Meaning and mechanism
When dealing with non-inverted sedimentary layers, the “superposition principle” can be applied, stating that the lower layer was deposited before the upper layer. It is essential to first verify the original depositional state.
The fault cutting into an intruded rock or layer was formed after the intrusion. By connecting multiple relationships, it is possible to establish the sequence of deposition, intrusion, and fault events.
Relationships and conditions
Discordance refers to the gap between observed data and expected values, which arises when erosion ceases or when an existing stratum has been eroded over a period of time. The fact that there is a gap, and the length of that period, are separate pieces of information.
When dealing with layers overturned by drainage or with deposited materials, a simple “up and down” assessment cannot be applied directly. In absolute dating, separate dating measurement data is required.
What to observe in the simulation
The order is determined by the law of superposition, law of penetration, and law of cutting. The unconformity represents time of loss of deposition and erosion.
It covers only the two sedimentary layers that have not been overturned and designated faulting, intrusion, and erosion events. Absolute age and duration of unconformity are not calculated. Tilt and displacement are for explanatory purposes only and do not restore the actual geological structure.
Key relationships and symbols
The cutting incident is younger than the cutting subject.
| Symbol | Meaning | Unit |
|---|---|---|
| Superposition | Lower layers form first in an undisturbed sedimentary sequence | Ordering relationship |
| Cross-cutting | A cross-cutting event is younger than what it cuts | Ordering relationship |
Worked example
What if a fault cuts the intrusive rock that cuts both A below and B above?
According to the law of superposition, B is younger than A, and according to the law of penetration and cutting, the cutting event is younger than the cut object.
Common misconceptions
- It is not possible to directly calculate the age of each layer simply by knowing the relative order.