GEO1115_2021_Activity9.2S
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Geology
Date
Dec 6, 2023
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THEME 9 – SEDIMENTARY ROCKS
[ 53/54 : A+ ]
A
CTIVITY
9.2S – T
ECHNICAL
QUESTIONS
(
/54)
Reference material:
-
Lecture of associated theme (Brightspace)
-
Glossary (
https://mysite.science.uottawa.ca/dumas/
)
-
Reference document “Section of the lab manual” in Brightspace
Activity:
1.
Explain why a quartz sandstone (clastic sedimentary rock composed of quartz grains) could react
when in contact with HCl.
Hint: Think of one of the two processes taking place during lithification.
(
/2)
One process during lithification is cementation, which means the chemical precipitation of other
minerals between sedimentary rock particles and then binding these particles together to form one
singular rock. This means that other pieces of rock such as limestone, which reacts strongly with HCl,
could be interspersed with the other pieces of quartz and sand during the lithification process.
2.
Refer to the figure below to answer the following questions.
(
/6)
a)
Knowing that chemical weathering is a process that starts on the
surface
of rocks and particles,
and then proceeds inwards, what do you think is the effect of particle size on the rate of
chemical weathering? Justify your answer (N.B.: the picture above was added to help you
answering the question). (
/4)
The smaller the particle size, the faster the same amount of rock will be affected by chemical
weathering. This is because it starts on the surface and moves inwards, and the smaller the
particles the more surface area it has for the same amount of rock. As we can see in the image
above, the larger particle has only 24cm
2
of surface area, whereas the same amount in smaller
particles has 48cm
2
surface area and therefore has double the available space to succumb to
chemical weathering.
b)
Thus, summarize in one sentence the role and/or influence of physical weathering on the rate
of chemical weathering. (
/2)
The more physical weathering that has occurred and broken the rock into smaller particles, the
more susceptible to chemical weathering it will be.
3.
Knowing that the sample below is a clastic (detrital) sedimentary rock, what is the most likely
mineral forming the pink/red grains giving this sample its pinkish colour? Justify your answer.
Hints: look back at the definition of a clastic sedimentary rock to figure where most of its sediment
comes from; think of the dominant composition of the rocks in this source area (i.e. felsic or mafic,
and their associated minerals based on the igneous rocks classification table).
(
/2)
The pink particles are likely potassium feldspar as clastic sedimentary rocks are composed mainly of
quartz and feldspar particles. Potassium feldspar is a felsic, pink rock and could be the cause of this
colour.
4.
Answer the following questions about the weathering and erosion of a granite.
(
/12)
a)
If a fresh granite was physically weathered, eroded, and transported on a very short distance,
of what composition would you expect the resulting sediment to be (name the three minerals
forming the majority of the clasts obtained)?
Hint: look back at the definition/composition of
a granite in the Igneous Rocks Classification Table in the Reference document “Section of the
lab manual” of Theme 8 in Brightspace.
(
/3)
Quartz, feldspars, muscovite(mica)
b)
If this same sediment was transported for a very long time and on a very long distance at the
Earth’s surface:
i.
which two of the three minerals named in “a)” would you expect to be altered?
(
/1)
Feldspars and Muscovite
ii.
to what type of minerals would you expect these two minerals to be altered?
(
/1)
Clay minerals
iii.
what is the name of the process responsible for the observed chemical changes?
(
/1)
Chemical weathering
iv.
which of the three original minerals named in “a)” will remain due to its high resistance to
the process named in “b) iii”?
(
/1)
Quartz
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c)
Compare the sorting and rounding (or angularity) of the sediment described in “a)” with the
one described in “b) iv” (i.e. a sediment composed almost entirely of grains of the mineral
named in “b) iv” after a long period and distance of transport). Justify your answer. (
/4)
The sediment in “a)”, granite, will be much more angular and have poorer sorting than that in “B)
iv”, granite composed almost solely of quartz which has undergone intense chemical weathering.
This is because everything left is made almost entirely of the same material and therefore has
about equal resistance to the weathering and should be approximately the same size. Also
chemical weathering begins on the surface of particles and makes its way inwards which means
all of the angular parts will be weathered away first and will in turn produce rounder particles
than the initial granite.
d)
Give another textural change that the individual particles in “b) iv” likely experienced during
their long period/distance of transport after since the beginning of their journey. (
/1)
The particles will likely also experience a change in size, becoming much smaller than they were
originally.
