NON-CLASSICAL TYPES OF LOESS

Transport of silt and clay in suspension by wind is an universal process. It occurs under all climates, mainly in desertic and sub-desertic regions. The accumulation of such materials originate loess, which is described as a structured loam or loamy silt. Especifically, a sequence of processes is necessary for the generation of loess: a) production of particles, b) deflation and transportation of dust, c) accumulation, d) formation of a structure. Each of those processes can occur in several ways. For example, particles can be originated in periglacial processes, piroclastic eruptions, salt cristalization in playas, etc. Deflation occurs in deserts, periglacial areas and other environments. Epigenetic structures can be produced by percolation of carbonates, iron oxides or silica.

Owing to historical reasons, loess studies have been focused in the type “periglacial origin -- cathabatic wind -- steppe -- carbonate structuring”. That can be considered as the classical case.

Other loess types are frequent in different regions, among them 1) The “cangahua” in Ecuador: piroclastic eruptions -- valley wind -- mountain praire -- carbonate structuring. 2) The tropical loess in South America: deflation of river and fan splays -- savanna -- iron sesquioxide structuring. 3) Gypsum loess in North Spain: destruction of anhidrite/gypsum layers in a dry an cold climate -- valley wind -- steppe -- gypsum structuring. 4) Trade-wind deposits in Venezuela: deflation in tidal flats -- trade wind along the Orinoco valley -- savanna -- iron hydroxide structuring. All this non-classical types conform the accepted loess definitions and they also share the most important field characteristics of loess such as grain size, friability, vertical or sub-vertical slopes in outcrops, subfussion and others. Other cases can probably appear under a systematic scrutiny.