Dotts classification

Dott's Classification of S.St

1. SEDIMENTARY PETROLOGY
2013

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Dott’sClassification of Sandstones
Most sandstone classifications are based on the composition of the rock. Dott’s classification
scheme is used in most courses. It is based on the relative proportions of:
Matrix: (fine-grained - <0.03mm - material that is associated with the sand grains).
Quartz
Feldspar
Rock fragments (sand grains that are made up crystals of two or more different minerals).

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In Dott’s classification, a triangular diagram is plotted between Quartz, feldspar and rock or lithic
fragments. To classify sandstones using Dott’s scheme the first step is to determine composition
of the rock.
Point counting is a method whereby a thin section on a petrographic microscope is examined by
stepping across the thin section at equal intervals and identifying the material (quartz, feldspars,
rock fragments or matrix) that lies immediately beneath the cross hairs. Counting 250 to 300
grains will accurately yield the proportion of each component.

Sandstones that contains 5 to 15% clay matrix are called arenites (note that the Spanish word for
sand is arena). These can be subdivided based on the percentages of Quartz, Feldspar, and unstable
lithic fragments (fragments of pre-existing rock).
Feldspar-rich sandstone is called an arkose. Lithic rich sandstones are called litharenites. Further
subdivisions are shown in the diagram. If the rock has between 15 and 75% clay matrix, the rock
is called a wacke. Quartz wackes have predominantly quartz surrounded by a mud or clay
matrix. In a feldspathic wacke, feldspar is more abundant, and in a lithic wacke, lithic fragments
are more abundant. The term greywacke is seldom used today, but was originally used to describe
lithic-rich sandstone with between 10 and 50% mica, clay, or chlorite matrix. Rocks with greater
than 75% clay matrix are called mudstones.
As the percentage of quartz increases, the mineralogical maturity of arenites increases. Also, as
the percentage of clay matrix decreases the degree of sorting increases, and thus the textural
maturity increases. Textural maturity also increases in the opposite direction as the % clay matrix
increases from 50 to 100%.
This classification is based on the major component of most sandstone and provides a basis for a
consistent nomenclature for sandstones. The names can be modified to reflect other components
of the rock:

3. SEDIMENTARY PETROLOGY
2013

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e.g., Calcareous quartz arenite: a quartz arenite with calcite cement.
Specific types of rock fragments may also be important in determining the history of the sediment.
Fragments of limestone or dolomite are simply classed as “rock fragments” using Dott’s scheme.
Such grains break down rapidly with transport so that their presence suggests that the sediment
was deposited very close to the area that it was produced.