Structural features of Pleistocene cryogenic deposits in the Lena and Amga Rivers Plain (Central Yakutia)

Dispersive perennially frozen Quaternary deposits, ca. 100 m thick, were intersected by a borehole drilled in the central part of the Lena – Amga River plain. By the lithogenetic and cryolithological properties of deposits, the section is divided into 6 units: I – seasonal freeze-thaw (int. 0–1.1 m); II – blanketing loams (int. 1.1–2.65 m); III – ice complex (int. 2.65–26.15 m); IV – lacustrine (int. 26.15–63.45 m); V – lacustrine-alluvial (int. 63.45–78.9 m); VI – alluvial (int. 78.9–94.5 m) deposits. Two upper units (I, II) are at the stage of incorporation into permafrost. Unit III is characterized by a low bulk volume, high levels of moisture, organic matter, and mineralization, the presence of wedge ice and a variety of cryostructures. These features indicate that diagenetic transformations of sediments are absent. Stable isotope content in ice wedges provides evidence that they had formed mostly from snow melt. Units IV–VI are characterized by undisturbed bedding, absence of ice layers, exclusive distribution of massive cryostructure, higher density, low levels of mineralization and organic content. The stated peculiarities of units IV-VI witness deeper transformation of sediments, corresponding to the termination of the initial stage of diagenesis. By a degree of sediment diagenetic transformations prior to freezing, unit III is related to syncryogenic, while units IV–VI refer to epicryogenic deposits. Changes in sedimentation and freezing of sediments on the Lena– Amga River plain are associated with climatic fluctuations and glaciations. Freezing of the major volume of epicryogenic stratum had taken place in the Karga-Sartan time of the Late Pleistocene.

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