Name(s):___________________ __________________________
Section (please circle one):
Lab 8: Relative and Absolute Geological Dating Lab: F14
To accompany Chapter 8 in the AGI Lab Manual by Busch and Tasa 10th ed. Read
the introduction and carefully review stratigraphic principles (superposition, original
horizontality, cross cutting, inclusions, fossil and strata correlation) on Pages 207-216 plus
figure 8.10, the Geological time & fossil (faunal) succession chart with range zones p.213.
Learn to recognize the three different types of unconformity and be able to estimate
the relative amount of missing geological time for each type.
1. Disconformity – erosion or non-deposition generally lasting years to millions of years.
2. Angular unconformity needs deformation & erosion, ~millions of years or more.
3. Nonconformity marks erosion clear down to crystalline basement rocks below the
sedimentary basin. Often this is the basal unconformity for an entire sedimentary basin.
The rocks below are deformed igneous and metamorphic that are hundreds of millions to
billions of years older than the overlying sedimentary succession. This requires uplift and
erosion on land of much of the upper continental crust.
Work through examples in figures 8.1 through 8.9 in the book so that you are able to
work out geological events and sequences and infer relative timings for events.
Activity 8.1 Geological Inquiry for Relative Age Dating
Stratigraphic layering represents a sequence of geological events. A bed marks an episode
and an environment (facies) of deposition. The contacts between the beds mark hiatus (a
gap) representing either a time of non-depostion or erosion. The next overlying bed
represents a new deposit with its own environmental characteristics, flow regime, transport
fluid etc.
The cartoon on the left represents all of Earth History, but really captures a thorough record
of the Phanerozoic Eon, with all of its Geological Periods and major changes in life forms
from the cartoon fossil record. In any place on Earth we tend to find less complete records,
but segments of shorter duration that preserve more detail for that interval of the
stratigraphic record. The photo above on the right is of the “Amherst Clay” from de-glacial
lake Hitchcock (~10 Ka). The dark and light grey layers here in Ben Keisling’s photo
(University of Massachusets) represent annual varves of dark fine grained winter clay and
lighter summer sands from meltdown of the ice. These are thought to capture El Niño
Climate cycles according to Rittenour et al.(2000).
A. Ink in the stratigraphic contacts between the “thumb” thick varves (1-2 cm) in the photo
on the right. The cleaned face has annual layers representing ________ years of deposits. (2)
B. Examine the photo at the bottom of page 217 in your manual. There are about 4 distinct
layers here. There is grass (1) is growing on a thin (~40 cm) brown soil (2). The soil is
developed on a 2.4 m thick columnar jointed brown basalt flow (3) mapped as 7 Ma Rim
Basalt near Sedona. Under this are Pangaea supercontinent red beds (4) (ancient continental
soils of Permian Age, Schnebly Hill Formation). This is a lot of geological history to pack
into a mere 5-6 metres of outcrop, but stratigraphy has more missing time than represented
by the layers themselves.
1. Even if I hadn’t told you the redbeds were the oldest, how can you look at this outcrop
and tell their relative age? _________________________________________________ (1)
What stratigraphic principle did you use to determine this relationship? _____________
______________________________________________________________________ (1)
2.a. The uppermost contact between the buried red soil and the overlying lava flow varies
from less than 10 cm to more than 20 cm thick and it is more intensely red than the
underlying soils. What kind of a contact is this, (what did the lava do to the soil)? _____
______________________________________________________________________ (1)
2.b. Why is the top of the solid rocky cliff-forming basalt flow uneven and how did it come
to be this way? __________________________________________________________ (1)
3. Notice the vertical fractures in the basalt flow. Are they younger or older than the basalt
a) younger b) older (1)
How do you know this? ___________________________________________________ (1)
4. Notice the angular brown clasts of basalt in the overlying brown soil. Are the clasts older
or younger than the soil which surrounds them?
a) younger b) older (1)
How do you know this? ___________________________________________________ (1)
Angular Unconformities:
Unconformities mark missing time in the local rock record. Angular unconformities are easy
to spot because the strata meet at an angle where the underlying beds have been eroded off
and capped by later sediments. All sediments are more or less deposited in horizontal strata.
