Alluvium

ALLUVIUM (alluvial deposits, formations, sediments), a geological term of genetic significance, denoting pebbles, sands, loams, etc., deposited by the action of water in more or less permanent terrestrial bodies of water. Initially this term was applied mainly to deposits of rivers (river alluvium), then it extended to deposits of lakes (lake alluvium) and to deposits of gullies and ravines (ravine alluvium). Alluvial deposits are formed from material that enters the given body of water from outside or is washed away by these waters from the shores and partly from the bottom, then is washed over, sorted, and deposited with decreasing speed of water movement. Depending on the size of the particles comprising it, alluvium may consist of pebbles of various sizes, gravel, sands, loams, and clays. The size of the deposited particles depends on the speed of water movement at which their deposition occurred; the greater the speed, the coarser the sediments. Thus, alluvial pebbles are deposited at the highest speed of water movement, while clays (silts) are deposited in standing water; intermediate in size grains, gravels and sands, are deposited at medium speeds of water movement. Alluvial deposits are characterized by fairly good mechanical sorting of their constituent particles. An exception is ravine alluvium, which in terms of sorting yields to river and lake alluvium and represents a transition to poorly sorted deluvial formations. Due to the constant variability of water movement speed in terrestrial bodies of water, alluvial deposits are characterized by extreme variability in mechanical composition in horizontal and vertical directions, e.g., rapid transition from pebbles to sands or loams, and back. Also characteristic of alluvium is irregular cross-bedding, lenticular occurrence, and rapid thinning out of layers and lenses. Alluvium often contains remains of freshwater and terrestrial animals and plants. Among alluvium, layers of peat, freshwater marl, accumulations of bog iron ore and some other mineral formations deposited in terrestrial bodies of water are sometimes found. By time of formation, modern alluvium and rarely accessible to direct observation fossil formations of alluvial type are distinguished. Modern alluvium, lying at the bottom of bodies of water and forming floodplain (inundated) shores, was formed and continues to accumulate under the conditions of the existing regime of the modern body of water. Ancient alluvium, which formed high terraced banks, was deposited in previous phases of the body of water's existence under a different regime. Fossil formations of alluvium type—presumed deposits in terrestrial bodies of water of ancient geological epochs—are usually covered by later sediments. Besides its genetic significance, the term alluvium was formerly used for age determination of deposits, denoting formations of the modern geological epoch, in contrast to 'diluvium,' i.e., the glacial epoch. Both these terms in the indicated meaning have long been obsolete. In practical terms, alluvial formations in some places represent certain interest as ballast material, as they very often consist of pebbles, gravel, and sand. Sometimes accumulations in alluvium of peat, bog iron ore, etc. have some practical significance. Alluvial deposits have somewhat greater significance from a hydrogeological point of view, as, being located in depressions of the relief and consisting mostly of loose porous material, they usually accumulate in themselves atmospheric precipitation falling in the area of their occurrence, and then play a fairly prominent role in water supply. Even in the central zone of the RSFSR, water-bearing horizons in modern or ancient alluvium that supply dug wells and springs are often encountered. However, the extent and thickness of alluvial water-bearing horizons are usually small, and the distribution of water in them, given the aforementioned variability of rocks, is often variable, which greatly reduces the practical significance of these horizons. In addition, alluvial waters, which mostly have good initial qualities, are poorly protected due to the easy water permeability of these deposits from surface contamination and therefore in sanitary terms are sometimes not entirely satisfactory. With significant water permeability of alluvium and in the absence of confining impervious layers (clays), waters of some other artesian or groundwater-bearing horizons can penetrate into it. In such cases, the quality of alluvial waters largely depends on the composition of waters of these latter horizons. N. Preobrazhensky, V. Khimenkov.