Duricrust

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Duricrust is the generic term for all crusts belonging to the group of terrestrial soils that arise outside the influence of groundwater. These are soil horizons that can be a few millimeters to a little more than 10 meters thick, and in which parts of such a horizon or even the entire horizon are solidified rock-like due to the precipitation of mineral substances from the pore water . Duricrusts occur not infrequently in rock sequences from earlier geological epochs and provide information on the environmental conditions that prevailed at their time of origin.

term

The word Duricrust is made up of the Latin words durus for hard and crusta for the crust . It was first used in Australia in connection with near or surface layers of laterites , bauxites and quartzites . The word component crust is a bit misleading here, because it does not stand for a solid shell or cover, as in "earth crust" or "bread crust" and therefore does not designate a landscape shape. Instead, the processes that lead to the formation of duricrusts usually take place below the actual surface of the terrain. Usually, a duricrust only comes directly to the surface through erosion and can then actually be relief-forming (see meaning ).

Some forms of Duricrusts not only denote the "petrified" soil horizon in a broader sense, but also represent extensive chemical changes within the soil and are therefore regarded as so-called extreme soil forms .

Classification and nomenclature

There is a nomenclature for Duricrusts that primarily clarifies the mineralogical differences:

  • Prefix Al- or Alu- for Duricrusts, in which aluminum minerals such as gibbsite or boehmite dominate
  • Prefix calcium for Duricrusts in which calcite (calcium carbonate, CaCO 3 ) dominates (z. B. Caliche )
  • The prefix Dol- or Dolo- for Duricrusts, in which dolomite (calcium-magnesium double carbonate, CaMg (CO 3 ) 2 ) dominates
  • Prefix ferric for Duricrusts, in which iron minerals such as hematite or goethite dominate (z. B. bog iron )
  • Prefix Gyp- for Duricrusts in which gypsum (calcium sulfate, CaSO 4 • 2H 2 O) dominates
  • Prefix Sal- for Duricrusts, in which Halite (sodium chloride, NaCl) dominates
  • The prefix Sil- stands for Duricrusts, in which quartz (silicon dioxide, SiO 2 ) dominates.

These prefixes are combined with the suffix -crete or -crust . A weakness of this nomenclature is that -cretes and -crusts can mean the same thing on the one hand, and on the other hand each stand for a more specific, albeit very similar, phenomenon. The terms Ferricrete and Ferricrust both stand for Duricrusts with a high iron content, but Ferricrete can also specifically refer to a clastic sediment cemented with iron minerals and Ferricrust to the hard crust of an iron concretion . Both the cementation of a sediment with iron minerals and the formation of iron concretions can be traced back to a duricrust, but can also take place in the context of other geological processes.

It should also be taken into account that there are transition forms between mineralogically different Duricrusts that arise under similar conditions (e.g. between Silcretes and Ferricretes or Ferricretes and Alucretes or Calcretes, Gypcretes and Salcretes).

In a current study of metal-containing Duricrusts (called "Orecretes", English: ore = ore) a slightly different nomenclature is used. In this case, only the anion of the dominant minerals in the duricrust is decisive, which is combined with the suffix -crete:

  • Oxicrete: oxides and (oxide) hydrates
  • Carbocrete: carbonates
  • Silicacrete: Silicates (mainly quartz or opal)
  • Halcrete: Halides (chlorides, iodides, fluorides and bromides)
  • Sulcrete: sulfates and APS minerals (aluminum phosphate sulfates)
  • Phoscrete: phosphates
  • Arsenocretes: arsenates
  • Vanadocrete: Vanadates

These names are then combined with the symbol or symbols of the metal or metals occurring therein, e.g. B. Silicacrete- (Fe-Al) .

Emergence

Silcrete layer exposed and heavily weathered by erosion ( Pennsylvania of New Scotland , Canada)
Calcrete (Caliche), both bulbous (predominant part of the picture) and layered (at the top) formed, in red siltstone and claystone of the Moydart Formation ( Silurian of New Scotland)

In general, duricrusts always arise with the participation of water and predominantly in connection with soil formation in areas with a tropical to subtropical climate and with at least locally flat relief. However, the water involved is by definition not groundwater, but groundwater, which comes from precipitation or nearby bodies of water.

Formed in a humid climate

Handpiece from Raseneisenstein, a Ferricrete that has recently been created in moderate latitudes. On top of the handpiece, which consists mainly of iron oxides and hydroxides, there is a thin greenish coating of the iron phosphate mineral Vivianite (origin unknown).

In humid climates , rocks are weathered to a large extent by chemical means . The area between the soil or terrain surface and the unweathered base material of a soil is called the weathering profile. In the humid tropics, which are aggressive in terms of chemical weathering, weathering profiles of up to 120 meters in thickness can arise.

In the course of chemical weathering, numerous minerals of the starting material (including feldspars ) are decomposed, with various chemical elements and compounds being dissolved ( mobilized ) in the soil water . The chemically modified starting material is now called saprolite .

After a shorter or longer vertical or lateral transfer ( migration ) of solutes, they are precipitated again as soon as the solution reaches an area of soil, providing the necessary chemical conditions, and it usually is a change from reducing to oxidative environment is . This leads to the formation of initially small, bulbous concretions, which over time, if the conditions remain the same, become larger and can grow together to form a continuous layer, with the previously existing loose material being completely displaced. In other cases, the precipitation of the minerals leads to a loose sediment being impregnated and thereby cemented.

