Geology 2
Exercise 4
Relative Time
In the following diagrams, place the age of formation of the units or features in their
correct chronological order. Place the letter of the oldest unit on the bottom of the list and
letter of the youngest unit on the top and fill in the rest appropriately. The order of some
of the units, particularly the most recent units in number D, may be ambiguous. Mark on
your list which units or events may have formed at the same time.
A.
B.
C.
What kind of unconformity separates unit B from unit P? A nonconformity
What kind of unconformity separates unit E from the underlying units? An angular unconformity.
D.
Part 2: Photographs
Answer the Questions for the following photographs, which will be provided for you in
lab.
1. Carmel Valley Gravels
a. What is the name of
the unit in the lower
right of this photograph?
Monterey Formation.
b. In what depositional
environment was this
sedimentary rock
deposited?
Deep Ocean
c. What is the
depositional
environment of the
gravels?
A river energetic enough
to transport those boulders.
d. What kind of contact separates the unit in the lower right from the gravels?
A Depositional Contact, or in even more detailed terminology, an unconformity
e. Is it a conformable contact? Why or why not?
No, for two reasons. First, 15 million years have passed between the deposition of the
Monterey fm and the deposition of the river gravels. Second, the deep sea depositional
environment of the Monterey fm is very different from the river gravel depositional
environment of the younger sediments; some uplift and erosion must have occurred to
bring the deep sea sediments to above sea level in order for the river gravels to be
deposited on top of them.
f. What is the geologic history of the rocks and sediments shown here?
1. Deposition of silica-rich mudstone or diatom-rich ooze in the deep sea to create the
Monterey Formation. 2. Uplift and erosion of the Monterey Formation to above sea level.
3. Deposition of the river gravels by a river that was almost certainly a predecessor to the
Carmel River. It was through the deposition of these gravel on top of an erosional surface
that the unconformity was created.
g. The fellow in the lower right with the red shirt on is Rich Keziran, a well-known
history teacher at MPC. What do you think he is thinking as he gazes up at these rocks?
Wow, I should really take an entire geology class. Then I could spend even more of my
weekends gazing at rocks and thinking about their history.
2. The Grand Canyon
The point of this image is to find the angular unconformity and understand its
significance in terms of the geologic evolution of the Grand Canyon. Note the two series
of sedimentary rocks: the Grand Canyon Series occurs in the lower right of the
photograph and is dipping towards the right. The Paleozoic series lies above the Grand
Canyon series and is flat-lying.
a. Draw a quick sketch that emphasizes the difference in dip between the two groups of
sedimentary rocks.
The lower group dips to the right of the photograph whereas the upper group has no dip;
it is horizontal.
b. What is the contact between the two groups of sedimentary rocks called?
An angular unconformity
c. What must have happened during the time period represented by this contact?
Folding of the lower group of sedimentary rocks to create the dipping beds, and erosion
of the lower group of rocks to create a horizontal surface for the upper group of rocks to
be deposited upon.
d. What is the geologic history of the rocks that comprise the Grand Canyon, as seen in
this image?
1. Deposition of the lower group of rocks.
2. Folding of the lower group of rocks to create the dipping beds seen today.
3. Uplift and erosion to create the flat surface that is the angular unconformity today.
4. Subsidence
5. Deposition of the upper group of rocks.
6. Uplift
7. Erosion by the Colorado River to create the Grand Canyon.
3. Plutons in Chile
The larger
diagram shows a
mountain-sized
view of the contact,
whereas the inset
shows a close-up
view of the contact.
a. Which of the
rocks, the light
rock or the dark
rock is younger?
What evidence is
seen on the closeup (the inset) to
support your
answer?
The lighter rock is the younger, intruding rock. On the close-up, small veins can be seetn
intruding from the light rock into the darker rock.
b. Where within or on the Earth did these plutonic rocks crystallize?
Beneath the surface within the crust.
c. What must happened after crystallization to expose these rocks at the Earth’s surface?
Erosion and uplift.
4. Inclusion
a. Which is older, the dark rock or the light rock?
The light rock is older.
b. How can you tell?
The dark rock contains an inclusion of the light rock.
The dark rock must be younger in order to contain an
inclusion of the older light rock.
5. Garrapata Beach
a. What is the origin
of the sedimentary
rocks in the lower
half of the outcrop?
Turbidites (undersea
landslides) deposited
on the continental
slope or rise.
b. Which way are
these rocks dipping
(the rocks in the
lower half of the
image)?
To the left and
slightly towards the
photographer.
c. What is the origin of the sedimentary rocks in the top half of the outcrop?
They are poorly consolidated (see the kind of erosion) beach sands; they were formed on
the beach.
d. What is the origin of the large boulders between the two sedimentary units?
Probably got to the beach in either a small river or were eroded from a nearby granite
outcrop and washed down the beach. The ocean waves subsequently rounded the
boulders and deposited them on the beach. Uplift then lifted the boulders and the beach
sands onto the terrace.
e. What is the geologic history of this place?
1. Deposition of the turbidites.
2. Uplift and erosion.
3. Deposition of the beach sands and the big granite boulders on the wave-cut platform.
4. More uplift to create the terrace and to isolate the beach sands and the big granite
boulders above the beach.
6. Contacts.
a. What kind of sedimentary
rock occurs above the granite?
A conglomerate
b. Assume the granite clast
(lower image) is the same
granite that occurs below the
sedimentary rock. Which is
older, the granite or the
sedimentary rock?
The granite must be older.
Older rocks are always the
inclusions. They are always
included within younger
rocks.
c. In these images, there is no
evidence of shearing or
faulting. What kind of contact
separates the sedimentary rock
from the granite?
A depositional contact. Since a
sedimentary rock rests on a
plutonic rock, the contact must
be a nonconformity.
d. Note that the slightly
weathered granite occurs on
both side if the fresh granite.
Which is older, the slightly
weathered granite or the fresh
granite? (Note: the difference in weathering is probably more due to a difference in
composition than a difference in age.)
The slightly weathered granite is older than the fresh granite. It looks like the fresh
granite intruded into the slightly weathered granite.
e. What kind of contact separates the slightly weathered granite from the fresh granite?
An intrusive contact.
f. Where did the granite form (circle one)?
1. On the surface of the Earth
2. Below the surface of the Earth?
g. What had to happen to get the granite to the surface?
Uplift and erosion.