The angle means the underlying rocks not only had time to be deposited and lithified, but
also time enough to become folded, uplifted and eroded. This is essentially a mountain
building episode and subsequent erosion all reduced to that single angular contact. When
James Hutton found the famed angular unconformity at Siccar Point, he realized the
significance and on that day, he discovered “Deep Time”.
The photo below shows: Monterrey diatomite turbidites of Middle 16 Ma to Upper 6 Ma
age. These deposits were laid down in deep cold offshore basins, then compacted, tilted
uplifted and eroded near Montaña del Oro State Park near Obispo County California. This
has been stripped bare along the modern beach, but is capped by older uplifted beach
deposits of > 125,000 year age from the penultimate sea level highstand. Here the 2
sedimentary layers not only happened at very different times, but in different facies or
environmments. This is close to the San Andreas Fault where the Pacific Plate is dragging
the edge of North America to the Northwest (recall lab 2.7 on p.66 & 67).
Ink in bedding planes & angular unconformity here!
Examine the photo, that on p.218 and Figures 8.4 & 8.5 p.210.
In your manual, the photo shows buff coloured horizontal gravel (Late Tertiary continental
fan deposits of Arizona, in modern North America) overlying Continental Red-bed
sandstones (pre-Middle Triassic 225 Ma of the Moenkopi Formation of Pangea) that became
folded later in Cretaceous time during the uplift of the Colorado Plateau.
C.1. In the photo above, ink bold BLUE lines between the diatomite layers.
Assuming that the cream coloured diatomites were originally more horizontal, Explain what
process caused them to become tilted (folded) this way. ___________________________
_______________________________________________________________________ (2)
C.2 Ink in the Angular unconformity on the photo above in Red and measure the angle
between the unconformity (horizontal) and the left dipping diatomite beds . angle ____ ° (2)
What sequence of events is represented by the line (planar surface) of the unconformity,
even though there is no rock layer present to record this? _________________________
_______________________________________________________________________ (2)
D. Reflect and discuss from your exercises in B and C and list the rules or guidelines a
Geologist must follow to determine what age is represented by rocks in a given layer and for
the ages of the gaps between those layers. ______________________________________
_______________________________________________________________________ (3)
Activity 8.2 - Determining the Sequence of Events in a Geological Cross
Examining Geological Cross Sections:
Cross sections of rock strata: canyon walls, cutbanks of streams, eroded cliffs, road cuts,
valley walls represent the layered view of the Earth. Often there is no convenient exposure
and a geologist has to make their own cross section from subsurface borehole or geophysical
information. In all of these sections, the uppermost layer is the youngest, but unless you are
standing up to your “@ρ$ə” in mud, the youngest layer is not a modern Zero-age deposit but
rather an unconformity of some kind eroded down to some prior layer and time in Earth
History. There is no single place on Earth thick-enough, nor complete-enough to record all
of geological time. Because of this, we need to piece it together, much like a jigsaw puzzle,
to build up the entire geological time column. There are widespread younger deposits, but
the older parts of geological record, like so many unconformities have often been entirely
eroded away or are too deep to be exposed at all.
A. There are 4 cross sections, 3 of these are hypothetical ones but the second one is real and
represents the Inner Gorge of the Grand Canyon of the Colorado. Note the legend blocks
below so you can tell igneous intrusions which baked the rocks they cut from eroded plutons
covered by gravel of their own detritus. Generally sediments are deposited in horizontal
layers. If they are dipping (tilted) there has been enough time after their deposition and
lithification to uplift and erode them in a mountain building episode. In geological
sequences, unconformities for erosion or non-deposition actually represent more missing
time than is represented by the deposition of a single layer or sequence. Therefore, we
include unconformities in the sequence as they represent the biggest blocks of time. Often
an unconformity is more than one type depending on what the underlying layer is like. For
example a flat lying strata that overlies both dipping beds and crystalline rocks is both an
angular unconformity and a non-conformity. The present day erosional surface
(nonconformity) on the Canadian Shield (and the same as “S” the one in the base of the
Grand Canyon of the Colorado) is missing on average 2.5 Ga or a little over half of Earth
history! List letters and unconformity types (Dis, Ang, Non), top downwards = Youngest to
Oldest, next to each cross section.
