Sedimentary rocks and depositional environments: the classification of clastic rocks by grain size (conglomerate and breccia, sandstone including arkose, greywacke and orthoquartzite, siltstone, mudstone and shale) and of chemical and biogenic rocks (limestone including oolitic, micritic and fossiliferous, chalk, the evaporites rock salt and gypsum, and coal); sedimentary structures (cross-bedding, graded bedding, ripple marks, desiccation cracks) as way-up and environment indicators; depositional environments (fluvial, deltaic, shallow marine, deep marine, desert); and diagenesis and lithification.

What this dot point is asking

Eduqas wants you to classify sedimentary rocks into clastic rocks (by grain size, with the sandstone types arkose, greywacke and orthoquartzite) and chemical and biogenic rocks (limestones, chalk, evaporites, coal), to interpret sedimentary structures as way-up and environment indicators, to assign rocks to depositional environments, and to describe diagenesis and lithification. It sits downstream of weathering and upstream of petroleum systems.

The answer

Clastic rocks, classified by grain size

Clastic rocks are made of fragments (clasts) of pre-existing rock, transported, deposited and lithified, and are named by grain size:

• Conglomerate (rounded gravel, over $2\ \mathrm{mm}$) or breccia (angular gravel of the same size); rounded means far-travelled, angular close to source.
• Sandstone (sand, $0.06$ to $2\ \mathrm{mm}$), refined by composition: arkose (feldspar-rich, over about $25\%$, immature), greywacke (dirty, angular grains in a clay-rich matrix, from turbidity currents, immature) and orthoquartzite (clean quartz with a silica cement, the most mature).
• Siltstone (silt, $0.004$ to $0.06\ \mathrm{mm}$).
• Mudstone or shale (clay, under $0.004\ \mathrm{mm}$); shale is the fissile (thinly splitting) variety.

Chemical and biogenic rocks

These form by precipitation from solution or from the remains of organisms:

• Limestone (calcium carbonate), in three varieties: oolitic (ooliths, tiny concentric spheres from warm, agitated shallow seas), micritic (fine carbonate mud from quiet water) and fossiliferous (rich in shell debris).
• Chalk: a soft, very pure, fine white limestone of microscopic algal plates (coccoliths), from warm, clear seas.
• Evaporites: precipitated as restricted seawater evaporates, in order gypsum then rock salt (halite).
• Coal: from plant material buried in oxygen-poor swamps (peat to lignite to bituminous coal).

Sedimentary structures as way-up and environment indicators

Sedimentary structures form at deposition; they reveal the environment and, crucially, the way up, letting you detect overturned beds:

• Cross-bedding. Inclined foresets formed as ripples or dunes migrate; truncated at the original top and curving to meet the base, recording a current direction (rivers, deltas, dunes).
• Graded bedding. A bed grading from coarse at the base to fine at the top as a waning turbidity current drops its coarsest load first; the coarse base gives the way up.
• Ripple marks. Symmetrical ripples form under waves; asymmetrical current ripples form under a one-way current and point downstream.
• Desiccation (mud) cracks. Polygonal cracks tapering downwards as mud dries in air; they indicate periodic exposure (floodplain, tidal flat) and give the way up.

Depositional environments

The combination of rock type and structures fixes the environment:

• Fluvial (rivers): cross-bedded sandstones and conglomerates, current ripples.
• Deltaic: interbedded sands, silts and muds where a river meets the sea, often coarsening upward.
• Shallow marine: well-sorted sandstones, wave ripples, fossiliferous and oolitic limestones.
• Deep marine: graded turbidite sandstones and fine muds, below wave base.
• Desert (aeolian): very well-sorted, frosted sand in large-scale dune cross-bedding, sometimes with evaporites.

Diagenesis and lithification

After deposition, diagenesis is the sum of the low-temperature physical and chemical changes that turn sediment into rock, the key part being lithification.

Compaction reduces the porosity and packs the grains; cementation glues them. Fine muds lithify largely by compaction, while sands rely on cementation. Because cementation fills pores, diagenesis degrades a reservoir over time.

Examples in context

Example 1. A turbidite sequence. Repeated graded beds, each coarse at the base and fine at the top with sharp bases, record successive turbidity currents pouring sediment into deep water: the graded bedding gives both the deep-marine environment and the way up.

Example 2. Evaporites as petroleum seals. Where a restricted sea evaporated, gypsum then rock salt precipitated. Because rock salt is impermeable and flows to seal fractures, evaporites make excellent cap rocks above hydrocarbon reservoirs, linking this statement to petroleum systems.

Try this

Q1. Name the three textural varieties of sandstone defined by composition, stating which is the most mature. [3 marks]

• Cue. Arkose (feldspar-rich, immature), greywacke (dirty, matrix-rich, immature) and orthoquartzite (clean quartz, the most mature).

Q2. Explain how graded bedding can be used both to identify the environment and to determine the way up. [3 marks]

• Cue. Graded bedding forms as a waning current (a turbidity current) drops coarse grains first, then finer ones, indicating a deep-marine setting; the coarse base and fine top give the original way up.

Q3. Name the two processes of lithification and state what each does to a loose sand. [2 marks]

• Cue. Compaction (the overburden squeezes grains together and expels pore water, reducing porosity) and cementation (precipitated minerals such as silica bind the grains into rock).