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. In this
type of exercise list the oldest event at the bottom of the page and progresses
upward to the youngest event.
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 in some cases a 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. Nonconfomities can also be formed as a
result of an igneous intrusion into sedimentary or metamorphic rocks.
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.