Sedimentary Rock Classification
Sedimentary rocks are classified by their mode of origin and composition into clastic, carbonate, and other chemical or biochemical groups, each with its own naming scheme.
Definition
The systematic naming of sedimentary rocks based on grain size, composition, and depositional texture, distinguishing clastic, carbonate, and chemical or biochemical types.
Scope
This topic covers the grain-size scale and classification of clastic rocks (conglomerate, sandstone, mudrock), the compositional classification of sandstones using frameworks such as Folk's and Dott's, the textural classification of carbonates by Folk and Dunham, and the categories of chemical and biochemical rocks including evaporites, chert, and coal.
Core questions
- How does grain size define the basic clastic rock names?
- How do compositional triangles classify sandstones?
- How do the Folk and Dunham schemes classify carbonates?
- What are the main chemical and biochemical sedimentary rocks?
Key theories
- Compositional classification of sandstones
- Sandstones are named by plotting the proportions of quartz, feldspar, and lithic fragments and by the abundance of fine matrix, distinguishing quartz arenites, arkoses, and litharenites from muddy wackes, as formalized in Folk's scheme.
- Dunham carbonate classification
- Carbonate rocks are classified by depositional texture, whether grain- or mud-supported and whether bound during deposition, yielding terms such as mudstone, wackestone, packstone, grainstone, and boundstone.
Clinical relevance
Standardized classification enables consistent description and correlation of strata, communication of reservoir quality in petroleum geology, and interpretation of depositional processes from rock names alone.
History
Folk's clastic and carbonate schemes and Dunham's 1962 carbonate texture classification, developed largely in petroleum-industry settings, became the standard frameworks for naming sedimentary rocks and remain in wide use.
Key figures
- Robert L. Folk
- Robert J. Dunham
- Francis J. Pettijohn
Related topics
Seminal works
- folk1980
- dunham1962
- boggs2009
Frequently asked questions
- What is the difference between sandstone and shale?
- Sandstone is made of sand-sized grains, while shale is a fissile mudrock made of silt- and clay-sized particles; grain size is the primary basis for distinguishing them.
- What is an arkose?
- A feldspar-rich sandstone, typically derived from the rapid erosion of granitic source rocks, indicating limited weathering and short transport.