Bioclastic Grainstone

A grainstone is a carbonate rock in which grains larger than 0.25 mm comprise more than 10 volume% of the rock and have a matrix of sparry calcite. They generally are deposited in moderate- to high-energy environments, but their hydraulic significance can vary. Dunham (1962) provided several suggestions for their origin: (1) they may be produced in high-energy, grain-productive environments where mud cannot accumulate, (2) they may be deposited by currents that drop out the grains and bypass mud to another area, or (3) they may be a product of winnowing of previously deposited muddy sediments. In cases where the grains are exceptionally large, Embry and Klovan (1971) designated these carbonates “rudstones.”

Dunham classification.

Folk classification.

Bibliography

• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• M. J. Hibbard (1994): Petrography to Petrogenesis. Prentice Hall editore

Photo
Bioclastic fragments in a matrix of sparry calcite. PPL image, 2x (Field of view = 7mm)	Bioclastic fragments in a matrix of sparry calcite. XPL image, 2x (Field of view = 7mm)	Bioclastic fragments in a matrix of sparry calcite. PPL image, 2x (Field of view = 7mm)
Bioclastic fragments in a matrix of sparry calcite. XPL image, 2x (Field of view = 7mm)	Red algae (Rhodolith) fragment. PPL image, 2x (Field of view = 7mm)	Red algae (Rhodolith) fragment. PPL image, 2x (Field of view = 7mm)
Bioclastic fragments in a matrix of sparry calcite. PPL image, 10x (Field of view = 2mm)	Bioclastic fragments in a matrix of sparry calcite. PPL image, 10x (Field of view = 2mm)	Bioclastic fragments in a matrix of sparry calcite. XPL image, 10x (Field of view = 2mm)
Fibrous Brachiopod fragmant. PPL image, 10x (Field of view = 2mm)	Fibrous Brachiopod fragmant. XPL image, 10x (Field of view = 2mm)	Annelid fragment. PPL image, 10x (Field of view = 2mm)
Annelid fragment. XPL image, 10x (Field of view = 2mm)	Sparry calcite filling a Foraminifera. PPL image, 20x (Field of view = 1mm)	Sparry calcite filling a Foraminifera. XPPL image, 20x (Field of view = 1mm)
Bioclastic fragments in a matrix of sparry calcite. PPL image, 10x (Field of view = 2mm)	Bioclastic fragments in a matrix of sparry calcite. XPL image, 10x (Field of view = 2mm)	Bioclastic fragments in a matrix of sparry calcite. PPL image, 10x (Field of view = 2mm)