GEOL 101 GCCCD Geology Events Sequence Lab Report

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GEOL 101

Grossmont Cuyamaca Community College District

GEOL

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Name: GEOLOGIC HISTORY This lab is being used and was modified with permission from Gary Jacobson of Grossmont Community College. INTRODUCTION: One of the most fundamental techniques in geology is dating rocks and putting geologic events in their proper sequence. There are two ways by which this can be accomplished. Absolute dating involves being able to date how many years old a rock is. Although this may sound simple, determining the numerical age of a rock is difficult to accomplish because several criteria must be met. Radiometric dating is the most commonly used way to absolute date a rock. The method is limited because you can only date a rock which has formed directly from cooling magma or a recrystallized metamorphic rock. However, even if you cannot tell the precise age of a rock, you can determine the order in which a series of geologic events occurred and place a relative age on that rock. This is called relative dating which is the most fundamental concept in geology. Absolute dating techniques have only been around since the late 1960’s, but geologists have been putting relative ages on rocks since the 1700’s. Early geologists used the principle of faunal succession and other principles of relative dating to determine the relative age of a rock. In fact, fossils and the principles of relative dating were used to create the geologic time scale long before we knew the absolute age of the earth. In this lab we are going to focus on principles of relative dating. PRINCIPLES OF RELATIVE DATING: A few foundational principles make distinguishing older from younger events relatively simple. Most of these make use of the common sense fact that if event “A” does something to event “B”, then event “B” must be older. Superposition: Rocks deposited on the earth’s surface form layers that are older on the bottom and younger on top. Thus in Figure 1 layer E is the oldest and A is the youngest. This is true for undisturbed sedimentary and extrusive igneous rocks. Superposition does not apply to intrusive igneous rocks and rocks that have been overturned by folding or displaced by reverse faults. Note that Sill F in Figure 2 would actually be younger than layers A, B and C, which lie above it. This is assuming Figure 1 is the starting condition for Figure 2. Original Horizontality: Rocks on the earth’s surface are originally deposited in essentially horizontal layers (Figure 1). Therefore non-horizontal rocks indicate that some younger event has disturbed their original horizontality. In Figure 2, folding would be younger than layers AE, but not necessarily younger than Sill F, because intrusive igneous rocks do not need to be originally horizontal. 1 Figure 2: Original Horizontality and Original Continuity Figure 1: Superposition Original Continuity: Rocks deposited on the earth’s surface form layers that continue laterally in all directions until they thin out as a result of non-deposition, or until they reach the edge of the basin in which they are deposited. Intrusive igneous bodies such as dikes, sills and laccoliths also have a degree of original continuity, but they may terminate by taperingout between the rocks that enclose them (note Sill F in Figure 2). Also, rocks that appear tilted or folded (Figure 2) indicate a tectonic or folding event has occurred and the event is younger than the rocks themselves. Cross-cutting Relations: Geological features are younger than the features they cut. The rule applies to intrusive igneous bodies, faults and erosion surfaces. Thus the erosion surface in Figure 3 is younger than the units it cuts. When a molten rock (magma) pushes through (intrudes) a body of rocks, the resulting igneous rocks must be younger than those rocks which were intruded. Sill F (Figure 2) must be younger than the units above and below it. When an earthquake breaks a group of rocks, a fault forms. More on faults later. Figure 3: Surface Erosion Figure 4: Angular Unconformity Unconformities: If a surface of erosion becomes buried, as G has done in Figure 4, then the feature is called an unconformity. An unconformity is a break in time. They can occur for a variety of reasons, but they always result from an interruption in sedimentation. There are three types of unconformities. 2 1. If the layers below the unconformity are non-parallel to the erosion surface, the structure is called an angular unconformity (Figure 4). Figure 5: Disconformity Figure 6: Nonconformity 2. If the layers below the unconformity are parallel to the unconformity but there is a break in time or an erosional surface, the structure is called a disconformity (Figure 5). In figure 5 we know that there is an unconformity above unit C because Dike x is stops at the top of unit C. Dikes and sills don’t normally stop intruding exactly at a contact between two units. Because of this we can interpret the contact between units C and G to be an erosional surface or unconformity, more specifically a disconformity. 3. A nonconformity overlies metamorphic or plutonic igneous rocks (Figure 6). In other words any place where sedimentary rocks come in contact with crystalline Rocks (metamorphic or igneous). Law of Inclusions: Inclusions are pieces or fragments of one rock type embedded in another. The cobbles in a conglomerate are inclusions of the conglomerate. Similarly, the sand grains in sandstone are inclusions of the sandstone. Plutonic igneous rocks may contain inclusions that form when pieces of a pluton’s wall rocks break off and become incorporated into the crystallizing magma. Inclusions are always older than the rocks they are contained in. In Figure 7 (basically an enlargement of Figure 6), unit G contains inclusions of the granite below it. Thus, the granite inclusions are older than G. Furthermore we can conclude that the granite is not intrusive to G, but that G was deposited on top of eroded granite. 3 Figure 7: The granite inclusions in G are older than G. If we were to put the geologic events that are shown in Figure 7 in order starting with the first event or oldest event it would look the like the table below. Notice that unconformities are listed as a geologic event. Youngest Oldest Deposit H Deposit G nonconformity Granite Reasoning: Because there are inclusions of granite in unit G, we know, based on the law of inclusions that the granite is older than unit G. Therefore, granite is the oldest geologic event. This rational also indicates that there is an erosional surface or unconformity between the granite and unit G. Because granite is a crystalline rock this unconformity is a nonconformity. Unit G is below unit H therefor G must be older then H. For the purposes of this lab we are going to focus on two types of faults, normal and reverse. 