- Related Topics:
- terrigenous sediment
For more than a century a number of partly conflicting and partly complementary hypotheses have been put forward to explain the origin of the silt fraction of loess. The mineral constituents of loess (quartz and feldspar, for example) are reduced to minute particles by weathering action, principally in semiarid and arid regions. But the source of dust may also be a silty, sandy unconsolidated sediment of Paleogene or Neogene age (formed from 65.5 to 2.6 million years ago), as, for instance, the Ogallala Group of the United States. The sand and silt may have been size sorted by the wind, which then transported and deposited the silt. Dust storms derived from such rock units are frequent even today in the continental deserts of Asia; the silt content of “continental loesses” is derived in this manner.
Another significant source of dust may have been the marginal zone of the Pleistocene ice sheet over Europe and North America. During the several ice advances, or glaciations, huge amounts of glacial till, rich in silt, could accumulate in the zone. The silt fraction could then be carried away by the westerly winds and deposited on the lee side of some relief feature; this is sometimes called the theory of glacial loess.
Silt may also be removed from its site of origin of first deposition and be sorted by fluvial processes. Abundant silt is borne by rivers and deposited in times of flood in basins and on alluvial fans that border mountain regions. Some of the silt is then removed from broad floodplains by winds; this results in a double sorting, once by water and once by wind, and hence a more sharply defined typical grain size. In this case loess accumulation is not necessarily connected with a glaciation; indeed, most of the alluvial loesses do not date back to a glaciation.
The reason for conflicting opinions on loess formation is that the silt fraction may have been deposited or reworked by any of the various processes; and, depending on environment and other circumstances, the dominant process may change through time.
There is a consensus that the accumulated dust fraction must have undergone diagenesis (physical and chemical changes after deposition) in order to turn into loess. Diagenesis includes the weathering of fine grains of lime, aluminum silicates, and other substances by hydration in arid climates. In near-surface silt the finest clay particles are cemented by migrating solutions of calcium carbonate and iron oxides and colloidal iron compounds; the quartz grains acquire crusts or coatings of lime or iron, and adjacent grains are cemented together. As a result of these processes, some of the finer particles also attain loess fraction size of 0.01 to 0.05 mm. It has been proved that a grain size of about 0.05 mm is the most favourable for thriving grassy vegetation. Essentially, diagenesis turns sediment deposited in the form of dust into a loosely cemented siltstone.
The peculiar features of loess are thus developed during a process of loess genesis that is akin to that of soil formation. This circumstance served as a basis for the pedogenic (soil origin) theory of loess formation, which basically held that loess is a product of weathering and pedogenesis under semiarid conditions in grasslands and wooded steppes. It would be difficult to explain the formation of a thick loess blanket subdivided by several horizons of paleosols in this way, however. More reasonable would be a polygenetic origin, in which dust can accumulate as the result of any process alternating in time and space, with loess formation resulting from pedogenetic processes acting under favourable climatic conditions.
These pedogenetic processes may take place in three different ways. Epigenesis is an accumulation of a mineral mass without loess properties, perhaps with a high silt and lime content, which under weathering and soil formation acquires loess properties and is transformed into loess. In syngenesis, the accumulation of a mineral mass that is mainly of eolian origin and the acquisition of all loess properties occurs simultaneously, under the influence of soil formation. In protogenesis the accumulated mineral matter already has all the main loess properties because transport occurred subsequent to weathering and soil formation.
The optimum conditions of loess formation are thought to have existed along the border of the continental ice sheet, in such areas as the cold steppes, wooded steppes, loess tundras, and in the steppes bordering the continental deserts. Under conditions other than optimum, the accumulated dust would likely turn into a loesslike deposit differing from typical loess, namely loess loam, limeless loess, brown earth, or reddish loam, or, alternatively, into a soil.
The formation of loess packets is correlated with the cold, dry climatic phases of the Pleistocene glaciations in regions marginal to the ice. The climatic phases, and the occasions of loess formation, recurred three to five times as within the last glaciation.