Sedimentary Breccia

Sedimentary breccia is a coarse clastic rock of sharp, angular fragments larger than 2 millimeters, cemented in a finer matrix. Its angular clasts show that the debris was buried close to its source.

🪨 Geology Guide Specimen: Open Sedimentary Breccia in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Sedimentary, and test your eye in LithoClade.

Rock fragments lose their sharp corners within a short distance of rolling in water. Breccia clasts that are still angular therefore traveled only a short way, often just falling or sliding downslope before they were buried and cemented by calcite, silica or iron oxide.

Sedimentary breccias form at cliff bases as talus, along active fault scarps, in debris flows and in collapse breccias where cave roofs or evaporite beds have fallen in. They are common in the sedimentary record of rifting basins, where steep faults shed coarse debris.

The test is shape: angular clasts make a breccia, rounded clasts make a conglomerate. Breccias with other origins include volcanic breccia and impact breccia, each recognized by its setting and by what the fragments are made of.

• Conglomerate — the rounded-clast equivalent
• Volcanic Breccia — angular fragments from volcanic processes
• Tillite — poorly sorted glacial deposits