Geologic history – relative age dating:

Usually this exercise is associated with a cross section or profile in which there are a series of stacked rock units composed of sedimentary, igneous, and/or metamorphic rocks as well as structures such as faults and folds (anticline, syncline) and unconformities (nonconformity, angular unconformity, disconformity). This type of exercise applies the principals of relative age dating you studied in the text to deduce the order of geologic events. Please review the 5 principles before you start the exercises. Normally, one would list the oldest event at the bottom of the page and progresses upward to the youngest event. In this case you will list the oldest event at the top and work toward the youngest.

The question that many people have is where do I start?

It is useful to identify the unconformities which represent breaks in the geologic time record. Usually in this type of exercises an undulating wave line indicates an erosional surface. Remember, in order to form an erosional surface like an angular unconformity, disconformity or nonconformity, the rock units have to be uplifted or the oceans levels fall so the rock is exposed at the surface of the earth before they can weather and erode. If you have subsequent deposition of marine sediments (limestone, shale and sandstone) then the land mass must subside or the oceans rise to cover the surface by water before you can have deposition of sediments. Remember that the rocks were deposited originally on a horizontal surface (or nearly so) so if they are no longer horizontal then something had to happen to them. If they are not horizontal but tilted then there was a period of uplift or deformation causing tilting; or if they are bent/folded then there was a period of deformation causing folding.

Example: cross-section view

unconformity created Youngest

Period of uplift to the surface and erosion forming modern day

surface

Deposition of sandstone X

Deposition of limestone

angular unconformity

Oceans rise followed by deposition of sandstone Z

Period of uplift to the surface and erosion

Period of deformation causing tilting of rock units

A fracture or fault forms and fluids carrying silica precipitate

quartz vein

Deposition of shale A

disconformity

Oceans rise followed by deposition of sandstone B

Period of uplift to the surface and erosion

Deposition of shale E

Deposition of sandstone F

Deposition of conglomerate

nonconformity

Oceans rise followed by deposition of conglomerate

Period of uplift to the surface and erosion

Gneiss forms

Oldest event

Suggestions:

One can use unconformities and structures to subdivide the cross section. If there is an unconformity then work on the oldest event up to that unconformity because this represents a break or a gap in the rock record. In this way you may compartmentalize what appears to be a very complex problem into smaller and more manageable parts. Work on only a small part of the cross-section’s geologic history then find the next higher unconformity and work on the events between the two unconformities. You will join the unconformities by linking them with periods of uplift and erosion followed by ocean rising and deposition of sedimentary rocks (if the rocks are marine in origin.