depressions filled by the Cenozoic weakly consolidated sediments.
The revealed attributes of the structures of paleocryogenic strata in shallow intermountain
depressions of Central Asia can be used for reconstruction of the thawed out and buried
permafrost in the continental freshwater basins (for example, Baikal and Khubsugul), also for the
studying of Arctic shelf permafrost.
Key words: Cryogenic strata, syngenetic and epigenetic permafrost, micromorphological
cryoindications
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