Dunham classification

The Dunham classification system for carbonate sedimentary rocks was originally devised by Robert J. Dunham (1924–1994)[1] in 1962,[2] and subsequently modified by Embry and Klovan in 1971[3] to include coarse-grained limestones and sediments that had been organically bound at the time of deposition.

[2] The original Dunham classification system was developed in order to provide convenient depositional-texture based class names that focus attention on the textural properties that are most significant for interpreting the depositional environment of the rocks.

Dunham specifically stated that, where appropriate, these six textural class names are intended to be combined with modifiers describing grains and mineralogy.

The most widely adopted of these has been that of Embry and Klovan (1971)[3] who recognized that the Dunham classification scheme lacked detail when it came to the description of organically-bound and coarse-grained limestones.

Embry and Klovan proposed the subdivision of the Dunham 'boundstone' category on the basis of the means by which the sediment was organically-bound, thus yielding three new classes within the Dunham boundstone class: Recognising that the identification of these structures is problematic at the limited scale of a petrographic thin section and typically requires examination of outcrop exposures or core, Embry and Klovan stated that where the mode of binding is not identifiable then the original Dunham classification term boundstone should be retained.

Diagram showing the Dunham carbonate classification scheme
Grainstone in the Dunham Classification ( Brassfield Formation near Fairborn, Ohio). Grains are crinoid fragments.
Grainstone with calcitic ooids , crinoid fragments, and sparry calcite cement; Carmel Formation , Middle Jurassic , of southern Utah, USA.
Thin section photomicrograph of a carbonate mudstone in plane polarised light.
Thin section photomicrograph of a fragmented bioclast wackestone in plane polarised light.
Thin section photomicrograph of a peloid packstone with fragmented bioclasts, plane polarised light.
Thin section photomicrograph of a calcite cemented ooid grainstone, plane polarised light.
Thin section photomicrograph of Orbitolinid foraminifera floatstone with a fragmented bioclast packstone matrix, plane polarised light