Typical for Duricrusts in weathering profiles of humid climates are Alucretes, Ferricretes and Silcretes. Here Alucretes u. a. from the weathering of nepheline syenite and ferricretes and tiallitic alucretes u. a. from the weathering of basic rocks . In this way, Alucretes form deposits of the aluminum ore bauxite , where they occur together with Ferricretes.

The SiO 2 bound in silcretes can definitely be mobilized again and deposited again either at the same point or at another location. This deposition process can take place at the same point at different times in chemical environments that differ to a certain extent and then leads to the formation of complex, polyphase SiO 2 duricrusts, which are referred to as multiple silcretes .

Formed in semi-arid and arid climates

In a warm, dry climate, chemical weathering only plays a very minor role. Due to the high evaporation rate, if the sedimentation is interrupted for a long time, primarily calcretes, gypcretes and salcretes are created by the separation of calcite, gypsum or table salt (geolog .: rock salt) from oversaturated solution when the water evaporates in the pore space of the upper soil layers.

When calcretes are formed, they also initially form tubers, which then grow together to form a closed layer. Not infrequently, however, the roots of plants are encrusted with calcite. The regular alternation of sedimentation phases and sedimentation pauses can result in series with several calcrete horizons following one another at a certain vertical distance. Calcretes that arise in an arid climate are also known as caliche .

Gypcretes and Salcretes are particularly typical of Playa and Sebcha sediments. In the soil, near the shore of an over-salted sea lagoon or a salt lake, pore water rises from the water-saturated area of ​​the soil towards the surface due to the capillary effect, whereby a high degree of gypsum and halite is deposited there due to the high evaporation rate. Due to the high water solubility of halite, salcretes are quickly dissolved again in the soil when the conditions become increasingly humid.

meaning

Table mountain, the plateau of which is formed by a duricrust ( Burkina Faso ).

Duricrusts are of particular importance for geomorphology, deposit science and geology as indicators of a particular paleo-environment.

Silcretes have a high resistance to weathering in dry climates . So it can u. It can happen that in a humid period a layer of silicate forms in the bottom of a valley floor, with the solutions that supply the SiO 2 coming from the mountains that rise above this valley and are exposed to relatively strong chemical weathering. If the climate changes towards drier conditions, physical weathering dominates, which the mountain material, which has meanwhile been quite heavily attacked, is less able to withstand than the valley floor, which is now well protected against further erosion by the silicate layer, which only badly weathered in a dry climate ("Armored") is. Progressive erosion in the region is now ensuring that the mountains turn into valleys and the former valley floor forms a table mountain . Such a process is commonly referred to as relief reversal .

For deposit science and thus also for the mining industry, alucretes are particularly important, as they form bauxite deposits that are mined for the extraction of aluminum.

During the exploration of Mars , the Rover Opportunity discovered a sequence of rocks in the Eagle crater of the Meridiani Planum, which contains gypsum and hematite-rich concretions and cementations, which could be duricrusts. This would be clear evidence of a prolonged presence of liquid water in near-surface sediments of Mars. However, these rocks are likely to be several billion years old.

References and comments

  1. ↑ geodz.com : GeoDZ.com. The Lexicon of the Earth: Duricrust (definition). Retrieved December 29, 2011
  2. ↑ Christiane Martin, Manfred Eiblmaier (ed.): Lexicon of Geosciences: in six volumes, Heidelberg [including]: Spektrum, Akad. Verl., 2000–2002
  3. ↑ To understand it, it is important to clearly define the term groundwater here. While z. B. in hydrogeology, according to DIN 4049, so that almost all water is meant that does not stand or flow directly on the earth's surface, in the definition of the Duricrust only water in deeper rock layers is referred to. Near-surface "ground" is used here to distinguish ground water called and not counted as actual groundwater.
  4. ↑ britannica.com : Duricrust (English). Retrieved December 29, 2011
  5. ↑ Klaus KE Neuendorf, James P. Mehl Jr., Julia A. Jackson: Glossary of Geology. American Geological Institute, Alexandria, Virginia, 2005
  6. ↑ Harald G. Dill, Berthold Weber, Reiner Botz: Metalliferous duricrusts (“orecretes”) - markers of weathering: A mineralogical and climatic-geomorphological approach to supergene Pb-Zn-Cu-Sb-P mineralization on different parent materials. In: New Yearbook of Mineralogy - Papers: Journal of Mineralogy and Geochemistry. 190 (2), 2013, pp. 123-195, doi : 10.1127 / 0077-7757 / 2013/0235
  7. ↑ The term "tiallitic" goes back to the nomenclature of Hermann Harrassowitz for the classification of laterites and means that the corresponding weathered soils have a relatively high titanium content, primarily in the form of the mineral anatase .
  8. ↑ SW Squyres, JP Grotzinger, RE Arvidson, et al .: In Situ Evidence for an Ancient Aqueous Environment at Meridiani Planum . In: Science . tape 306 , 2004, pp. 1709–1714 , doi : 10.1126 / science.1104559 .

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