How do I tell a river deposit from a shallow-marine deposit?

Use the sedimentary structures and fossils together. Asymmetrical ripples and cross-bedding from a one-way current, imbricated pebbles and plant debris point to a fluvial (river) environment. Symmetrical ripples from oscillating waves, well-sorted sand or carbonate, marine fossils and bioturbation point to shallow marine. Ripple symmetry is the single most diagnostic structure: symmetrical means waves, asymmetrical means a one-way current.

What does graded bedding tell you?

Graded bedding is a single bed that is coarse at the base and fines upward. It forms from a waning current, specifically a turbidity current that loses energy and drops its coarsest grains first, then finer grains as it slows. So graded bedding indicates a deep-marine environment (turbidites interbedded with background mud), and because the fine end is the original top, it also gives the way up in folded rocks.

What is accommodation space and why does it matter?

Accommodation space is the available vertical space for sediment to accumulate in a basin, between the basin floor and the base level (usually sea level). It is created by subsidence (which lowers the floor) and by a rise in relative sea level, and reduced by a fall. Whether a basin deepens, shallows or fills is the balance between the rate at which accommodation space is created and the rate of sediment supply, so accommodation space controls the whole architecture of a basin fill.

What is the single most common mistake in this topic?

Saying both P and S waves are blocked by the outer core. Only S waves are blocked, because they cannot travel through liquid; P waves pass through but are refracted, creating a separate P wave shadow zone. Also common is confusing the basin types' subsidence mechanisms: rift basins start with fault-controlled stretching then thermal subsidence, passive margins are mainly thermal, and foreland basins are flexural (loaded by mountains).

EnglandGeology

OCR A-Level Geology: Earth structure, sedimentary environments and basin analysis overview

Q: How do seismic waves reveal the Earth's internal structure?

Seismic waves change velocity abruptly at boundaries between layers of different density, and these velocity changes (and the refraction they cause) define the layers, for example the velocity jump at the Moho (the crust-mantle boundary). The state of the core comes from shadow zones: S waves cannot travel through liquid, so the S wave shadow zone on the far side of the Earth proves the outer core is liquid, while the refracted P waves form a P wave shadow zone that defines the core's size.

A deep-dive OCR A-Level Geology guide to Earth structure, sedimentary environments and basin analysis. Covers the layered Earth and the seismic evidence (Moho and shadow zones), facies and sedimentary structures, the main depositional environments and logs, and basin types, subsidence and accommodation space, with the exam patterns OCR repeats.

Generated by Claude Opus 4.818 min readOCR-H414-Module-3-4-7Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this topic actually demands
Earth internal structure and seismic evidence
Sedimentary environments and facies
Basin analysis and subsidence
How this topic is examined
Check your knowledge

What this topic actually demands

Earth structure and geophysics groups three OCR strands that all rely on reading evidence: the deep structure of the Earth from seismic waves, the reconstruction of ancient environments from sedimentary facies, and the analysis of the sedimentary basins those environments fill. The examiners test interpretation above recall: deducing the core's state from shadow zones, an environment from structures, and a basin's evolution from its fill.

This guide walks through the three clusters in a sensible order, then sets out the exam patterns OCR repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Earth internal structure and seismic evidence

The Earth is layered: a thin crust (oceanic thin, dense, basaltic; continental thick, less dense, granitic), a solid but slowly convecting mantle, a liquid iron-nickel outer core and a solid inner core. Because we cannot drill to the core, the structure is read from seismic waves, which change velocity at boundaries; the velocity jump at the Moho marks the crust-mantle boundary.

The state of the core comes from shadow zones. S waves cannot travel through liquid, so the S wave shadow zone proves the outer core is liquid; P waves pass through but are refracted, forming a P wave shadow zone that defines the core's size. The liquid outer core generates the Earth's magnetic field, linking this strand to palaeomagnetism.

Sedimentary environments and facies

A facies is a rock body whose lithology, structures and fossils reflect a depositional environment. The key skill is reading sedimentary structures: cross-bedding (a one-way current, gives flow direction), graded bedding (a waning turbidity current, deep marine, gives way up), symmetrical ripples (waves, shallow marine) versus asymmetrical (rivers), and desiccation cracks (subaerial drying). Each environment (fluvial, deltaic, shallow marine, deep marine, desert) leaves a characteristic facies.

