3.3 Sedimentary & metamorphic petrology

Sedimentary Rocks

Sedimentary rocks form at the Earth's surface through weathering, erosion, transport, deposition, and lithification. They blanket roughly 75% of continental surfaces and host most fossils, groundwater, coal, and petroleum. Two major groups exist: detrital (clastic) and chemical/biochemical.

Clastic (Detrital) Rocks: Classified by Grain Size

Clastic rocks are built of transported solid fragments, and the primary classification variable is grain size (the Wentworth scale):

Texture also records transport history. Sorting (uniformity of grain size) and rounding (abrasion of grain edges) both increase with transport distance and energy. A well-sorted, well-rounded quartz sandstone is texturally and compositionally mature. Sediment maturity reflects the progressive removal of unstable grains (feldspar and rock fragments) and the concentration of stable quartz. Textural maturity (sorting and rounding) and compositional maturity (quartz enrichment) are independent, and both influence the porosity and permeability that make sandstones important aquifers and petroleum reservoirs. Sandstones are subdivided by composition into quartz arenite (over 90% quartz), arkose (over 25% feldspar), and graywacke or lithic sandstone (muddy matrix with rock fragments).

Chemical and Biochemical Rocks

These precipitate from solution or accumulate from organic material:

• Limestone (CaCO3, calcite) — biochemical (fossil shells, reefs) or chemical; it effervesces in dilute HCl. Dolostone is the magnesium-bearing equivalent, CaMg(CO3)2.
• Evaporites — precipitate as water evaporates: rock salt (halite), rock gypsum (CaSO4 with water), and anhydrite. Evaporite sequences follow the solubility order of the dissolved salts.
• Chert — hard, microcrystalline silica (SiO2).
• Coal — biochemical, formed from compacted and altered plant matter (organic).

Carbonates are further described by texture: micrite (microcrystalline lime mud), sparite (coarse calcite cement), chalk (fine biogenic ooze), and coquina (loosely cemented shell fragments). Coal itself matures through ranks of increasing carbon content and heat: peat, then lignite, then bituminous coal, and finally anthracite (so highly altered it is effectively metamorphic).

Diagenesis and Sedimentary Structures

Lithification converts loose sediment into rock through compaction (grains pressed together, porosity reduced) and cementation (silica, calcite, or iron oxides precipitate in pore spaces). Diagenesis encompasses all low-temperature (below about 200 C) physical and chemical changes after deposition, up to but not including metamorphism. Sedimentary structures — bedding, cross-bedding, graded bedding, ripple marks, and mud cracks — record the depositional environment and the original way-up direction. Graded bedding (coarse grading to fine upward) records waning currents such as turbidity flows, while cross-bedding preserves the migration direction of dunes and ripples. Porosity is progressively reduced as burial depth increases, and where temperatures exceed roughly 200 C diagenesis grades into metamorphism.

Metamorphic Rocks

Metamorphism alters a pre-existing rock (the protolith) in the solid state through heat, pressure (both uniform lithostatic pressure and directed differential stress), and chemically active fluids, all without melting. If melting occurs, the process becomes igneous instead. The three agents work together: heat drives recrystallization and new mineral growth, directed tectonic stress produces foliation, and chemically active fluids speed reactions and can even change bulk composition through metasomatism. Prograde metamorphism proceeds with rising temperature and pressure; retrograde metamorphism partially reverses it during cooling, though it is usually incomplete.

Foliated vs. Non-Foliated

• Foliated rocks develop a planar fabric when platy minerals (micas) grow aligned perpendicular to directed stress. With increasing metamorphic grade, a shale protolith follows a classic progression:

  slate to phyllite to schist to gneiss

  Slate (low grade, very fine, splits along slaty cleavage) grades to phyllite (a silky sheen), then schist (visible aligned micas defining schistosity), then gneiss (high grade, with compositional banding of segregated light and dark minerals).

• Non-foliated rocks form from single-mineral protoliths or under uniform stress: marble (from limestone), quartzite (from sandstone), hornfels, and anthracite. Large metamorphic crystals such as garnet that grow within a finer matrix are called porphyroblasts.

Metamorphic Grade and Facies

Grade describes the intensity (temperature and pressure) of metamorphism, from low grade (slate) to high grade (gneiss). In metapelites, index minerals mark rising grade: chlorite, then biotite, garnet, staurolite, kyanite, and sillimanite. Metamorphic facies (zeolite, greenschist, amphibolite, granulite, blueschist, eclogite) are mineral assemblages that indicate specific pressure and temperature conditions; blueschist marks high-pressure, low-temperature subduction settings, while greenschist and amphibolite mark increasing temperature.

Types by Setting

• Regional metamorphism — large-scale, driven by heat and directed stress at convergent boundaries and orogenic belts; it produces foliated rocks over broad areas.
• Contact (thermal) metamorphism — localized heating in the aureole around an igneous intrusion; with little directed stress it produces non-foliated hornfels.
• Dynamic metamorphism (fault zones, producing mylonite) and burial metamorphism are additional types.

Where temperatures climb high enough for partial melting to begin, the rock becomes a migmatite — a mixed rock transitional between metamorphic gneiss and igneous granite that marks the upper limit of metamorphism. Knowing protolith relationships (shale to slate, limestone to marble, sandstone to quartzite, basalt to greenschist or amphibolite) and the foliation grade progression is heavily tested on both the FG and PG exams.