4 1. Normal faults form under extension or you could think of it as rocks being pulled apart like they are at divergent boundaries. When extension occurs the hanging wall block, or the side of the fault that makes an acute angle (less than 90°) with the lands surface, moves down relative to the footwall block, or the side of the fault that makes an obtuse angle (greater than 90°) with the lands surface (Figure 8). Figure 8: Cartoon block diagram of a normal fault. Note that the bold angled line in the center is the fault. The hanging wall block (left side) moves down relative to the footwall block (right side), as indicated by the arrows on either side of the fault, under extension. 2. Reverse faults form under compression or you could think of it as rocks being pushed together like at convergent boundaries. When compression occurs the hanging wall block moves up relative to the footwall block (Figure 9). Figure 9: Cartoon block diagram of a reverse fault. Note that the bold angled line in the center is the fault. The hanging wall block (left side) moves up relative to the footwall block (right side), as indicated by the arrows on either side of the fault, under compression. 5 1. Put all 8 geologic events depicted in figure 10 in the proper sequence from oldest to youngest in the table provided. If an unconformity is present indicate what type, angular unconformity, disconformity, or nonconformity. Figure 10: The tilting beds located below unit B indicate an erosional surface or unconformity is present. These tilting beds were originally deposited horizontally, they are tilting now because of a folding event. Youngest 8. 7. 6. 5. 4. 3. 2. Oldest 1. 2. What type of unconformity is L? How do you know? 3. Which rock unit is older J or T? How do you know? 6 4. Put all 14 geologic events depicted in figure 11 in the proper sequence from oldest to youngest in the table provided. Use the event bank below to complete the table, each event will only be used once and all the listed events should be used. You should only fill out boxes with the numbers 1 through 14 (the box labeled ‘Youngest’ is not a box you should be filling in). Figure 11: Granite F is an igneous intrusion with a dike extending towards the surface. Notice that all the beds that granite F cuts are tilted, indicating that the folding event that tilted these beds is most likely due to the intrusion of granite F. Because of this relationship the folding event must be younger then the intrusion of granite F. Event Bank: Deposit H Deposit J Deposit C Deposit B angular unconformity Deposit E Deposit D Deposit K Granite F disconformity Deposit I Deposit G Deposit A folding Youngest 14. 13. 12. 11. 10. 9. 8. 7. 6. 5. 4. 3. 2. oldest 1. 7 Refer to figure 12 to answer the following questions. 5. Are there inclusions of granite in unit A? Which is younger granite or deposition of unit A - H? 6. Based on cross cutting relationships which is older, Fault X or granite? 7. Based on cross cutting relationships which is older, Fault Y or Fault X? 8. What type of fault is Fault Y? 9. What type of fault is Fault X? Figure 12: There are no inclusions of granite in unit A and units A – H are all folded the same, indicating the granite intruded into units A – H after they were deposited and most likely caused the folding event. The basalt cross cuts all other geologic features and therefore must be the youngest event. Event Bank: Deposit A - H Deposit J Granite angular unconformity Fault X Basalt folding Deposit K Deposit I Deposit L Fault Y 8 10. Put all 11 geologic events depicted in figure 12 in the proper sequence from oldest to youngest in the table provided. Use the event bank to complete the table, each event will only be used once and all the listed events should be used. You should only fill out boxes with the numbers 1 through 11 (the box labeled ‘Youngest’ is not a box you should be filling in). youngest 11. 10. 9. 8. 7. 6. 5. 4. 3. 2. oldest 1. THE PRINCIPLE OF FAUNAL SUCCESSION Throughout earth history, organisms have evolved and succeeded each other in a definite and determinable order, a concept known as the principle of faunal succession. By knowing what fossils are contained within a rock, you can determine the age of the rock. The principle of faunal succession is the primary basis for the geologic time scale (Appendix 1). All of the divisions within the geologic time scale are based in large part by the appearance of, the dominance of or disappearance of key fossil groups. These fossils are known as index fossils, and index fossils have a very narrow age range. An age range is the part of geologic time during which a certain fossil species is known to have existed (Figure 13). As is shown in Figure 13, the five fossils each existed during a specific time in the geologic past. You always name the beginning age and then the end age for a fossil’s age range. So for fossil V, the age range is Silurian-Triassic, for fossil W the age range is Triassic-Cretaceous, for fossil X the age range is Ordovician-Jurassic, for fossil Y the age range is Jurassic- Quaternary, and for fossil Z the age range is CambrianDevonian. Figure 14: Examples of four different rock groups containing key index fossils. Use figure 13 to identify age ranges for each group. Figure 13: An example of age ranges for specific index fossils. 9 Ideally, if you can find a rock with an index fossil in it, it is very easy to date the age of the rock. However, many times you don't have a key index fossil present, but usually you have several fossils, each with their own age range. By observing the overlap in age ranges for the fossils you have, you can narrow down the age of the rock in question. This technique is known as biostratigraphy, and it is an extremely important tool in determining the relative age of a rock. For example, in figure 14, the age of rock A is Ordovician-Devonian because that is the only time in the geologic record when both fossils existed. For rock D, the age of the rock can be narrowed down to Jurassic because all three of the fossils only existed concurrently at that time. Once you have determined the age of different rock units, you can correlate different rocks together based on age, and then begin to piece together a detailed history of some area (Figure 15). Figure 15: Example of correlating rocks between outcrops. Notice that there are fossils present other than the index fossils we used to correlate the three outcrops. Because we know the ages of the rocks containing the index fossils we can interoperate the ages of the other fossils, as long as they fall between two index fossils. 11. Use the age ranges in figure 16 to determine the age of the three rocks below. Age: Age: Age: 12. List rocks E, F, and G in order from youngest on the left to oldest on the right. 10 Figure 16: Age ranges for questions 11, 12, 13, and 14. Figure 17: Correlating three outcrops. 13. Fill in the blanks below using figure 17. Layer in outcrop A, correlates to layer correlates to layer in outcrop B. Layer in outcrop B, in outcrop C. 14. Which layer (1 – 12) in outcrops A, B, or C represent rock F from question 11? 11 Appendix 1: Geologic Time Scale 12
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Name:
GEOLOGIC HISTORY
1. Put all 8 geologic events depicted in figure 10 in the proper sequence from oldest to youngest
in the table provided. If an unconformity is present indicate what type, angular unconformity,
disconformity, or nonconformity.

