Duricrust is a term applied to all types of surface or near-surface, hard, roughly horizontal layers produced by the transportation and subsequent deposition, or by the precipitation, of dissolved or airborne salts and other minerals within soils and unconsolidated sediments, and decomposed or permeable rocks. Exposed duricrusts have a protective effect similar to that of a resistant cap rock in sedimentary sequences, and they can produce topographically significant landforms in much the same way. These landforms include plateaus, mesas, and buttes ("breakaways" in Australia) in flat-lying units, and hogbacks or cuestalike forms in tilted or folded units. Duricrusts are named for their major constituent. Those discussed here are:
Other saline crusts cemented by salts other than calcium sulfate are common, but because they are not sufficiently resistant to erosion to persist as landscape features, they are not considered to be duricrusts. Many duricrusts show evidence of replacement of one type by another (e.g., calcrete replaced by silcrete), indicating a complex history of formation under changing conditions of climate and chemical processes. Because duricrusts are a barrier to the infiltration of water, they increase surface runoff--thereby contributing to flooding.
Among pedologists, chemists, and geologists, agreement is not complete as to the mode of formation of the duricrusts. In general, they are formed beneath the surface, and result from the deposition, and/or precipitation, of transported salts and other minerals that come from the physical and chemical weathering of soils and rocks. The salts and minerals can be carried as airborne dust and carried down into the surface materials by rainfall, or they can be carried in solution in the ground water. The solutes and suspended micro-matter can be moved up and down by an oscillating water table and redeposited or precipitated at different levels in the soils, unconsolidated sediments, and permeable rocks at the same site, or they can be carried down the drainage system and deposited or precipitated far from the source. Thus, the composition and character of the resulting duricrust is determined by atmospheric and climatic conditions, hydrologic conditions, nature of the source materials, topography, and local physical/chemical conditions within the host materials and the invading solutions.
Duricrust layers can be extensive in area, and within a unit vary in thickness and properties. Duricrusts form in lowlands during long periods of stability that allow the redeposited or precipitated materials to slowly accumulate in place. Many duricrusts are "fossil" deposits unrelated to present conditions.
Duricrust layers at or near the surface can provide good conditions for travel. Even though covered with a sand/gravel layer, up to a foot or so in thickness, a duricrust unit can provide the needed support for passage of heavy trucks, tanks, etc., by limiting the distance a vehicle can sink into the soft surface materials. As an exposed unit, the surface characteristics can vary from a relatively smooth cover of gravel-sized fragments that allows for relatively high speed travel, to a surface so broken and dissected and littered with such large rock fragments that vehicular traffic is not feasible. In advanced cases of dissection and long-term weathering, the duricrust winds up as islands of rubble in an intricate, and sometimes angular, network of channels or wadis filled with finer cuts of gravel as well as sand and silt. Barring rains and floods, these channels provide the preferred routes for travel. In time, the islands of rubble become further reduced in particle size and can be traversed.
The more water soluble duricrusts, such as calcrete and gypcrete, can have solution holes or pits. Where covered with a veneer of sand and gravel, these pits, now filled with loose material, cannot be seen, and become obstacles to cross country movement. For a line of vehicles advancing, some will miss the holes and proceed on; others will hit the holes and become stuck in loose sand. Buried layers of duricrust, or hardpan, can restrict access to ground water, and can be a problem in engineering excavations. They can be so hard that they can be attacked only with heavy duty power equipment.
(Common names are in bold) Billy, calcrete, caliche, carbonate platform, croute calcaire, croute de nappe (Algeria and Tunisia), croute gypseuse, ferricrete, grey billy, gypcrete, gypsite, hardpan, ironstone, kankar, kunkur, silcrete, gres polymorphe, porcellanite, sarsen, puddingstone, meulieres.
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