How do clastic, biogenic and chemical sediments form and turn into rock?
The formation of clastic, biogenic and chemical sedimentary rocks, the processes of diagenesis (compaction, cementation, recrystallisation), and the use of sedimentary structures, including those formed by infrequent processes such as turbidity currents, as evidence of conditions.
A focused WJEC and Eduqas A-Level Geology G1 answer on how clastic, biogenic and chemical sedimentary rocks form, the diagenetic processes that lithify and alter them, and how sedimentary structures, including graded bedding from turbidity currents, are read as scientific models of conditions that are hard to observe directly.
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What this dot point is asking
G1 asks you to explain how sediments become rock and how the rock records the conditions of its formation. WJEC stresses that some processes (such as turbidity currents) are rarely seen directly but can be understood through scientific models, and that sedimentary structures are read as evidence. This underpins stratigraphy, environments and the way-up problems in map work.
The answer
The three classes of sedimentary rock
Sedimentary rocks form in three ways, by origin of the particles:
- Clastic: from fragments of pre-existing rock (gravel, sand, mud), classified by grain size into conglomerate, sandstone and mudrock.
- Biogenic (organic): from the remains of organisms, such as shelly or reef limestone (calcite from shells and corals) and coal (compressed plant matter).
- Chemical: from minerals precipitated from solution, such as rock salt and gypsum from evaporating seawater (evaporites) and some ironstones.
Diagenesis: turning sediment into rock
Diagenesis is the set of low-temperature changes that act on sediment after deposition and during burial, including lithification.
Common cements are calcite, silica and iron oxide; the cement and the porosity it leaves control whether a rock can later hold oil, gas or water, which matters in G4.
Sedimentary structures as evidence
Structures preserve the conditions at deposition and are the geologist's main tool for reading environments and way-up:
Turbidity currents are the headline example of an infrequent, hard-to-observe process understood by modelling: a dense, sediment-laden flow rushes down the continental slope (often earthquake-triggered) and deposits a graded turbidite bed as it slows. The model is supported by laboratory flows and by the regularity of the deposits.
Examples in context
The Aberystwyth Grits in west Wales are a thick pile of stacked turbidites, each a graded bed, classic evidence of repeated turbidity-current deposition on an ancient slope. The Zechstein evaporites beneath the North Sea formed by seawater evaporation and now seal hydrocarbon reservoirs, linking chemical sedimentation to resources. Carboniferous limestones of the Mendips and Pennines are biogenic, built from crinoid and coral debris, and show cross-bedding and bioturbation recording shallow tropical seas.
Try this
Q1. Classify rock salt, shelly limestone and sandstone as chemical, biogenic or clastic. [3 marks]
- Cue. Rock salt chemical (evaporite); shelly limestone biogenic; sandstone clastic.
Q2. Explain how cross-bedding records the direction of an ancient current. [2 marks]
- Cue. Ripples or dunes migrate down-current, building foresets that dip in the direction the current flowed.
Q3. State how a graded bed shows that strata are the right way up. [2 marks]
- Cue. A turbidite fines upward, so coarse at the base and fine at the top is the original (right-way-up) orientation.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20206 marksExplain how a turbidity current forms a graded bed, and how a graded bed can be used to deduce the way up of strata.Show worked answer →
Describe the current first, because the bed is its product.
A turbidity current is a dense, sediment-laden flow that moves downslope under gravity, for example triggered by an earthquake on a continental slope. It carries a mixture of grain sizes in turbulent suspension.
As the current slows and finally stops, the largest, densest grains settle first, then progressively finer grains, building a single bed that grades from coarse at the base to fine at the top (a Bouma sequence).
This is a way-up indicator because the grading only runs one way: coarse grains at the bottom, fine at the top, in the original deposit. If a graded bed is found coarse-up, the strata have been overturned by folding.
Markers reward the dense gravity-driven flow, settling by decreasing grain size to give a fining-up bed, and the use of coarse-base to fine-top as the original way up.
WJEC Eduqas 20184 marksDescribe two diagenetic processes that convert loose sediment into a sedimentary rock.Show worked answer →
Name and explain two processes, because two are asked for.
Compaction is the squeezing together of grains under the weight of overlying sediment. It expels pore water and reduces porosity, packing the grains more tightly.
Cementation is the precipitation of minerals (such as calcite, silica or iron oxide) from pore fluids into the spaces between grains, binding them into a solid rock.
A third process, recrystallisation, changes the crystal sizes or mineralogy after burial, for example altering aragonite shells to calcite, but compaction and cementation are the core lithifying pair.
Markers reward compaction as packing under load expelling water, and cementation as mineral precipitation binding grains, with correct named cements.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)