Figure 10: The tilting beds located below unit B indicate an erosional surface or unconformity is
present. These tilting beds were originally deposited horizontally, they are tilting now because of a
folding event.
Youngest 8.
7.
6.
5.
4.
3.
2.
Oldest 1.

Earth’s surface
N
B
L
T
P
A
J

2. What type of unconformity is L? How do you know?
Angular unconformity
In the diagram, it seems like the horizontal pararel strata of the earth’s layers have been laid in deposits
on top of tilted eroded layers of sedimentary rocks. This has led to the creation of a type of angular
disorganization.
3. Which rock unit is older J or T? How do you know?
J is older. Before the top layer of sedimentary rocks got deposited, J was below T.
4. Put all 14 geologic events depicted in figure 11 in the proper sequence from oldest to youngest
in the table provided. Use the event bank below to complete the table, each event will only be
used once and all the listed events should be used. You should only fill out boxes with the
numbers 1 through 14 (the box labeled ‘Youngest’ is not a box you should be filling in).
1

Figure 11: Granite F is an igneous intrusion with a dike extending towards the surface. Notice that all
the beds that granite F cuts are tilted, indicating that the folding event that tilted these beds is most
likely due to the intrusion of granite F. Because of this relationship the folding event must be younger
then the intrusion of granite F.
Event Bank:
Deposit H
Deposit J
Deposit C
Deposit B
ang...


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