Lithological discontinuity (WRB)

Lithological discontinuity is one of the diagnostic properties used, in the WRB system, to discriminate some soils from others.

Description

The term "Lithological discontinuity" (from Greek lithos, stone, and Latin continuare, to continue) refers to significant changes in particle-size distribution or mineralogy that represent differences in lithology within a soil. A lithological discontinuity can also denote an age difference.

Criteria

A lithological discontinuity requires one or more of the following :

• an abrupt change in particle-size distribution that is not solely associated with a change in clay content resulting from pedogenesis

or :

• a relative change of 20 percent or more in the ratios between coarse sand, medium sand, and fine sand

or :

• rock fragments that do not have the same lithology as the underlying continuous rock

or :

• a layer containing rock fragments without weathering rinds overlying a layer containing rocks with weathering rinds

or :

• layers with angular rock fragments overlying or underlying layers with rounded rock fragments

or :

• abrupt changes in colour not resulting from pedogenesis

or :

• marked differences in size and shape of resistant minerals between superimposed layers (as shown by micromorphological or mineralogical methods)

Additional characteristics : in cases, a horizontal line of rock fragments (stone line) overlying and underlying layers with smaller amounts of rock fragments or a decreasing percentage of rock fragments with increasing depth may also be suggestive of a lithological discontinuity, although the sorting action of small fauna such as termites can produce similar effects in what would initially have been lithologically uniform parent material.

See also

• The FAO reference text, (2007 version)