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Surface paleosols on Moscow glacial till in the center of the Russian plainтезисы доклада
- Авторы: Makeev A., Kust P., Lebedeva M.
- Сборник: Proceedings, XIII-th Int. Symposium and Field Workshop on Paleopedology (ISFWP)
- Тезисы
- Год издания: 2014
- Место издания: Torun, Poland
- Первая страница: 18
- Последняя страница: 19
- Аннотация: Ground moraines of Moscow (Late Saalian, Warthe, MIS6) glaciation form important component of landscapes in northern Europe, including the center of the Russian Plain. Soils of ground moraine areas are formed on glacial and periglacial sediments (base tills, fluvioglacial deposits, mantle loams). Glacial and periglacial sediments are the oldest soil-forming rocks here. Within an altitude of 130-180 m above sea level glacial tills are often covered only with a thin veneer of sands, sandy and silty loams, so that surface soils are formed on bipartite parent material. The study of such soils as pedo-sedimentary sequences allows identifying a set of lithological, pedogenic and cryogenic features that had been formed during various stages within several climatic cycles: Pre-depositional. The lower units of bipartite sediments are reddish-brown diamictons (sandy clays or sandy heavy clay loams). The bright reddish-brown color (up to 5YR 6/6) and high birefringence of plasma through the whole strata due to high weathering is typical for base tills of Moscow ageю These features are inherited from sediments mobilized by the glacier: older tills, weathering crusts, hard rocks, etc. Uniform composition of tills indicates effective mixing and homogenization of material along the ice flow path. Stones are dispersed in the matrix and presented by a big array of rock debris – granites, limestones, breccia, etc. Over consolidated matrix supported fabric with low porosity is a result of big pressure under the glacier. 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. Syndepositional. During and immediately after deposition diamictons were overlain by veneer of fluvioglacial sands with an aeolian admixture. The minimal thickness of fluvioglacial cover layers is about 45 cm and it increases in the direction of insequent meltwater gullies. The lower unit (diamicton) changes coherently: in the direction of the gullies it is becoming rewashed, streaked and wedged out. Aeolian input results either in a separate cover layer, patches of silty material or in the admixture of silty particles in the upper parts of cover layers. Such pattern indicates that aeolian input accompanied deposition of melt-water sediments and those final depositional stages occurred in arid environments. Prismatic structural units that could be traced throughout the whole strata of glacial till had been formed most likely due to shrinking during sediment stabilization. Pedogenic, stage I. Impact of a long pedogenesis, presumably occurring during the last interglacial (MIS5) is well seen in the lower unit of bipartite sediment. 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. Though many lithological features have not been overprinted or obliterated by pedogenesis. Cryogenic. Platy structure due freeze-thaw cycles may be observed within the upper meter of diamicton. Irregular network of frost fissures indicate severe freezing. Fissures have sandy or silty filling, depending on the cover layer. Frost features include also stone lines on the border of cover layer and diamicton, signs of cryoturbation (lenses and pockets) and circles of sand grains typical for modern arctic environments. Cryogenic features are responsible for intermixing of cover layer and till deposits, resulting in the complicated morphology of EBt horizons that is typical for Glossisols. The sand wedge filling consists of aeolian sand and loamy streaks of different sizes, originating from the host material. Streaks of Bt horizon are presented in the frost wedge fillings, while clay cutans are absent here, indicating that cryogenic stage succeeded stage I pedogenesis, presumably during Valday (Wurmian) glacial time. Pedogenic, stage II. Pedogenic features of the last interglacial have not been overprinted by Holocene pedogenesis. For this reason it is difficult to differentiate the impact of Holocene and last interglacial pedogenesis in the lower unit. Holocene soil formation 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). The abundance of relict features and clear record of final stages of sedimentation within profiles of day-surface soils on glacial tills allow regarding them as pedolithocomplexes and surface paleosols. Their study may help to unravel complex interaction between lithological, cryogenic and pedogenic features since Moscow time thus deriving reliable palaeoclimatic interpretation from the last interglacial - glacial cycle till present. Upland soil-sedimentary sequences need to be further studied in detail, their stratigraphy, chronology and relative contribution of different pedogenic and sedimentary processes are still a matter of discussion. Correlation of these profiles with other Quaternary continental records – loess/paleosol sequences of European loess belt and loess area of Southern part of Russian Plain and periglacial cover beds of European low mountains open additional attractive research perspectives.
- Добавил в систему: Макеев Александр Олегович