Geological Section 1
(21 points)
Inner Gorge Grand Canyon - Geological Section 2 (see Activity 8.2 p 219
(17 points)
B. This is a cross section of the bottom, innermost layers of the Grand Canyon near Bright Angel
Falls. There are more layers on top of this all the way up to the Permian Kaibab Limestone of the
South Rim. After you have completed all 4 cross sections, come back to this point to discuss with
lab partners, what “Geological laws” of Relative Dating you would need to piece in the missing
time and rock layers from the 2 unconformities in your section above of the inner Gorge. On the
section above ink in a series of dashed lines for the contacts of the missing layers. Discuss the Laws
here and ink in the dashed contacts above. ___________________________________________
__________________________________________________________________________ (10)
Geological Section 3
(18 points)
Geological Section 4
(27 points)
Activity 8.3 Using Index Fossils to Date Rocks and Geological Events
(replacement for labbook p.221 questions.
The geological time column (Figure 8.10 above) is divided into Periods (several tens of Ma)
with names like Ordovician (for a tribe along the coast of Wales) or Cretaceous (for the age
of chalk). The Tertiary Period is further subdivided into: Paleocene, Eocene, Oligocene,
Miocene and Pliocene.
Fossils are unique impressions or mineral replacements of once living biota. The average
lifetime of a species, be it a microscopic species of plankton or a massive mammal, is about
2 Ma. Thus recognizing and telling one fossil species from another gives a pretty well
defined interval of time. Groups of closely related animals live longer than individual
species. For example there have been horse-shoe crabs since the Lower Paleozoic, but there
have been hundreds if not thousands of different species. Index fossils are widespread but
short lived and thus are very useful to tell the relative “Geological ages” of different strata,
for example Middle Cambrian versus Late Devonian. Marine strata preserve the best fossil
records. Before radiometric age dating became possible in the 1960’s, geologists knew that
Cambrian was older than Permian, but not by how much or when either of them was in
terms of absolute age. Paleontologists use the overlapping and sequential range zones of
different groups of fossils. Identifying species with easily distinguished shapes is the key to
interpreting relative ages. Now that we have used volcanic ash beds or lavas intercalated
with sediments to assign absolute radiomentric ages, we know the Cambrian began 543
million years ago. Be as precise as you can in assigning the Period names such as Early
Middle or Late, depending on when the fossil’s range zone starts or ends. Finally, using the
absolute age scale along the left edge of the diagram, assign an absolute age to the fossil
assemblage pictured. When Paleontologists use many species to constrain this the precision
improves down to the level of +/- 0.5Ma in long well studied fossiliferous stratigraphic
sequences. Here we are using long lived groups and our precision is less but the general idea
is the same.
For example, if a stratum contained Shark teeth and the brachiopod Chonetes, we can say it
was sometime between Late Devonian and Latest Permian for relative age and between 384
Ma and 255 Ma from the overlap of the 2 range zones for its absolute radiometric age.
Examine the photos in the lab manual and answer the following questions:
• This trilobite is frequently rolled up and has
bulbous eye spots
• These brachiopods resemble symmetric wings
A. Analyse the fossils in the top frame and these of the same types. All of these fossils are
typically found in fine grained micrites (microcrystalline limestone mud).
1. What are the names of the 2 index fossils ________________ & ______________. (2)
To what 2 groups of marine invertebrates do they belong? _________ & __________. (2)
2. Based on the overlap of the range zones of these 2 index fossils, their strata belong to the
(circle one) Early Middle Late ________________________ (geological time period). (2)
3. What is the absolute age range, from the oldest possible age to the youngest possible age
when both of these index fossils coexisted? ____________ Ma to _____________ Ma (2)
Straight cephalopod (walking stick rock)
Fossil oysters (devil’s toenail)
B. Analyse the fossils in the middle frame, hall cabinets and these images above of the same
types. These fossils are typically found in both limestones and detrital sandstones across
much of the world including parts of Canada from Vancouver Island to Saskatchewan.
1. What are the names of the 2 index fossils __________________ & ______________. (2)
2. Based on the overlap of the range zones of these 2 index fossils, their strata belong to the
(circle one) Early Middle Late ________________________ (geological time period). (2)
3. What is the absolute age range, from the oldest possible age to the youngest possible age
when both of these index fossils coexisted? ____________ Ma to _____________ Ma (2)
• 2 different contemporaneous brachiopods with
• 2 images of a trilobite from the same strata
C. Analyse the fossils in the top frame and these of the same types. All of these fossils are
typically found in fine grained micrites (microcrystalline limestone mud).
