Sedimentary and Metamorphic Petrology
What the domain covers
Sedimentary and Metamorphic Petrology covers the origin, classification and textures of sedimentary rocks and the agents, types and grades of metamorphism. Questions test process sequences, clastic versus chemical rocks, foliated versus non-foliated textures, and protolith recognition.
Core rules
- Sedimentary rocks form by weathering, erosion, transport, deposition, compaction and cementation. Clastic rocks are classified by grain size: conglomerate or breccia, sandstone, siltstone and shale.
- Textural features include sorting, roundness, matrix and cement. Sedimentary structures include bedding, cross-bedding, graded bedding, ripple marks and mud cracks.
- Chemical and biochemical sedimentary rocks include limestone, chert, rock salt and coal.
- Metamorphism involves heat, pressure and chemically active fluids. Contact metamorphism is local around intrusions; regional metamorphism affects large areas during mountain building.
- Foliated rocks show planar fabric: slate, phyllite, schist and gneiss. Non-foliated rocks include quartzite, marble and hornfels. Index minerals indicate metamorphic grade.
Worked example
A shale is buried and subjected to increasing regional metamorphism. Low-grade metamorphism produces slate, with microscopic mica and slaty cleavage. Higher grade produces phyllite, then schist with visible micas, then gneiss with banding. The index minerals chlorite, biotite, garnet, staurolite, kyanite and sillimanite mark increasing grade. The protolith is shale throughout, but the texture and mineral assemblage change.
Traps
Conglomerate has rounded clasts; breccia has angular clasts. Marble is metamorphosed limestone, not a sedimentary rock. Quartzite is non-foliated even though it may show recrystallization. Do not call slate high-grade; it is low-grade.