Download Geologic Time Part I: Relative Dating

Geologic Time Part I:
Relative Dating
Unit B
Unit A
James Hutton conceived of the Principle of Uniformitarianism at Sicar Point, Scotland in
1785. Think about how much time would pass to create the rock record shown above.
What sequence of events would occur to produce the above record?
Unit B
Unit A
1. Deposition of sediment comprising Unit A (Sedimentation rates in the deep ocean
range from .0005 - .001 mm per year). How long would it take to accumulate 200
meters of sediment?) 2. burial compaction & lithification of Unit A, 3. deformation
(folding) of Unit A, 4. uplift and erosion of Unit A, 5. Deposition of sediment (Unit B), 6.
burial compaction & lithification of Unit B, 7. deformation (folding) of Unit B, 8. uplift and
erosion of Unit B. These geologic events require millions of years of time!
Principle of Original Horizontality: Layered sedimentary rock is typically deposited
horizontally, but can be deformed by tectonic processes. Can you think of a depositional
environment where sedimentary layers are not deposited horizontally?
Geologists use sedimentary structures to determine whether sedimentary layers or
beds are right-side up, vertical or overturned.
The Principle of Uniformitarianism states that the “present is the key to the past.” Those
processes operating at the earth’s surface today are inferred to have operated in the
past, such as the mudcracks forming in a playa lake today. Note the blowing sand in the
background will fill in the cracks.
The mudcracks shown above formed over 1.1 billion years ago in a dry lake basin
are now found in Glacier National Park, Montana.
Raindrop impressions are preserved in 1.1 billion year old sedimentary rocks in
the Belt Group rocks of Montana. Why do you think raindrop impressions were
preserved in this sediment? Think about the life record on Earth at this time.
The Principle of Superposition states that younger sedimentary rock layers will
overlie older sedimentary rock layers, providing the rock unit has not been
overturned.
The principle of Cross-Cutting Relations states that any structure or rock unit crosscutting another structure or rock unit will be younger. The fault line shown above
cross-cuts a glacial moraine. The seismic event that cause uplift along the fault postdates the formation of the moraine? Based on this same logic which event is older,
the offset along the fault or the stream deposition that covers the fault.
C
B
A
Which formed first and last? A. The micaceous schist, B. the plagioclase dike or C.
the fault that offsets the dike?
Use the principle of cross-cutting relations to figure out this mess! If you can numerically
date the dikes, you can reconstruct the deformation history of this region of Washington.
Which is older? The roof pendant comprised of contact meta-sedimentary rock or the
plutonic rock?
A
B
C
The Principle of Inclusion states that the included body will be older than the rock
containing the inclusion (in this case the clasts within the conglomerate).
Which clast age will be the closest constraining age for the conglomerate? A (55 Ma), B
(75 Ma), or C (100 Ma). Ma (million years).
The Principle of Faunal Succession utilizes evolutionary changes in the preserved life
record to assign relative ages to rock units. Trilobites were dominant during the
Cambrian Period (543-490 Ma) and went extinct by 245 Ma. Radiometric dating
techniques have been used to assign numerical ages to evolutionary time periods.
Ferns first evolved during the Carboniferous Period (363-290 Ma) and were dominant
terrestrial plants. Conifers, flowering plants and grasses evolved later.
During the Mesozoic Era dinosaurs flourished on the continental land masses. This
Allosaurus footprint impression is found in Jurassic aged rock.
Diplodocus was the largest land animal to walk the surface of the earth. It
measured 90 feet long from head to tail. It lived during the Jurassic Period.
Horses evolved 55 million years ago during the Eocene Epoch of the Tertiary Period.
Warm-blooded mammals dominate the Cenozoic Era.
Geologic Time Scale
Do not memorize the geologic time
scale. Note that the major
subdivisions such as eons represent
major developments in the Earth’s
history. The three eras (Paleozoic,
Mesozoic and Cenozoic) are all
contained within Phanerozoic eon
(largely defined by fossil preservation
in the life record). Periods are
defined by major evolutionary
changes, such as the evolution of
early fish in the Ordovician period
(505-435 Ma ago) of the Paleozoic
era (545-245 Ma).
Why do you think that the more
recent portion of the geologic time
scale has greater detail (defined by
epochs)?
Unconformities represent
breaks or missing time in
the rock or geologic record.
Unconformities, such as the angular
unconformity shown on the left, form
because of erosional hiatuses or
long periods on non-deposition
between depositonal events.
Think about Mercer Island. When
where our glacial sediments
deposited? About 15,000 years ago.
When do you expect to see more
sediment deposited over University
Hill? 60,000 years in the future
during the next glacial cycle.
That’s 75,000 years between
depositional events!
Three major types of unconformities:
unconformities represent missing time
between the two rock units.
1. Nonconformity: A break separating
older metamorphic or intrusive
igneous rocks from younger
sedimentary layers.
2. Angular Unconformity: Tilted or folded
sedimentary rocks under more flatlying layers
3. Disconformity: mostly horizontal break
in sedimentary record: both layers
above and below the unconformity are
relatively horizontal. This type of
unconformity can be very hard to
recognize in the field.
How could you determine in the field the
difference between a nonconformity and an
igneous intrusion contacting country rock?
Disconformity
Angular Unconformity
Stages of formation of a disconformity and an angular unconformity.
Formation of a Nonconformity
A.Magma intruded into pre-existing
sedimentary “country rock. Note
that a contact metamorphic zone
will form where the magma
contacted and reheated the country
rock.
B. Uplift and erosion of country
rock exposing the intrusive body to
the surface of the earth. Note that
country rock has been eroded.
C. Deposition of new sedimentary
layers over the eroded surface
of the igneous (granitic)
intrusion or metamorphic rock.
D. How would you differentiate a
nonconformity from a contact
metamorphic zone (both have
plutonic igneous rock adjacent
to sedimentary rocks)?
What type of unconformity is shown above? How do you recognize it?
What type of unconformity exists at
Sicar Point, Scotland, where James
Hutton developed ideas related to the
Principle of Uniformitarianism?
Draw the unconformity.
What type of unconformity exists at
Sicar Point, Scotland, where James
Hutton developed ideas related to the
Principle of Uniformitarianism?
Draw the unconformity.
Is this where you would map it?
What type of unconformity exists at the base of the Grand Canyon. The lower rock unit is crystalline plutonic
and metamorphic. The rock unit lying above is layered meta-sedimentary deposited in coastal setting.
You have already determined why disconformities are the most difficult
unconformities to recognize in the field. What types of geologic evidence might you
use to recognize disconformities? Fossil evidence or buried paleosols (old soils or
weathering zones that require time to form prior to burial).
The Grand Canyon has several unconformities recognized within its rock record.
Five well-known unconformities are shown on the above cross-section of the Grand
Canyon.
Relative dating techniques are based on stratigraphic principles can be used to
differentiate the relative age rock units and landforms. Relative dating techniques by
themselves cannot be used to assign independent numerical ages.
Numerical dating allows geologist to assign actual ages to a deposit or landform. Many
of the methods are based on the ratio radioactive isotopes and their respective decay
products measured in a material to be dated.
In many cases relative dating techniques are used in tandem with numerical dating
techniques to constrain the age of a landform or deposit.
Unit A (225 m.y. old)
Unit B (how old is it?)
Unit C (275 m.y. old)