1. What are the names of the 2 index fossils __________________ & _________________. (2)
2. Based on the overlap of the range zones of these 2 index fossils, their strata belong to the
(circle one) Early Middle Late _____________________________ (geological time period). (2)
3. What is the absolute age range, from the oldest possible age to the youngest possible age when
both of these index fossils coexisted? ___________________ Ma to _________________ Ma (2)
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D. Using Figure 8.10 to identify the fossils and their relative ages, examine the cross section
figure 8.2 above and answer the following questions.
1. There is a disconformity between beds _________________ and _____________________ (2)
2. In sedimentary geology, a “system” is the bed or succession of beds deposited during a
particular time interval. At the very least, what system is missing from cross section 8.2? (Period)
__________________________________________________________________________. (1)
3. With fossils, we do not know from just one occurrence whether we have the very earliest
occurrence of a fossil or the very latest, or somewhere in between. This introduces some error or
uncertainty in figuring the interval of missing time. Assuming the lower trilobite was the very
first of its kind and the upper trilobite was the very last of its kind, what is the maximum amount
of time missing: in relative ages from Early Middle Late ____________________________ to
Early Middle Late _________________________________________________________. (4)
4. The uncertainty is that these fossils might have been the last and first of their kinds
respectively. What is the maximum and minimum age missing between these 2 beds in absolute
radiometric time? Minimum ___________________________________________________ Ma
and Maximum ___________________________________________________________ Ma (4)
E. Examine Figure 8.3 above and note that the lower strata are dipping, while the upper strata are
flat lying.
1. The bold black wavy line labeled E is a/an:
Angular Unconformity
2. Check the fossil ages on figure 8.10 and note that the lower dipping beds are Paleozoic while
the upper flat lying beds are Tertiary. At the very least 2 important geological processes occurred
during the missing time represented by E. What were they? ____________________________ &
___________________________________________________________________________ (2)
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Activity 8.4 Absolute Dating Rocks and Fossils using radioactive decay pairs.
The absolute time scale is based on long lived radioactive parent elements (isotopes) and their
daughter elements (isotopes). A small proportion of the isotopes of certain elements are
intrinsically unstable and prone to radioactive decay. This occurs at a fixed rate for each isotope
pair and is expressed as half-lives, where half of the initial parent isotope decays in 1 half- life.
Read the instructions on Absolute Dating p 214 (fig repeated below) and answer the following
A.1 How many half-lives have transpired for the lava whose zircons have: 71% 235U and 29% 207Pb.
____________________________________________________________________________ (2)
2. Calculate the age of the lava flow having zircons with 71% 235U and 29% 207Pb and show your
______________________________________________________________________________ (2)
3.a. Give the absolute radiometric age for a layer immediately above the lava flow. ___________ (1)
b. Give the absolute radiometric age for a layer immediately below the lava flow. ____________ (1)
B. 1. Based on equal proportions of daughter 206Pb and parent 238U in the oldest meteorites, what is
the numerical value of their parent/daughter ratio _____________________________________ (2)
how many half-lives have elapsed since their formation? _______________________________ (1)
2. How many years is this for the age of the Earth? ___________________________________ (2)
& during what Eon did Earth first form? ______________________________________
C.1. A buried (not modern) peat bed has 6.000 % 14C parent remaining and 94.000 % 14N daughter.
Calculate the parent/daughter ratio for the peat bed to 4 decimal places __________________? (3)
2. How many half-lives have elapsed since the peat was deposited________________________? (1)
3. What is the absolute age of the Peat bed in years? __________________________________. (3)
4. What geological Period does this peat bed belong to? ________________________________ (1)
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5. Why must you avoid contamination of the peat by either younger living plant roots or older dead
carbon such as limestone or oil spills? ________________________________________________
_____________________________________________________________________________ (2)
6. What are the oldest deposits that can be dated by the radiocarbon method? __________________
______________________________________________________________________________ (1)
7. Explain why this is so:
______________________________________________________________________________ (1)
D. Zircon (ZrSiO4) is a trace mineral in most igneous rocks, making up less than a few percent. It
forms as the magma cools and solidifies. From this time onwards the small contaminant of Uranium
decays to lead according to the radioactive decay law and the half-life for each P/D isotope pair.
