How are unconformities and combined structures used to reconstruct a tectonic history?
The types of unconformity (angular, disconformity, nonconformity) and their significance, the structures of mountain belts (nappes, thrust stacks), and the use of cross-cutting relationships and superposition to reconstruct the sequence of tectonic events.
A focused WJEC and Eduqas A-Level Geology G2 answer on the three types of unconformity and what each records, the structures of orogenic belts such as nappes and thrust stacks, and how cross-cutting relationships, superposition and unconformities are combined to reconstruct the sequence of folding, faulting, intrusion, uplift and erosion in a region.
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What this dot point is asking
This dot point pulls G2 together by asking you to reconstruct history from structure. WJEC wants the three types of unconformity and what each records, the large-scale structures of mountain belts, and the use of the dating principles (superposition, cross-cutting) to put folding, faulting, intrusion, uplift and erosion in order. This is exactly the skill tested by cross-section questions in T2.
The answer
Unconformities: gaps in the record
There are three types, each recording a different earlier history:
Structures of mountain belts
Mountain building (orogeny) at convergent margins produces the most intense structures: tight, overturned and recumbent folds, stacked thrust faults, and nappes (large recumbent fold-and-thrust sheets transported many kilometres). These stack and thicken the crust, and later uplift and erosion expose the deep structure.
Reconstructing the sequence of events
The deformation dating principles let you order events:
- Superposition: in an undisturbed pile, lower beds are older.
- Cross-cutting relationships: any feature that cuts another (a fault, a dyke, an intrusion) is younger than what it cuts.
- Inclusions: fragments are older than the rock that contains them.
- Unconformities: the surface and everything below it are older than the beds above.
Examples in context
Hutton's unconformity at Siccar Point in Scotland is the classic angular unconformity, where gently dipping Old Red Sandstone rests on steeply tilted Silurian greywackes, the exposure that helped establish deep time. The Moine and Glencoul thrusts of north-west Scotland stack older rocks over younger in a classic thrust belt. Alpine nappes record recumbent fold-and-thrust sheets carried far north during the collision of Africa and Europe.
Try this
Q1. Name the three types of unconformity and state what lies below the surface in each. [3 marks]
- Cue. Angular (tilted or folded beds below), disconformity (parallel beds below), nonconformity (igneous or metamorphic rock below).
Q2. State the sequence of events recorded by an angular unconformity. [2 marks]
- Cue. Deposition, folding or tilting, uplift and erosion, then renewed deposition.
Q3. A dyke cuts a folded sequence. State which is younger and the principle used. [2 marks]
- Cue. The dyke is younger, by the principle of cross-cutting relationships.
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 what an angular unconformity is and describe the sequence of events it records.Show worked answer →
Define the structure, then unpack the history it implies, because the marks track the sequence.
An angular unconformity is a surface where younger, more or less horizontal strata rest on older, tilted or folded strata, with the bedding planes meeting at an angle across the surface.
The sequence it records is: first the lower beds were deposited, then they were folded or tilted by earth movements, then they were uplifted and eroded to produce the unconformity surface, and finally the area subsided or sea level rose so that the upper beds were deposited on top of the eroded surface.
So an angular unconformity records a complete cycle of deposition, deformation, uplift, erosion and renewed deposition, and is strong evidence of an episode of mountain-building between the two sets of beds.
Markers reward younger horizontal beds on older tilted beds, and the ordered history deposit-deform-uplift-erode-deposit.
WJEC Eduqas 20185 marksA granite cuts across folded sediments and is itself overlain by an unconformity. Reconstruct the sequence of geological events.Show worked answer →
Apply cross-cutting relationships and superposition in turn, because the order is the answer.
The folded sediments must be the oldest features: they were deposited first and then folded by compression.
The granite cuts across the folds, so by cross-cutting relationships the intrusion is younger than the folding; the sediments were already folded when the magma intruded.
The unconformity overlies the granite, so the region was then uplifted and eroded (exposing the granite at the surface) before the younger beds above the unconformity were deposited.
So the sequence is: deposition of the sediments, folding, intrusion of the granite, uplift and erosion forming the unconformity, then deposition of the younger beds.
Markers reward deposition then folding (oldest), intrusion after folding by cross-cutting, then uplift, erosion and renewed deposition above the unconformity.
Related dot points
- The concepts of stress and strain, the difference between compressional, tensional and shear stress, elastic, brittle and ductile behaviour, and the factors (temperature, confining pressure, strain rate, rock type and fluids) that control how a rock deforms.
A focused WJEC and Eduqas A-Level Geology G2 answer on stress and strain, the three stress regimes, elastic, brittle and ductile behaviour, and the controls (temperature, confining pressure, strain rate, rock competence and fluid pressure) that decide whether a rock fractures or flows when deformed.
- The geometry of folds (limbs, axial plane, hinge, fold axis, interlimb angle), the classification of folds (anticline, syncline, symmetrical, asymmetrical, overturned, recumbent, isoclinal) and the use of fold style to interpret the direction and intensity of compression.
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- The classification of faults (normal, reverse, thrust, strike-slip) by the relative movement of the hanging wall and footwall and by the stress regime, the terminology of fault planes (dip, throw, heave, slickensides), and the recognition of faults in the field and on maps.
A focused WJEC and Eduqas A-Level Geology G2 answer on the classification of normal, reverse, thrust and strike-slip faults by hanging-wall and footwall movement and stress regime, the terms used to describe fault planes (dip, throw, heave, slickensides, fault breccia), and how faults are recognised in the field and interpreted on geological maps.
- The principles of relative dating (superposition, original horizontality, cross-cutting relationships, included fragments and unconformities) and how they are combined to establish the sequence of geological events.
A focused answer to WJEC and Eduqas A-Level Geology F3 on relative dating, covering the principles of superposition, original horizontality, cross-cutting relationships, included fragments and unconformities, and how they are combined to reconstruct the order of geological events in a sequence.
- The construction of a geological cross-section from a map, the projection of dipping beds, folds, faults and unconformities into the section, and the reconstruction of the full sequence of geological events of an area from the map and section.
A focused WJEC and Eduqas A-Level Geology T2 answer on constructing a geological cross-section from a map, projecting dipping beds, folds, faults and unconformities into the section, and reconstructing the full sequence of geological events of an area using superposition, cross-cutting relationships and unconformities.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)