How do sedimentary basins form and fill, and how do we analyse them?
Basin analysis: the definition of a sedimentary basin and the mechanisms of subsidence (thermal subsidence after lithospheric stretching, flexural loading and sediment loading); the concept of accommodation space and its control by subsidence and sea-level change; the main basin types (rift, passive-margin and foreland); the use of vertical facies successions and burial-history curves to reconstruct basin evolution.
A focused answer to the OCR H414 dot point on basin analysis. Covers the definition of a sedimentary basin, mechanisms of subsidence (thermal, flexural and sediment loading), accommodation space and its control by subsidence and sea level, the rift, passive-margin and foreland basin types, and the use of facies successions and burial-history curves to reconstruct basin evolution.
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What this dot point is asking
OCR wants you to define a sedimentary basin, to explain the mechanisms of subsidence (thermal, flexural and sediment loading), to explain accommodation space and how subsidence and sea level control it, to describe the rift, passive-margin and foreland basin types, and to reconstruct basin evolution from facies successions and burial-history curves.
The answer
Sedimentary basins and subsidence
The crust subsides by three main mechanisms:
- Thermal subsidence. After the lithosphere is stretched and thinned (and heated), it slowly cools, becomes denser and contracts, sinking over tens of millions of years.
- Flexural loading. A heavy load (for example a growing mountain belt) bends the lithosphere down beside it, creating a basin (a foreland basin).
- Sediment loading. The weight of accumulating sediment itself pushes the crust down, adding to the subsidence.
Accommodation space
Whether a basin deepens, shallows or fills depends on the balance between the rate at which accommodation space is created (subsidence plus sea-level rise) and the rate of sediment supply. If supply outpaces space, the basin shallows and fills; if space outpaces supply, it deepens.
The main basin types
- Rift basin. Forms by extension (stretching) of the lithosphere, bounded by normal faults; subsides first by faulting, then by thermal subsidence. Fill often evolves from coarse alluvial and fluvial deposits to lacustrine or marine. (For example the East African Rift.)
- Passive-margin basin. Forms on a trailing continental margin after rifting has opened an ocean; subsides mainly by long-term thermal subsidence and sediment loading, accumulating thick marine sequences. (For example the Atlantic margins.)
- Foreland basin. Forms by flexural loading in front of a growing mountain belt, as the lithosphere bends down under the weight; fills with sediment shed from the mountains. (For example the basin in front of the Alps.)
Reconstructing basin evolution
Geologists reconstruct a basin's history from:
- Vertical facies successions. The upward change in facies (for example coarse to fine, continental to marine) records the changing environment as the basin evolved (deepening, shallowing, transgression or regression).
- Burial-history curves. Plots of depth (or sediment thickness) against time show how fast the basin subsided and was buried, which is critical for predicting whether buried source rocks reached the temperatures needed to generate hydrocarbons.
Examples in context
Example 1. The North Sea as a rift basin. The North Sea formed by lithospheric stretching, subsiding first by faulting and then thermally; its burial history matured organic-rich source rocks, making it a major hydrocarbon province, which ties basin analysis directly to petroleum systems.
Example 2. A foreland basin beside a mountain belt. As a mountain belt grows, its weight flexes the adjacent lithosphere down into a foreland basin that fills with sediment eroded from the mountains, recording the rise of the orogen.
Try this
Q1. Define accommodation space. [2 marks]
- Cue. The available vertical space for sediment to accumulate in a basin, between the basin floor and the base level (often sea level).
Q2. Name the subsidence mechanism that forms a foreland basin. [1 mark]
- Cue. Flexural loading (the lithosphere bends down under the weight of a growing mountain belt).
Q3. Explain what controls whether a basin deepens or fills. [2 marks]
- Cue. The balance between the rate at which accommodation space is created (subsidence plus sea-level rise) and the rate of sediment supply; if supply exceeds space, the basin fills and shallows.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/02 20206 marksExplain how a rift basin forms and subsides, and describe the type of sedimentary fill you would expect as the basin evolves.Show worked answer →
A level-of-response answer; cover the rifting, the subsidence mechanism and the changing fill.
