Sedimentary rocks: the stages from sediment to rock (deposition, compaction and cementation as lithification); the classification of sedimentary rocks into clastic (by grain size, from conglomerate to mudstone), chemical (precipitates such as evaporites) and biogenic or biochemical (limestone and coal); the description of clastic texture using grain size, sorting and roundness.

What this dot point is asking

OCR wants you to describe how loose sediment becomes rock by lithification (deposition, compaction and cementation), to classify sedimentary rocks into the clastic, chemical and biogenic (biochemical) classes, to use the grain-size scale to name clastic rocks (conglomerate to mudstone), and to describe clastic texture using grain size, sorting and roundness.

The answer

From sediment to rock: lithification

Loose sediment becomes solid rock through lithification, which OCR treats as two linked processes after deposition:

Compaction reduces the pore space and packs the grains; cementation glues them. Fine muds lithify mostly by compaction (they have abundant pore water to expel), while sands rely heavily on cementation to bind their larger grains.

Classifying sedimentary rocks

There are three classes, defined by how the sediment originated:

• Clastic. Made of fragments (clasts) of pre-existing rock, transported and deposited. Classified by grain size:
  • Conglomerate (rounded gravel, over $2\ \mathrm{mm}$) or breccia (angular gravel).
  • Sandstone (sand, $0.06$ to $2\ \mathrm{mm}$).
  • Siltstone (silt, $0.004$ to $0.06\ \mathrm{mm}$).
  • Mudstone or shale (clay, under $0.004\ \mathrm{mm}$).
• Chemical. Formed by direct precipitation from solution. Examples: evaporites (halite and gypsum, left when seawater evaporates) and some limestones.
• Biogenic (biochemical). Formed from the remains of organisms. Examples: most limestone (from shells and coral, calcium carbonate) and coal (from compacted, altered plant material).

Describing clastic texture

For clastic rocks you describe three textural properties, which together record the transport and energy history:

• Grain size records the energy of the environment: gravel needs high energy, mud settles only in still water.
• Sorting records the transport medium: well-sorted means a consistent medium (river, wind) separated the grains by size; poorly sorted means rapid dumping (ice, debris flow).
• Roundness records the transport distance: well-rounded grains have travelled far and been abraded; angular grains are close to source.

Examples in context

Example 1. Coal as a biogenic rock. Plant material accumulating in oxygen-poor swamps is buried and compacted to peat, then lignite, then bituminous coal: a biogenic sedimentary rock whose formation links directly to the coal-bearing sequences studied in economic geology.

Example 2. Evaporites as chemical rocks. Where a restricted sea or lake evaporates faster than it is replenished, dissolved salts precipitate in order (gypsum then halite), forming evaporite beds that act as excellent seal rocks for hydrocarbons.

Try this

Q1. Name the two processes of lithification and state what each does. [2 marks]

• Cue. Compaction (overburden squeezes grains together and expels pore water) and cementation (precipitated minerals bind the grains).

Q2. Classify the following by origin: sandstone, limestone, halite. [3 marks]

• Cue. Sandstone is clastic; limestone is biogenic (biochemical) or chemical; halite is chemical (an evaporite).

Q3. State which textural property of a clastic rock records the transport distance, and explain why. [2 marks]

• Cue. Roundness; grains become more rounded with distance as abrasion knocks off their corners.