Talus Breccia

Sedimentary breccias are a type of clastic sedimentary rock which are made of angular to subangular, randomly oriented clasts of other sedimentary rocks. They are formed by either submarine debris flows, avalanches, mud flow or mass flow in an aqueous medium. Technically, turbidites are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow. The other derivation of sedimentary breccia is as angular, poorly sorted, immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified colluvium. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in grabens.
Sedimentary breccias can be deposited as talus (scree), mass flows, such as landslides and mud flows, or submarine debris flows. Post-depositional breccias can also form by dissolution of carbonate rocks or as collapse breccias, such as cave collapse.

A conglomerate, by contrast, is a sedimentary rock composed of rounded fragments or clasts of pre-existing rocks. Both breccias and conglomerates are composed of fragments averaging greater than 2 millimetres (0.079 in) in size. The angular shape of the fragments indicates that the material has not been transported far from its source. Breccias indicate accumulation in a juvenile stream channel or accumulations because of gravity erosion. Talus slopes might become buried and the talus cemented in a similar manner.

Talus Breccia environment.

Breccia. From Natalie Teager, Arizona State University.

Volcanic Breccia. From Natalie Teager, Arizona State University.

Bibliography

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

Photo
General structure of the rock. PPL image, 2x (Field of view = 7mm)	General structure of the rock. XPL image, 2x (Field of view = 7mm)	General structure of the rock. PPL image, 2x (Field of view = 7mm)
General structure of the rock. XPL image, 2x (Field of view = 7mm)	General structure of the rock. PPL image, 2x (Field of view = 7mm)	General structure of the rock. XPL image, 2x (Field of view = 7mm)
fragments of various nature in a breccia. PPL image, 2x (Field of view = 7mm)	fragments of various nature in a breccia. XPL image, 2x (Field of view = 7mm)	sedimentary fragment with Foraminifera. PPL image, 10x (Field of view = 2mm)
Oolith sedimentary fragment. PPL image, 10x (Field of view = 2mm)	Oolith sedimentary fragment. XPL image, 10x (Field of view = 2mm)	Oolith sedimentary fragment. PPL image, 20x (Field of view = 1mm)
Oolith sedimentary fragment. XPL image, 20x (Field of view = 1mm)