loess
- Related Topics:
- terrigenous sediment
loess, an unstratified, geologically recent deposit of silty or loamy material that is usually buff or yellowish brown in colour and is chiefly deposited by the wind. Loess is a sedimentary deposit composed largely of silt-size grains that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs. The word loess, with connotations of origin by wind-deposited accumulation, is of German origin and means “loose.” It was first applied to Rhine River valley loess about 1821.
Thick loess blankets are composed of loess packets 1 to 5 m (about 3 to 16.5 feet) thick, each of which contains intercalated strata of loessial and loesslike sediments, paleosols (ancient soils), sand layers, and similar material. The totality of these constitutes the loess complex. There are several regional variants of loess that, together with true loess, constitute a loess series, including loessial sand, sandy loess, loess loam, and clayey loess. The individual elements of the loess series are hard to distinguish, and the several sediment types composing it are interpreted differently by workers in different regions or countries.
Loess covers extensive areas in Asia, Europe, and North America. Most widespread in today’s temperate zones and in the marginal semiarid zones of the deserts, loess covers about 10 percent of the land surface of the Earth. Loess usually exhibits a surface cover of fertile soil that is conducive to intensive agriculture. The capacity of loess to retain vertical or even overhanging walls is especially evident on the Loess Plateau in China, where some loess bluffs stand 150 m high and contain innumerable cellarlike dwellings excavated by the local inhabitants. In semiarid regions people such as the Pueblo Indians made houses and fortresslike closed edifices from loess-based adobe.
Physical and chemical properties.
The dominant grain-size fraction of loess, called the loess fraction, ranges from 0.02 to 0.05 mm (0.0008 to 0.002 inch) and includes grains of coarse and medium-grained dust. Grain-size analyses by various methods indicate that the abundance of this fraction is about 50 weight percent. Clay-sized particles (less than 0.005 mm [0.0002 inch]) make up another 5 to 10 percent. In some loess regions, the grain-size distribution shifts toward finer grains with increasing distance from the source of dust (e.g., eastward from Sand Hills, Neb.).
Loess typically exhibits a low moisture content of 10 to 15 percent that increases as porosity decreases. Its porosity is 50 to 55 percent, decreasing slightly downward to a depth of about 10 m (33 feet). Below this depth, porosity varies as a function of the grain-size distribution. If the loess is enriched with clay, then the porosity may decrease to 34 to 45 percent. The porosity of sandy loess is about 60 percent.
Loess contains 60 to 70 percent quartz with extremes of 40 and 80 percent. Feldspars and micas make up 10 to 20 percent and carbonates 5 to 35 percent. About 2 to 5 percent of the silt is composed of such heavy minerals as amphiboles, apatite, biotite, chlorite, disthene (cyanite), epidote, garnet, glauconite, pyroxenes, rutile, sillimanite, staurolite, tourmaline, and zircon. Grains are typically slightly weathered. In the finest grain-size fractions (below 0.002 mm [0.00008 inch]), such clay minerals as montmorillonite, illite, and kaolinite predominate over the detrital (fragmental) constituents. Clay minerals may be formed by various processes during and after the accumulation of loess.
The mineralogical composition of loess is fairly uniform, but there are some local deviations due to differences in grain size and area of origin. The area of origin of the dust fraction is revealed by the heavy mineral assemblage, and research has shown that the dust sources may be local, neighbouring, or distant.
The chemical composition of loess most often falls within the following percentage ranges: silica, SiO2, 50 to 60; alumina, Al2O3, 8 to 12; iron oxide as Fe2O3, 2 to 4; iron oxide as FeO, 0.8 to 1.1; titanium dioxide, TiO2, and manganese oxide, MnO, about 0.5; lime, CaO, 4 to 16; and magnesium oxide, MgO, 2 to 6.
The characteristic carbonate content of loess depends on the nature of the dust source, on geochemical and biological processes that occur during and after deposition, and on precipitation and leaching by groundwater. Carbonates are present in loess in a variety of forms, primarily as incrustations on quartz grains and clay-particle aggregates and as small granules and shell fragments. Secondary concentrations include concretions of nodules (Loess-doll) and layers of lime accumulation (caliche). Lime forms frequent tubular incrustations along decayed plant roots, fissure fillings, and similar avenues in loess.
Loess is a rather ill-consolidated sediment of low compressive strength. It is stable, however, as long as it remains dry. Parting surfaces are vertical because capillary incrustations of lime, developed around the roots of a grassy plant cover, lend a vertical texture to loess.
Soaked and loaded loess, however, is liable to collapse and slumping. Wetting decreases cohesion between grains by two-thirds, and the angle of internal friction also decreases (e.g., from 32° to 20°). Groundwater flow in loess will carry away fine, insoluble mineral particles, and this mechanical separation in loess can be accompanied by solution of mineral particles. This process gives rise to depressions, sinkholes, loess wells, and collapse ravines and is much accelerated by gully erosion.