Relative Dating
Correlation
Geological Time
Outline
Relative dating methods use geological principles to place events in
chronological order.
- The principle of superposition states that older beds are covered
by younger beds so in a sedimentary sequence the youngest unit is at the top.
- The principle of fossil succession states that organisms evolve
through time so that particular forms can be used as age markers wherever they
are found.
- Cross-cutting relationships Cross-cutting rocks such as igneous
intrusion are younger than the rocks they cut.
- Inclusions Any included pebbles and fragments
must be older than the host rock containing them.
- Deformation Any rocks effected by a deformation event
(folding or tilting) must pre-date the deformation episode.
Events can be ordered using the various methods discussed even when no absolute
ages can be measured. Where there is a break in sedimentation, a period of
erosion or an episode of deformation, the rock layers record the break as a
surface called an unconformity. Unconformities range from minor erosional
breaks to strong angular discordances in bedding. An unconformity indicates a
period where no rock record is accumulated. They are time-breaks of
indeterminate length. The last two diagrams illustrate various kinds of
unconformities.
Correlation
Geological Time
Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan,
57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9