Types of Unconformities
What is a disconformity?
A disconformity is a minor irregular surface separating parallel strata on opposite sides of the surface. Sedimentary layers are deposited, then the deposition stops and the layers are eroded. After a period of erosion, new sedimentary layers are deposited again.
What is a fault?
A fault is a fracture in the rock. The rocks on both sides of the fault have moved.
What is a joint?
A joint is a fracture/crack in the rock. The rocks on both sides of the joint do not move
What causes a joint?
A joint may be due to compression, tension or shear stress. A joint is a form of brittle strain
What is a nonconformity?
A nonconformity is a break surface that developed when igneous or metamorphic (not sedimentary layers) rocks were exposed to erosion, and younger sedimentary rocks were subsequently deposited above the erosion surface.
What is an angular unconformity?
An angular unconformity is a tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata. Layers were deposited, then deformed or tilted and then eroded. New layers were deposited on top of the eroded surface.
What is an unconformity?
An unconformity is a surface between strata layers that represents a break in the time record. It results from an interval when deposition was interrupted or stopped for a while. Then, the top of the layer was eroded and then deposition began again, forming more new layers.
What causes a fault?
Caused by tension or compression
How can a unconformity be identified in cross section?
In a cross section, an unconformity can be identified by a wavy, eroded layer.
How do joints occur?
Joints occur in sets, not alone as a single joint -- usually parallel joints.
Unconformity
Surface that represents a break in the rock record
What are the three main types of unconformities?
The three main types of unconformities are: Disconformity Angular unconformity Nonconformity
What evidence must there be for a fault?
To be a fault, there must be evidence that the rocks have moved on both sides of the fault Other evidence is: Crushed rock found in the fracture The sides of the fault have been polished smooth and may have striations (grooves) on them due to the rocks rubbing past each other The features on the ground or man-made features (like a fence) have moved from their original positions
Disconformity
When an underlying and overlying sedimentary rock strata are parallel and the contact plane is an erosional surface
Angular Unconformity
When an underlying rock and an overlying rock strata show some angle
Nonconformity
When underlying rocks are igneous or metamorphic and the overlying rocks are sedimentary
Unconformity
gap in the rock layer that is due to erosion or periods without any deposition.
Disconformity
layer of horizontal rock once exposed and eroded before younger rocks formed over it
Angular Unconformity
rock layers are tilted, and younger sediment layers are deposited horizontally on top of the eroded and tilted layers.
Nonconformity
sedimentary rock forms over eroded metamorphic or igneous rock