A sedimentary log records a succession vertically, and reading it bottom to top reveals environmental change: fining- and coarsening-upward patterns, and shifts between continental and marine facies that record transgression (sea advancing) and regression (sea retreating). The exam often gives a log and asks you to narrate the environmental history.

Basin analysis and subsidence

A sedimentary basin is a region of long-term subsidence where thick sediment accumulates. The crust subsides by thermal subsidence (cooling and contraction after stretching), flexural loading (bent down by a mountain load) and sediment loading. Accommodation space (the room for sediment, set by subsidence and relative sea level) and the rate of sediment supply together control whether a basin deepens, shallows or fills.

The main basin types are rift (extension, then thermal subsidence, with a fill evolving from coarse continental to finer or marine), passive-margin (mainly thermal, thick marine sequences) and foreland (flexural, beside a mountain belt). Facies successions and burial-history curves reconstruct a basin's evolution and, crucially, whether its source rocks reached the temperatures to generate hydrocarbons.

How this topic is examined

A typical OCR profile for Earth structure and geophysics:

Seismic-evidence questions (Paper 1). Explaining the shadow zones and the Moho, and contrasting oceanic and continental crust.
Facies and log interpretation (Paper 3). Identifying an environment from structures and fossils, and reconstructing environmental change from a sedimentary log.
Basin questions (Papers 1 and 2). Explaining a basin type's formation and subsidence, accommodation space, and reading a burial-history curve.
Level-of-response extended answers (Paper 2). How a rift basin forms and fills, and interpreting a multi-unit log as a transgression, are predictable extended questions.

Check your knowledge

A mix of recall and application questions covering the whole topic. Attempt them under timed conditions, then check against the solutions.

Explain how the S wave shadow zone shows the outer core is liquid. (3 marks)
State two differences between oceanic and continental crust. (2 marks)
Define a facies. (2 marks)
Explain how symmetrical and asymmetrical ripples indicate different environments. (2 marks)
State what graded bedding indicates and why. (2 marks)
Define accommodation space and name two things that control it. (3 marks)
Explain how a rift basin forms and subsides. (4 marks)
A basin subsides at 0.3 mm per year (steady sea level) and sediment is supplied at 0.1 mm per year. Predict whether it deepens or fills, and explain. (3 marks)

Solutions

Q1: S waves are shear waves that cannot travel through a liquid. They are absent over a wide region on the far side of the Earth from an earthquake (the S wave shadow zone), which shows they have been blocked by a liquid layer at depth: the outer core. So the outer core is liquid.
Q2: Any two of: oceanic crust is thinner ( $5$ to $10\ \mathrm{km}$ ), denser, basaltic and younger; continental crust is thicker ( $30$ to $70\ \mathrm{km}$ ), less dense, granitic and older. Oceanic crust subducts; continental crust does not.
Q3: A facies is a body of rock with a distinctive set of characteristics (lithology, sedimentary structures and fossils) that reflects a particular depositional environment.
Q4: Symmetrical ripples form from oscillating wave action, indicating a shallow-marine environment. Asymmetrical ripples form from a one-way current, indicating a river (fluvial) environment.
Q5: Graded bedding (coarse at the base fining upward) indicates a waning current, specifically a turbidity current, so it points to a deep-marine environment; it also gives the way up because the fine end is the original top.
Q6: Accommodation space is the available vertical space for sediment to accumulate in a basin, between the basin floor and base level. It is controlled by subsidence (which lowers the floor) and relative sea-level change (which raises or lowers base level).
Q7: A rift basin forms by extensional stretching and thinning of the lithosphere, producing normal faults that drop down a fault-bounded basin. It subsides first mechanically (by faulting), then by slower thermal subsidence as the stretched lithosphere cools and contracts. The fill often evolves from coarse alluvial and fluvial deposits to finer lacustrine or marine sediments.
Q8: With steady sea level, accommodation space is created at the subsidence rate, $0.3\ \mathrm{mm\ yr^{-1}}$ , which exceeds the sediment supply of $0.1\ \mathrm{mm\ yr^{-1}}$ . Because space is created faster than sediment fills it, the basin deepens.