Geol 370: Sedimentology and Stratigraphy Topic 4: Classification of Sandsdtone

1. There have been over
fifty classification
schemes proposed for
sandstone. Most of
those currently in use
involve a ternary QFL
(quartz, feldspar, lithic)
plot for framework
grains and a major
division based on the
relative amount of
matrix.
Classification of Sandstone

2. Dott Classification
Because of its simplicity and added detail, I prefer this version (with
some modification of terminology) of the Dott classification.

3. Point Counting
Sandstone composition is typically determined microscopically by
identifying mineral composition at 300 equally spaced points
across a thin-section.

4. Total Rock Composition
Component Proportion
(%)
Quartz 78
Feldspar 60
Rock fragments 36
Matrix 126
Total: 300
Total Q, F, and L: 174
Framework Grain Composition
Component Proportion
(%)
Quartz (78/174) 45
Feldspar (60/174) 34
Lithic (36/174) 21
Total: 100
Sample Point Count
Composition at 300 points is identified. The relative abundance of
framework grains to matrix determines whether the rock is an arenite, a
wacke, or a mudstone. The framework composition is normalized to
determine the type (quartz, feldspathic, lithic) of arenite or wacke.

5. Ternary Diagrams
The normalized framework percentages are plotted on ternary diagrams
to determine rock composition.
This sample plots as
lithofeldspathic. Because of its
high matrix content, it would
be a lithofeldspathic wacke or
lithic arkose wacke.

6. Quartz
Monocrystalline
Polycrystalline
Quartz grains occur in three forms, monocrystalline (igneous source),
polycrystalline (metamorphic source) and chert (sedimentary source).
Some workers consider chert to be a lithic fragment.
Chert

7. Feldspars
Orthoclase crystal undergoing dissolution
Feldspar grains occur in four forms, orthoclase, plagioclase,
microcline, and sanidine and repesent a variety of igneous and
metamorphic sources. Feldspar rapidly alters to clay (kaolinite,
sericite) and calcite and is highly subject to dissolution.
Plagioclase altering to calcite
Dissolution along cleavage planes

8. Lithic Grains
Volcanic Rock Fragment
Chert Plutonic Rock Fragment
Metamorphic Rock
Fragment
Sedimentary Rock
Fragment (shale)
Lithic fragments are grains
that are recognizable as
particles of pre-existing
rocks and are classified as
igneous, sedimentary, or
metamorphic. Some
workers consider plutonic
and gneissic rock fragments
to be feldspar grains.

9. Provenance of Sandstone
(framework grains)
Influence of Source Rock Influence of Climate
The primary control on sandstone composition is the source rock.
However, climate and transport distance also have significant impacts.

10. Provenance of Accessory Minerals in Sandstones
Igneous Metamorphic Ig/Mc mixed
Aegerine Actinolite Biotite
Augite Andalusite Enstatite
Chromite Chloritoid Hornblende
Ilmenite Cordierite Hypersthene
Muscovite Diopside Magnetite
Topaz Epidote Sphene
Garnet Tourmaline
Sedimentary Glaucophane Zircon
Calcite Kyanite
Chert Rutile
Sillimanite
Staurolite
Tremolite
Accessory minerals
typically have a much
more restricted range of
formation than quartz
or feldspar; therefore,
even minor occurrences
can be very useful in
determining the
sediment source.
Provenance of Sandstone (accessory
[mostly heavy] minerals)

11. Matrix
The matrix portion of sandstone consists of mud-size particles,
particularly of clay and mica in composition, though quartz and
feldspar can play a significant role.

12. Cement
Common cements:
• Quartz
• Calcite
• Dolomite
• Siderite
• Hematite
• Limonite
• Mn oxides
• Pyrite
• Gypsum
• Barite
Cement is a diagenetic precipitate formed mostly during burial and
reflects changes in temperature, pressure, and ion concentration.
Quartz sandstone with calcite, silica, and pyrite cements

13. Quartz Arenite
• At least 95% quartz
• Most common along passive continental margins and in
epicontinental seas
• Often found in eolian, beach, and shelf depositional systems
• Are typically well-cemented and texturally mature to supermature
• Usually represent multiple recycling events (polycyclic)
• Can be first generation if the result of intense weathering

14. Feldspathic Arenite/Wacke (arkose)
• At least 25% feldspar (any type)
• Arenites have less than and wackes more than 15% matrix
• Most common in proximity to granitic source terranes, particularly
those in arid or cold climatic regions or humid regions with high relief
• Often found in alluvial fan depositional systems
• Are typically immature to sub-mature texturally
• Usually from first generation sediment

15. Lithic Wacke (greywacke)
• At least 25% lithic framework grains
• At least 15% matrix
• Most common in proximity to volcanic source terranes,
particularly those in arid or cold climatic regions or humid regions
with high relief
• Often found in alluvial fan depositional systems
• Almost always immature to sub-mature texturally
• Nearly always from first generation sediment

16. Glauconitic Sandstone
Glauconite is an iron-rich clay mineral related to illite that forms on
marine shelves under mildly reducing conditions and very slow
sedimentation rates prior to burial, often coating the surfaces of shells,
fecal pellets, and sedimentary (clastic and carbonate) particles.
Deposits are often some times referred to as “green sands.”

17. Phosphatic Sandstone
Phosphate nodules and cements develop through dissolution of
organic materials, such as bones and teeth, under conditions of low
pH. Phosphate is commonly found in association with glauconite.

18. Porosity
Porosity is a measure of the open space within a rock and can be either
primary (intergranular) or secondary (dissolution) in nature.
Primary Porosity Secondary porosity

19. Porosity and Grain Size
Grain size and sorting significantly affect the primary porosity of sand.