Instructions: The mode of a rock refers to the relative percentages of different
ID: 289716 • Letter: I
Question
Instructions: The mode of a rock refers to the relative percentages of different minerals or particles that make up that rock. For example, a typical basalt mode might be 10% olivine, 5% clinopyroxene, 20% plagioclase, 65% groundmass. All types of rocks, including igneous, sedimentary, and metamorphic rocks, can be classified according to their modes. A common way of classifying sandstones is by using a QFL (quartz-feldspar-lithic) diagram, also called a Krynine Diagram. The attached table presents modes for various sandstones. For each mode on the table:
[30 points] Plot these modes on the attached QFL diagrams (Figures 1 and 2).
In order to do this, you will need to sum up all of the quartz/silica components (quartz + chert), feldspars (alkali + plagioclase), and lithics (all rock fragments). You will also need to normalize the sand-sized particles to 100%, as only sand-sized particles are plotted on these figures. Please show your work. If a clastic sedimentary rock contains abundant fossils, append the adjective "fossiliferous" to the front of its name. A rock that contains <25% sand should not be plotted on a QFL diagram.
[10 points] Name the rock (Figures 1 and 3).
A sedimentary rock is named based on the mode of its particles (Figure 1), as well as the distribution of grain sizes (Figure 3). Once you have determined the position of your sample on the QFL diagram (Figure 1), you will need to project this composition onto Figure 3 according to the relative proportion of matrix (mud-sized particles, does not include cement). For example, a "clean" sandstone made up of 90% quartz sand grains is called a quartz arenite, while a "dirty" sandstone containing 50% lithic fragments and 30% silt-sized particles would be called a lithic graywacke. Note: If your rock is a sandstone (>85% sand-sized particles), please use Figure 1 to determine the name – it is more detailed for sandstones than Figure 3.
[10 points] Determine the provenance (origin) of the sediments (Figure 2).
The mode of a sandstone is also useful for determining the origin of the sediments that formed it. For example, fine-grained volcanic rocks from a volcanic arc tend to form sandstones that are rich in lithic fragments, while eroded granites from continental interiors tend to form sandstones that are rich in feldspar and quartz. Use Figure 2 to determine the provenance of the sediments that make up each sample. Remember that if a rock does not contain significant sand-sized particles, it cannot be plotted on a QFL diagram, and therefore the provenance cannot be determined using this diagram.
[40 points] In many cases, we can infer the origin, transport, and deposition conditions of a sediment, simply by observing the components of a sedimentary rock. Explain in words how you think the sediment in each rock may have formed. Take the amount and types of lithic fragments into consideration, as well as the provenance you just determined in part c. For each sample, make an educated guess about 1) The parent rock or rocks (what is the original material that eroded to form this sediment?); 2) Transport conditions (how far has the sediment traveled from its site of origin? In other words, how mature is the sediment?); 3) Deposition conditions (was the sediment deposited in a high or low energy environment? Marine or terrestrial?)
Note: make sure you understand the factors that determine sediment maturity. And be careful with your words - maturity is not the same as age! Don’t forget to include all of the samples. Even if a sample doesn’t plot on Figure 2 due to low sand content, you can still say something about how it formed.
[10 points] Mudrocks are composed of >60% clay minerals. Based on this mineralogy, and what you have learned about weathering, why do you think mudrocks are the most common sedimentary rocks, accounting for ~65% of the volume of sedimentary rocks on the earth's surface? From what kinds of parent rocks do you think they might be formed?
Table 1. Modes of sedimentary rocks
sample #
1
2
3
4
5
sand:
quartz
14
86
10
41
chert
5
2
3
plagioclase
4
30
36
alkali feldspar
12
shale
1
4
3
granite
17
rhyolite
2
4
basalt
2
20
2
schist
1
1
fossils
8
mud:
silt
29
19
10
clay
30
63
6
cement:
carbonate
8
18
siliceous
9
Figure 1 below
Figure 2 below
Figure 3 below
sample #
1
2
3
4
5
sand:
quartz
14
86
10
41
chert
5
2
3
plagioclase
4
30
36
alkali feldspar
12
shale
1
4
3
granite
17
rhyolite
2
4
basalt
2
20
2
schist
1
1
fossils
8
mud:
silt
29
19
10
clay
30
63
6
cement:
carbonate
8
18
siliceous
9
Quartz Chert + Polyquartz -Quartzarenite Lithic Lithic /'Feldspathic ,, .. Arkose arenite… Feldspar LithoclastsExplanation / Answer
1)
2)
Normalized values of QFL
sample #
1
2
3
4
5
Quartz (Q total)
34.15
100.00
100.00
15.48
50.00
chert
Plagioclase (F total)
9.76
50.00
43.90
alkali feldspar
Shale (L total)
56.10
34.52
6.10
granite
rhyolite
basalt
schist
Fossils
82
Total after normalization
100
100
100
100
100
3 and 4)
sample #
Rock Name
Origin
1
Litharenite
Recycled Orogen or volcanic in origin
2
Quartzarenite
Craton Interior or continental origin
3
Fossiliferous mudstone
Marine in origin
4
Arkosic graywacke
continental origin
5
Arkose
Basement Uplift
5) Sample # 1- Litharenite: Parent rocks may be pre-exiting igneous, metamorphic and sedimentary rocks due to presence of numerous rock fragments. I can say this rock is not mature enough on the basis of considerable amount of clay and slit material. This rock must be deposited near to its parent rock. Deposition environment of this rock ranges from fluvial, deltaic and alluvial sedimentary environment mostly associated with the tectonically active margins such as volcanic arc, continental collision, subduction zones etc.
Sample # 2- Quartzarenite: Parent rocks may be quartzite. This is a fine grained sandstone and I can say this is a mature sedimentary rock due to absence of any other material except sand. The parent rock of quartzarenite must be far away from it’s depositional environment. It deposited in high energy depositional environment through aeolian processes.
Sample # 3- Fossiliferous mudstone: Parent rocks may be mica nad feldspar rich igneous and metamorphic rocks. This is a fine grained sedimentary rock. It deposited in low energy depositional environment through marine and fluvial processes.
Sample # 4- Arkosic graywacke: Parent rocks may be granitic rocks. This is a detrital sedimentary rock and it is not mature enough due to presence of >25% feldspar and high clay and slit material. It is detrital sedimentary rock. It deposited in low energy depositional environment.
Sample # 5- Arkose: Parent rocks may be granite. This is a detrital sedimentary rock and it is not mature enough due to presence of >25% feldspar and high carbonate material. This rock formed due to weathering of granitic rocks. It deposited in low energy depositional environment through weathering processes.
Last question is not answered because of I have already answered the four questions as the guidline of Chegg India.
Normalized values of QFL
sample #
1
2
3
4
5
Quartz (Q total)
34.15
100.00
100.00
15.48
50.00
chert
Plagioclase (F total)
9.76
50.00
43.90
alkali feldspar
Shale (L total)
56.10
34.52
6.10
granite
rhyolite
basalt
schist
Fossils
82
Total after normalization
100
100
100
100
100