TRACES OF MODERN METAMORPHISM IN PETROGRAPHY OF GROUND ICE
E.A. Slagoda
Tyumen Research Centre, Siberian branch of the Russian Academy of Sciences, Earth Cryosphere Institute, Tyumen, Russia
Keywords: ice texture and structure, metamorphism, exogenous processes
Abstract
A number of controversial issues on the genesis and metamorphism of ground ice are being addressed using structural-genetic methods and modern image analysis techniques. Ancient ground ice in natural outcrops is often transformed by seasonal cryogenic processes. The aim of this study is to identify features of the ancient ground ice metamorphism under the impact of modern processes. Ground ice and host sediments were studied at two key areas - the Pur-Taz interfluve and western Yamal - in the continuous permafrost zone of Western Siberia. At the Tazovsky geocryological station, ice wedges in thick Holocene peatlands and associated new ice formations beneath the thawed and frozen active layer were studied in 2017-2019. At the Marre-Sale station in western Yamal, data on the structure of ice wedges, wedge-shaped ice, massive ice beds, and associated newly formed ice bodies in sandy, loamy sandy, and loamy substrates were obtained in 2011-2019. In the upper part of the Marre-Sale coastal bluffs, the active layer overlying permafrost contains records of seasonal and long-term variations in thawing and freezing depths. At the foot of the coastal bluff, massive ice beds overlain with thawed and frozen scree sediments and a recently buried snowpack were described. The impact of modern processes on ground ice was assessed using cryolithological methods applied to host and overlying sediments. Thin sections were prepared from oriented ice core samples preserved in the frozen state. Structural and textural features of ice bodies of different geneses - wedge ice, massive segregational ice beds, injection-segregational ice, wedge-shaped ice - were determined in thin sections using transmitted and polarized light. Superimposed structural and textural features of these ice bodies were identified in the zone of their transformation by modern processes. The study made it possible to identify and visualize macrostructure and characteristic features of the transformation of texture and structure in thin sections of different genetic types of ground ice.
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