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Cryogenic structuring of superficial moraine sediments in low mountains of Kola Peninsula, Northwestern Russiaтезисы доклада
- Автор: Garankina E.
- Сборник: Late Pleistocene Glacigenic Deposits from the Central Part of the Scandinavian Ice Sheet to Younger Dryas End Moraine Zone. Excursion guide and abstracts of the INQUA Peribaltic Working Group Meeting
- Тезисы
- Год издания: 2011
- Место издания: Geological Survey of Finland, Finland
- Первая страница: 92
- Последняя страница: 93
- Аннотация: The Kola Peninsula located close to the Scandinavian center of ice, was a region of a significant glacial activity in the Pleistocene. Due to combination of lowland and mountainous relief there can be found moraines deposited by ice sheets, valley and cirque glaciers and also their collisions. After last deglaciation superficial moraine sediments tend to be reworked by different geomorphic agents. In subarctic climate conditions of low mountains cryogenic processes begin to act one of the main roles in alteration and structuring of these sediments. The Khibins and the Lovozeskie tundras are low mountains elevated around 0.9-1 km above the sea-level and located in the central part of the Kola Peninsula. These massifs represent small round alkaline intrusions into the frame of gneiss-granitoid rocks. The massifs are greatly tectonically dissected, reworked by glaciers and water erosion due to their relatively low resistance to the surface environmental conditions and partially covered by a complex diamicton layers (Regionalnaya cryolitologiya, 1989). Large troughs cross the massifs and a lot of smaller glacial valleys with cirques in their heads erode central parts of the mountains. These depressions usually carry certain amounts of alpine moraine sediments, thickness of which depends not only on the primary deposition rates, but also on the intensity of erosion in the particular places. These sediments consist generally of badly sorted coarse debris of local alkaline material (nepheline syenites). Though the low marginal zones of the mountainous massifs (outward slopes and valleys opened to the adjacent plains) are usually buried with continental moraines of the last glaciation. These mixed sediments (both rounded and coarse debris of derived rocks and local material) are found up to 600-700 m height in mountain valleys, but their occurrence hardly depends on the exposition of the valleys and the direction of the ice flow during the continental glaciations. Some erratic boulders are recorded on the tops of the mountains (on widely spread plateaus higher than 1 km) corresponding the influence of older and thicker glaciations. Detailed fieldworks provided in this region during last five years (2006-2010) and thorough analysis of literary materials (Baranov, 1958; Gladcin, 1936; Washburn, 1969) allow determining some particular features in distribution, development and variety of types of cryogenic structuring of widely spread moraine sediments. Lots of excavation works have been done during this period which can help to understand also the specifics of inner structure of cryogenic microrelief and to reveal some concealed patterns of its growth. Furthermore a station for measuring the modern rates of sediment movements on the slopes was installed. Impact of cryogenic processes on the ground surface in subarctic mountains declines with the lowering of its elevation. Therefore cryogenic microforms are common only for two geomorphic situations: 1) cirque’s floors and bottoms of mountain valleys in their upper parts covered with alpine moraines and 2) upper parts of mountainous slopes covered with continental moraines. In other geomorphic positions cryogenic structuring of superficial moraines also can occur, but they are much less typical for the studied territory and are not considered in this paper. It has to be said that in the first case abundance and variety of types of cryogenic microrelief are far greater. It is caused by the various local geomorphic conditions those occur due to modern erosion activity and differential draining of the territory. In the second case drainage of the sediments is usually much more even and depends mostly on the inclination of the slope. One of the main factors in the intensity of cryogenic structuring of the sediments is a fine-grained content. Alpine moraines in the studied field area usually include small amount of fines so the most accurate cryogenic forms originate in the local accumulation zones (like small shallow depressions or ephemeral lakes in the river valleys’ bottoms). These conditions in addition to the high level of moisture of the sediments lead to the appearance of well-sorted medium size (from 0.4 to 1.5 m in diameter) high-centered polygons and circles (with small boulders to pebbles in the ridges and silts to loams in the centers). With the increase of inclination (on the slopes of the depressions) these rounded cells of microrelief transform to a linear ones (Mudrov, 2007), so the ridges oriented across the slope come apart and disappear, while the ridges spread along the slope remain as a parallel stripes of fine and coarse grained material. In the vertical profile material is also sorted into prominent cells (0.2-0.6 m depth) and turns to the unsorted mass lower than 0.7 m from the surface. The surface of moraine ridges often also demonstrates signs of cryogenic structuring. But better drainage of the sediments and lower fine-grained content explain much less evident cryogenic microrelief. In these terms small flat forms with diameter less than 0.5 m appear. Their centers consist of gravel and small pebbles with a little amount of sand or sandy loam and ridges include also pebbles and sparsely boulders. Lateral sorting of material is much worse and in depth structuring never reaches 0.1 m. Sometimes the mixed composition of the material acts is the point of the visual evidence of these microforms. Derived (and much better rounded) material tends to gather in the ridges of microforms, while the coarser local material remains in their centers. It is important to mention, that if the upper margin of cryogenic structuring of moraine sediments defines by the height of distribution of the glacial deposits, then the lower margin of cryostructuring is controlled by the appearance of vegetation with the declining of height and softening of the climate conditions. Thereby these two different types of the cryogenic structuring of moraine sediments are only two extreme cases, whereas in natural environment we can observe a great variety of forms which combine the signs of both types and have got other particular features. So we have to point that the character of the cryogenic microrelief is controlled mainly by local conditions like regime of moistening, extent of vegetation, local amplitudes within the scope of microforms, inclination of the slopes and etc. It has to be considered during the studies focused on moraine explorations because of highly uneven processing of these sediments by cryogenic agents.
- Добавил в систему: Гаранкина Екатерина Вадимовна