5.
Refer to the picture below to answer the following questions.
(
/6)
a)
Based on the picture, did this structure form under a unidirectional, or a bidirectional current?
Justify your answer. (
/2)
Bidirectional current since the pattern within the waves of the rock is symmetrical with no clear
direction of crests and troughs.
b)
Give the name of the bedform associated to this sedimentary structure, describe its
morphology, and relate it to the direction(s) of the current responsible for its formation. (
/4)
The bedform is an oscillatory ripple, it is characterized by straight and parallel crest as well as its
symmetrical structure which is formed by bidirectional currents. This shows that the current, in
reference to the image specifically, flowed both to the left and the right in an oscillating pattern.
6.
The picture below shows a rock outcrop on roadside.
(
/4)
a)
In the above picture, what was the current direction relative to the picture (i.e. towards the
right, left, top, or bottom)? Briefly justify your answer. (
/2)
The current was to the left as the cross stratification is angled down to the left with implies the
sediment was moved over an edge and then fell towards the left.
b)
Do you think the structure in this picture resulted from the migration of ripples, or dunes?
Justify your answer. (
/2)
Dunes as ripples are generally much smaller(height ≤ 5cm) and this is a large roadside outcrop.
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7.
Refer to the picture below to answer the following questions.
(
/9)
a)
Give the name of the sedimentary structure shown in the red rectangle. Indicate whether it
resulted from the migration of ripples, or dunes, and justify your answer. (
/3)
Cross-Stratification resulting from the migration of ripples as dunes are generally much
larger(must have a height > 5cm) and this is much smaller.
b)
Based on the picture, did this structure form under a unidirectional, or a bidirectional current?
Justify your answer. (
/2)
Unidirectional since the pattern seen in the ripples is asymmetrical and is following a clear crest
and trough pattern.
c)
Give the name of the bedform associated to this sedimentary structure, describe its
morphology, and relate it to the direction(s) of the current responsible for its formation. (
/4)
The bedform is a ripple formed from a unidirectionally current, it is small with a height of 5cm or
less. It follows an asymmetrical pattern of crests and troughs which indicate the direction of flow
of the wind or air current, in this case, in reference to the picture specifically, towards the left.
8.
Refer to the figure below to answer the following questions.
(
/10)
a)
Assuming that the grains forming the sedimentary rocks depicted in the two previous
questions (i.e. Q6 and Q7) are of medium sand size, which of the two structures (Q6 or Q7)
formed under the highest conditions of energy (flow velocity)? Justify your answer. (
/2)
The rocks in Q6 would have been formed under higher energy conditions than that of Q7 as the
former was created by a dune and is much larger and therefore would have taken more energy
for the sediment to be moved over. As opposed to the latter which is formed by a ripple and is
much smaller, taking much less energy to move the sediment across that space.
b)
If the flow velocity had been higher (for the same grain size), what other two structures could
have formed based on this diagram? (
/2)
Upper flat bed and Antidunes
c)
Based on the phase diagram above, would you expect ripples to form in very coarse sand? If
so, at what velocity? If not, what structures would you expect to form in very coarse sand in
increasing order of current velocity? (
/3)
No, in increasing order of current velocity acting on very coarse sand I would expect No
movement, lower flat bed, subaqueous dunes, upper flat bed, and antidunes
d)
Based on the phase diagram above, would you expect dunes to form in very fine particles such
as silt? If so, at what velocity? If not, what structure would you expect to form in such a fine-
grained sediment for current velocities between 20 and 60 cm/s? (
/3)
No, I would not expect subaqueous dunes to form in very fine particles such as silt, only
anitdunes at extremely high energy levels. At a current velocity between 20 and 60 cm/s I would
expect silt to form ripples.
9.
Observe the change in grain size from bottom to top in the deposit illustrated in the picture below.
Knowing that the grain size is related to the flow velocity, what could you say about the change in
energy (flow velocity) during the depositional event that formed the deposit depicted in this
picture? Justify your answer.
(
/3)
Due to the grain size at the bottom being larger than the grain size at the top, I can say that when the
earlier sediment was being deposited it was a much higher energy environment with higher flow
velocity. Over time the energy levels decreased and therefore the size of sediment able to be
transported and deposited decreased as well resulting in the smaller particles on top.
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