Lab 8 - Geologic Time - Handout.docx (1)

GEOL101 Dynamics of the Earth – Fall 2023 Name: Emily Thomson Laboratory 8 Geologic Time . . . keeps on ticking, ticking Section: Learning Outcomes: ● Differentiate absolute and relative dating ● Describe and apply the six principles of relative dating to reconstruct geologic events ● Differentiate between the various types of unconformities ● Determine the relative age of a sample by comparing its fossil composition using the principles of faunal succession Introduction One of the most fundamental techniques in geology is dating rocks and putting geologic events in their proper historical sequence. There are two ways by which this can be accomplished. Absolute dating involves determining the age of the rock in years (e.g., 125,000 years; 2.6 billion years). The most common form of absolute dating is radiometric dating, where specific minerals are analyzed to determine (1) the amount of remaining original “parent” isotope (e.g., Uranium-235) incorporated during the rock’s formation to (2) the amount of initially-absent “daughter” isotope (e.g., Lead-207) produced by the progressive radioactive decay of the parent over time. This method involves complex instrumentation and careful selection and screening of samples. We won’t cover absolute dating more in this laboratory, but it is important to realize that geoscientists apply this technique to various mineral-isotope systems to estimate the absolute age of geologic events on timescale ranging from thousands to billions of year. Even if you cannot tell the precise age of a given 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. Geologic Time Scale The geologic time scale, shown on the next page, reflects how scientists from all over the world have worked as a community for over 150 years to divide the ancient record of life and our planet into highly resolved intervals of geologic time. Most of the major boundaries between these time intervals are based upon major changes in the fossil record; for example, the Cretaceous-Paleogene (K-P) boundary is defined by the extinction of the dinosaurs from a meteorite impact over 65 million years ago. Thus, the geologic time scale acts as both a calendar and a file cabinet for our growing understanding of the history and dynamics of our complex Earth system. Notice that the time scale on the next page is divided into finer and finer time intervals from right to left; for example, the Cenozoic Era includes the Paleogene , Neogene , and Quaternary Periods , while the Paleogene Period includes the Paleocene , Eocene , and Oligocene Epochs . An interesting fact about the geologic time scale is that most of it was developed long before the discovery of radioactivity – a discovery that subsequently allowed geoscientists to add absolute dates to all the boundaries largely defined initially by fossils. This lab is being used and was modified with permission from Gary Jacobson of Grossmont Community College.

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 have already been in existence and therefore must be older. Below are six principles to help you order geologic events in time. 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 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 A-E, but not necessarily younger than Sill F, because intrusive igneous rocks do not need to be originally horizontal. 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 tapering-out 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.

Figure 5: Disconformity Figure 6: Nonconformity 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. Figure 7: The granite inclusions in G are older than G. If we were to put the geologic events in Figure 7 in order starting with the oldest event, then the order would be: granite formation, non-conformity, G deposition, H deposition. Notice that the unconformity is considered a geologic event since it involves the erosional removal of some amount of material. Why this ordering? Because granite inclusions are found 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 rationale also implies a non-conformity between the granite and unit G. Unit G is below unit H, and therefore unit G must be older than H. For the purposes of this lab we are going to focus on two types of faults, normal and reverse. 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 land’s surface, moves down relative to the footwall

block, or the side of the fault that makes an obtuse angle (greater than 90°) with the land’s surface (Figure 8). Figure 8: Cartoon block diagram of a normal fault. 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. 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. 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.

Question 3. Which rock unit is older J or T? How do you know? J is older because this layer is the farthest from earth's surface where the youngest unit would be. The way it is tilting shows this is the bottom layer making it older. Question 4. Put all 14 geologic events depicted in Figure 11 in the proper sequence from oldest to youngest in the table below. Use the event bank below to complete the table; each event will only be used once. 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 than 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 10. C 5. A Youngest 14. H 9. angular unconformity 4. granite F 13. D 8. folding 3. G 12. E 7. K 2. I 11. disconformity 6. B Oldest 1. J

Refer to Figure 12 to answer the following questions. Question 5. Are there inclusions of granite in unit A? Which is younger granite or deposition of unit A - H? no there are inclusions of granite in unit A Question 6. Based on cross cutting relationships which is older, Fault X or granite? granite is older Question 7. Based on cross cutting relationships which is older, Fault Y or Fault X? fault x is older Question 8. What type of fault is Fault Y? normal fault Question 9. What type of fault is Fault X? reverse fault

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 above, 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. Question 11. Use the age ranges in figure 16 to determine the age of the three rocks below. Age: Cambrian to Cretaceous Age: Premian to quaternary Age: cambrian to pennsylvanian Question 12. List rocks E, F, and G in order from youngest on the left to oldest on the right.

Figure 16: Age ranges for questions 11, 12, 13, and 14. Figure 17: Correlating three outcrops. Question 13. Fill in the blanks below using figure 17. Layer _2_____ in Outcrop A correlates to Layer___8___ in Outcrop B. Layer _5_____in Outcrop B correlates to Layer __12____ in Outcrop C. Question 14. Which layer (1 – 12) in outcrops A, B, or C represent rock F from Question 11. Layer 10 in outcrop C