Calcrete
Prior
to the late Silurian, there were only inorganic, shallow, microbial protosoils. Soil formation (pedogenesis)
properly began in the Silurian as chemical weathering of high ground resulted
in minerals and clays being deposited in basins. Early organic soils containing
plant material have been found in the Early
Devonian Rhynie Chert, near Aberdeen, where fossils of early vascular plants
have also been found. The development of deeper and
stronger plant rooting systems on Archaeopteris, Lepidendropsis/Protostigmaria), and Rhacophyton in the Late
Devonian resulted in deeper soils containing more organic material by the
process of pedoturbation.
Clays from the smectite group are
found in Devonian palaeosols. These clays take up water and swell in wet
conditions and so shrink when they dry out because of evaporation. This results
in vertical fissures which allow fluids to penetrate the soil. With each
hydration / evaporation cycle, the concentration of calcium and magnesium carbonate minerals increases and these
saturated mineral solutions react with the soil altering the aluminium based clays
to either calcretes or dolocretes. This alteration is post deposition as can be
seen by the limited penetration down from the surface. Older
calcretes tend to
have been altered to dolocrete.
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