- Formation by rifting
- A rift basin forms where the lithosphere is stretched and thinned by extensional (tensional) stress, producing normal faults that drop blocks of crust down to create a fault-bounded basin.
- Subsidence
- There are two phases. Initial subsidence is by mechanical stretching and faulting. Then, as the stretched lithosphere cools over time, it becomes denser and contracts, causing slower, longer-term thermal subsidence.
- The sedimentary fill
- Early in rifting, steep fault-bounded margins shed coarse, immature sediment (alluvial fans and fluvial conglomerates and sandstones) into the basin. As the basin deepens and widens, finer sediments accumulate: lacustrine (lake) muds, and, if the sea enters, shallow-marine and then deeper-marine deposits. So the fill often coarsens at the base then fines and deepens upward.
Top-band answers link extensional stretching and normal faulting to the basin, the two-stage (mechanical then thermal) subsidence, and a fill that evolves from coarse continental to finer or marine deposits.
OCR H414/01 20194 marksExplain what is meant by accommodation space and how it is controlled by subsidence and sea-level change.Show worked answer →
Define the term, then give the two controls.
- Definition
- Accommodation space is the available space for sediment to accumulate in a basin, that is, the vertical space between the basin floor and the base level (often sea level) up to which sediment can be deposited.
- Control by subsidence
- Subsidence lowers the basin floor, creating new accommodation space, so more sediment can accumulate. Faster subsidence creates space faster.
- Control by sea-level change
- A rise in (relative) sea level raises the base level, increasing accommodation space; a fall lowers the base level, reducing it (and may cause erosion). The balance between the rate at which accommodation space is created and the rate of sediment supply determines whether the basin deepens, shallows, or fills.
Markers reward accommodation space as the space available for sediment, and both controls: subsidence lowering the floor and sea-level change raising or lowering base level.
Related dot points
- Sedimentary environments: the concept of facies as a body of rock reflecting a particular depositional environment; sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks and desiccation cracks) and their interpretation; the characteristics of the main environments (fluvial, deltaic, shallow marine, deep marine and desert); the construction and interpretation of sedimentary logs to reconstruct environmental change.
A focused answer to the OCR H414 dot point on sedimentary environments. Covers facies, sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks, desiccation cracks) and their interpretation, the fluvial, deltaic, shallow-marine, deep-marine and desert environments, and how sedimentary logs reconstruct environmental change.
- Hydrocarbons: the petroleum system (source rock, maturation, migration, reservoir rock, trap and seal); the formation of oil and gas from organic-rich source rocks by burial and heating; the properties needed in a reservoir (porosity and permeability) and a seal (low permeability); the types of trap (structural and stratigraphic); the formation of coal from plant material with increasing rank.
A focused answer to the OCR H414 dot point on hydrocarbons. Covers the petroleum system (source rock, maturation, migration, reservoir, trap and seal), the formation of oil and gas by burial and heating, the porosity and permeability needed in a reservoir, low-permeability seals, structural and stratigraphic traps, and the formation of coal with increasing rank.
- The geological record: the hierarchy of the geological time scale (eon, era, period, epoch) and the major divisions (Precambrian and the Phanerozoic eras); correlation of strata by lithostratigraphy (matching rock units) and biostratigraphy (matching fossils and biozones); the use of marker horizons such as volcanic ash bands; the distinction between rock units (systems) and time units (periods).
A focused answer to the OCR H414 dot point on the geological time scale and correlation. Covers the eon, era, period and epoch hierarchy and the major divisions, correlation by lithostratigraphy and biostratigraphy, the use of marker horizons such as ash bands, and the distinction between rock units and time units.
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
- 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.
A focused answer to the OCR H414 dot point on sedimentary rocks. Covers lithification (deposition, compaction and cementation), the clastic, chemical and biogenic or biochemical classes, the grain-size scale from conglomerate to mudstone, and how clastic texture is described using grain size, sorting and roundness.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)