From this we can date the cooling age of magmatic rocks.
1. As you walk on a modern beach here like Dallas Road or Island View, the sand will contain some
small amount of zircons. Could zircon crystals in such modern beach sands be used to date the time
of sedimentary deposition (like the last storm)?
Circle one: Yes No (1)
Explain why? ____________________________________________________________________
______________________________________________________________________________ (2)
• Zoned metamorphic rims as overgrowths on an
original igneous core
• The igneous core is metamict (radiation damaged)
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2. For Precambrian rocks (up to billions of years ago, before fossils) the only way to date events line
intrusions or metamorphism is by radiometric dating using appropriate minerals and isotope pairs.
While igneous crystals are simple and unzoned, metamorphic crystals contain an original igneous
core and one or more rims for subsequent metamorphic events. With a special tool called SHRIMP,
basically a laser beam aimed through a microscope and hooked up to a mass spectrometer, zircons
can now be dated one layer at a time, like peeling an onion! In this case could zircon crystals in a
very old Precambrian metasediment (metamorphosed sedimentary rock ) be used to date the time of
deposition of that sediment?
(circle one): Yes No
Explain why? ____________________________________________________________________
______________________________________________________________________________ (2)
2. When geologists need to date minerals or rocks they need to know certain things before attempting
to do this. Now that you have done some relative and absolute dating, what 3 geological rules should
be followed in attempting to date any rock using the isotopes in one of its minerals? ___________ (3)
E. An E-bay special offers an “authentic dinosaur bone” confirmed by radiocarbon dating to be 400
Ma. What 2 things are wrong with the “scientific claims” the seller makes? __________________(2)
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Activity 8.5 Geological History from a New Mexican Outcrop:
• Note the relative age
of the Fagopsis leaf &
• Obtain the Absolute
age from the Zircon
A.1. What is the relative age for the sedimentary rocks in this exposure (use fossil to estimate
Period/Epoch)? ___________________________________________________________________
_____________________________________________________________________________ (2)
2.a Mass spectrometry on the zircon from the sill gave 98.9% parent 235U and 1.1% 207Pb. Find this
value on the decay curve. Geochronologists quickly calculate how far they are along the decay curve
by diving the % of the Parent by the % of the daughter to get a simple ratio. To begin with this ratio is
infinite. After 1 half-life it is 1.0 while after 2 half-lives it is 0.333 etc. ________________________
b. Compute the value of the Parent/Daughter ratio in the Zircon?
______________________________________________________________________________ (3)
c. About how many half-lives have elapsed since the sill emplaced? ________________________ (2)
d. What is the half-life of the parent isotope 235U? __________________________________ Ma (1)
e. What is the absolute age of the sill based on the crystallization age of the zircon crystals? To do
this, multiply the half life of the parent isotope by the number of half lives elapsed. Show your
calculations. ____________________________________________________________________(3)
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f. What is the relative geological age (Period) for the sedimentary rocks below the sill? ________
___________________________________________________________________________ (2)
g. What is the minimum absolute radiometric age the shale just below the sill? _______________
____________________________________________________________________________ (2)
h. Explain when the sill became faulted and eroded and how you know this? _________________
____________________________________________________________________________ (2)
3. Measure the total displacement across this fault. __________________________________ m.(1)
B. On this list for the sequence of events youngest to oldest for this strip mine outcrop from New
Mexico. Use the Epoch names of the relative ages, the date for the absolute age, the flow, the shale
and sandstone and coal beds and the unconformity on top.
