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Periods of extreme pedogenesis recorded in micro features in soils on Moscow (Saalian) glacial tillтезисы доклада
- Авторы: Макеев А.О., Куст П.Г., Лебедева М.П.
- Сборник: transactions of the 15th International Conference on Soil Micromorphology
- Тезисы
- Год издания: 2016
- Место издания: UNAM Mexico
- Аннотация: Base tills of Moscow (Late Saalian, Warthe, MIS6) glaciation form important component of landscapes in northern Europe, including the center of the Russian Plain. They are often covered only with a thin veneer of sands, sandy and silty loams, so that surface soils are formed on bipartite sediments. The study of such soils as pedosedimentary sequences allows identifying a set of lithological, pedogenic and cryogenic features that had been formed during various stages within several climatic cycles. The lower units of bipartite sediments are reddish-brown diamictons. The bright reddish-brown color and high birefringence of plasma is typical for base tills of Moscow age and are inherited from sediments mobilized by the glacier. Uniform composition of tills indicates effective mixing and homogenization of material along the ice flow path. Glacial till architecture includes sand lenses and heterogenic fragments due to the filling of small subglacial cavities. Glaciotectonic deformation structures include shearing features, folding, thrusting and rotational structures, tension fractures, till wedges and other evidences of emplacement of matrix within the mobile sediment. During and immediately after deposition diamictons were overlain by veneer of fluvioglacial sands with an aeolian admixture with the minimal thickness of 45 cm. Aeolian input results either in a separate silty layer, patches of silty material or in the admixture of silty particles. Such pattern indicates that aeolian input accompanied deposition of melt-water sediments and final depositional stages occurred in arid environments. Prismatic structural units that could be traced throughout the whole strata of glacial till had been formed by shrinking during sediment stabilization. Impact of a long pedogenesis, presumably occurring during the last interglacial (MIS5) is well seen in the lower unit of bipartite sediments. It is resulted in the formation of pedogenic structural architecture (cracks, superimposed on earlier prismatic units, subangular blocky peds and porosity) and well-developed multi-layered clay cutans. These lead to the formation of a sequence of Bt horizons. Platy structure due freeze-thaw cycles may be observed within the upper meter of diamicton. Irregular network of frost fissures indicate severe freezing. Frost features include stone lines on the border of cover layer and diamicton, signs of cryoturbation (lenses and pockets) and sand grain circles. Cryogenic features are responsible for intermixing of cover layer and till deposits, resulting in the complicated morphology of EBt horizons. Streaks of Bt horizon are presented in the frost wedge fillings, while clay cutans are absent here, indicating that cryogenic stage succeeded pedogenesis, presumably during Valday (Wurmian) glacial time. Holocene pedogenesis being mostly of eluvial - illuvial character, probably strengthens initial lithological discontinuity of bipartite parent material. Soil horizonation is clearly seen within the upper unit (a sequence of A and E or Bw horizons). Clear record of final stages of sedimentation and ancient pedogenesis within profiles of day-surface soils in glacial tills may help to derive reliable palaeoclimatic interpretation from the last interglacial - glacial cycle till present. Correlation of these records with other archives in glacial and periglacial areas opens attractive research perspectives. *Research is supported by the Russian Science Foundation, Grant #14-27-00133. '
- Добавил в систему: Макеев Александр Олегович