1. unconformity type and age: _____________________________________________________ (1)
2. fault type and age: ____________________________________________________________ (2)
3. igneous sill age: _______________________________________________________________ (1)
4. Tilting/folding age:_____________________________________________________________ (1)
5. grey shale and coal with fagopsis leaf age: __________________________________________ (1)
6. white sandstone age: ___________________________________________________________ (1)
7. Lowest grey shale age: _________________________________________________________ (1)
C. What kinds of questions remain unresolved concerning the geological history at this location?
Write down one such question and discuss what kind of additional information you would need to
obtain to answer it. ______________________________________________________________
___________________________________________________________________________ (4)
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Activity 8.6 CSI (Canyon Scene Investigation) Grand Canyon, NE Arizona
A. Lower Grand Canyon of Colorado River, view of North Rim near Canyon Village. Refer back to
your Cross Section 2 for the names of the rock units. The lowest rock outcrops are dark brown and
pink with rugged, irregular weathering. They were sedimentary and volcanic rocks originally
deposited between 1.75 to 1.73 Ga (Giga Annum = billion years) which later became regionally
1. Label these rocks Vishnu Schist & draw a wavy line (“S”) along their uppermost surface. This is
called “The Greatest Unconformity” in this part of North America.
Keep in mind that the Vishnu Schist comprises crystalline metamorphic rocks.
The wavy line you just drew is a/an: a) disconformity b) angular unconformity c) non-conformity.(1)
2. On top of this lower unconformity is a gently NE dipping package of rocks called the Grand
Canyon Supergroup. The lowest member of the GCS is the Bass Dolomite, a buff coloured, cliff
forming set of 7-9 layers. Label the Bass
Above the Bass is a recessive weathering red shale which forms gentle slopes. Label this Hakatai (1).
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3. At a gentle angle, there is another buff/light brown weathering cliff forming unit about 6-9 beds
thick. This is the Lower Cambrian (~545 Ma) Tapeats Sandstone. Label the Tapeats on the photo
above (1) and draw another wavy line at its base. This wavy line is your familiar upper unconformity
(“R”) from Cross Section 2. Keep in mind that the Cambrian Tapeats Sandstone and underlyng Grand
Canyon Supergroup are all sedimentary rocks. This is called “The Great Unconformity” here.
The wavy line you just drew is a/an: a) disconformity b) angular unconformity c) non-conformity. (1)
4. (3 in manual) Assuming the Vishnu Schist (below unconformity S) has an absolute age of about
1740 Ma, & the overlying Tapeats Sandstone has an absolute age of 545 Ma (above unconformity R),
a) What is the total time span that is missing between these 2 unconformities? ____________ Ma (2)
b) Reflect and Discuss whether all of this time you just calculated is represented by the deposition of
the Grand Canyon Supergroup, why or why not? _______________________________________
______________________________________________________________________________ (2)
If you decided “not”, explain where the missing time went and locally, what kinds of geological
processes occurred during the rest of this interval. ______________________________________
_____________________________________________________________________________ (2)
B. In the manual on p 226 and Below is a nearby photo showing more of the North Rim at the bottom
of the Canyon. Here all of the rocks shown are sedimentary.
of the Canyon. Here all of the rocks shown are sedimentary.
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1. The East dipping strata near the lowest part of the canyou belong to the Grand Canyon Supergroup,
here the beds are the red Hakatai shales up through the Unkar Group Shales, the brown Cardenas
Basalt and the stripey, light coloured Nankoweap sandstones and shales here making the uppermost
east dipping beds. Label the Grand Canyon Supergroup ________________________________ (1)
Above the GCS is “The Great Unconformity”. Label this with a wavy line and the words : Great
Unconformity. _________________________________________________________________ (2)
2. The wavy line you drew is a/an: a) disconformity b) angular unconformity c) non-conformity. (1)
C. Above the Great Unconformity is the entire Paleozoic Section as recorded in this part of North
America. It is all flat lying. It goes from the Tapeats Sandstone (brown cliff former) at its base up to
the the Permian Kaibab limestone (White cliff former) with a bit of brown Triassic Moenkopi
formation at the back skyline. In the photo above, it only catches the Tapeats sandstone (cliff former),
recessive Bright Angel Shale and a stack of 3 cliff forming limestone formations: Cambrian-Muav,
Devonian-Temple Butte and Mississippian-Redwall at the upper limit of the photo. For the Paleozoic
sedimentary rocks here, do you think they represent continuous deposition? a) yes or b) no
Do you think there is any missing geological time in this stack of strata and if so, what kind of
unconformities must be present here?
a) Disconformity b) Angular c) Nonconformity
If you can figure out where one of these is located, sketch it in on the photo above and suggest which
geological system (period) is missing here? __________________________________________ (2)
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Activity 8.1 Geological Inquiry for Relative Age Dating