Numerical and Relative Age Dating Assignment (4)

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Georgia State University *

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1122

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Geology

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Apr 3, 2024

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RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) When you arranged historic photos of GSU in order from oldest to youngest, you were applying principles of relative age dating, just as you would in determining the history of a sequence of rocks. You analyzed photos of GSU locations as older or younger in comparison to other photos and you looked for clues in the photos such as hairstyles, clothes, and cars from previous decades just as you might study a rock layer for fossils to provide insight as to a specific moment in history. Here you’ll learn about the principles of relative age dating in more depth and how to apply them to determine the order of events when studying a cross-section of rock layers. Just as you did in class for historic GSU photos, you’ll write the letters of rock layers and related events from oldest to youngest. PRINCIPLES OF RELATIVE AGE DATING Relative dating involves putting geologic events in order to determine the age of a rock relative to other rocks, that is, assigning whether a rock is older or younger than other rocks. The entire geologic time scale is actually based on the principles of relative dating because until recently, methods had not yet been developed to calculate numerical ages for rocks. Instead, ages of rocks were dated as “Jurassic” or “Devonian”, names which refer to certain periods in earth history when specific groups of fossils are known to have existed. The principles of relative dating are the rules that guide geologists when they interpret the geological history of a series of rocks. Principle of Superposition: Sedimentary rocks form when sediments such as sand, silt, or clay settle out of the water column and fall to the bottom of a basin, creating layers that become lithified. In any sequence of undeformed sedimentary rocks, layers at the bottom are going to be older than the ones near the top, because rocks at the bottom of a sequence must have been deposited first (in order for other rocks to have formed on top of them). See Figure 1a where A is the youngest rock and E is the oldest rock. P rinciple of Original Horizontality: Sediments under the influence of gravity, are almost always laid down in horizontal layers called beds (as they fall to the bottom of a basin when settling out of the water column). Thus, if you see rock layers that are tilted, it can be assumed that they have been changed from their original position (Figure 1b). Figure 1: Principles of relative dating show that the rocks in a) are horizontal compared to b) after having been tilted due to a tectonic event. Comprehension Check #1: a) Is rock C older than D in Figure 1? Explain. No Superposition says the closer the top, the younger the layer. Figure 2 : Principles of relative dating focusing on cross-cutting relationships. (a) Faulting of rocks. Rocks A-E are older than the fault as it cuts through the rock layers. (b) Igneous intrusions and lava flows. These igneous features are younger than rocks A-E. Principle of Cross-Cutting Relationships: Rock layers that are changed due cross-cutting phenomena include faults that break through rocks and igneous intrusions. The first scenario happens when a fault forms due to earthquake activity, and so rocks are then broken up. Thus, a fault that cuts through a rock is going to be younger that the rock itself as it is a newer event (Figure 2a). The second scenario is when molten rock (magma) pushes through (intrudes) a body of rocks, and so the resulting igneous rocks must be younger than those rocks which were intruded (Figure 2). Likewise, lava flows must be younger than rocks they cover up. Comprehension Check #2: a) Is rock B or D older in Figure 2a? Explain. I believe B is older due to cross cutting. Also, its below D b) Is the lava flow or rock E younger in Figure 2b? Explain. The lava flow is the youngest sense it says that lava flows must be younger than rock they cover up and its covering up E.

© 2008 Kendall Hunt Publishing Company and Brent Zaprowski RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) Principle of Inclusions: Rocks that have broken apart from their original source rock and are later preserved in a different rock layer are called inclusions. The layer with the inclusions is younger than the inclusions (having originated from older rock). Thus, inclusions are older than the surrounding rock in which they are preserved. Unconformities: The surface between distinct rock units is called a contact. Sometimes, contacts between layers are what are called unconformable, which indicate a gap in sedimentary deposition. There are 3 major types of unconformities. Angular unconformities are when newer sedimentary layers are situated on top of older tilted sedimentary rocks (Figure 4). Disconformities are when sedimentary deposition is on an erosional surface that is roughly parallel to the underlying undeformed sedimentary layers such as seen in Figure 3 whereby D and E are separated by a wavy surface indicating erosion. Nonconformities are when sedimentary rocks overlie a surface under which are older igneous or metamorphic rocks. Hint: Each of the following geologic cross-sections features one or more unconformities demonstrating a ‘change’ in between the expected layering of rock units over time. 1) Of the rocks A-F, which of the rocks was deposited first and is the oldest ? D 2) Of the rocks A-F, which of the rocks is the youngest and was deposited last? F 3) Which occurred first: The tilting of A, B, C and D or the deposition of E? The tilting happened first 4) Which occurred first: The erosion of F or deposition of A, B, C, D? The deposition of A, B, C, D 5) Write the order of events for the entire sequence including any tilting, erosion, or unconformities. Start with the oldest rock. First deposition of D, C, B, A, the tilt of DCBA, Deposition of E, F, then the erosion of F last Figure 4 Earth’s surface F E A B C Figure 3: Principles of relative dating. The inclusions in rock layer E are visible above the erosional surface between D and E. This indicates that the inclusions originated from rock layer D and so are older than the layer E in which they are preserved. G F E D C B A Comprehension Check #3: a) Is rock F or the inclusions in rock E older? Explain. Rock E because the inclusions are older. b) Are the inclusions in rock E or rock D older? Explain. Rock D is older because it makes up Rock E Use Figure 4 to answer the following questions now that you have learned principles of relative age dating:

RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) D

© 2008 Kendall Hunt Publishing Company and Brent Zaprowski RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) Figure 5 Answer the below questions based on Figure 5: Note: E is an unconformity. 6) Which of the rocks was formed first and is the oldest ? M 7) Which of the rocks is the youngest and was formed last? H 8) Which occurred first: The folding of M, F, P, X or the deposition of F and G? Deposition of F but I don’t see G 9) Which occurred first: The erosion that led to the surface E or the intrusion by dike H? Erosion to surface E 10) Write the order of events for the entire sequence including any folding or erosion. Start with the oldest rock. Deposition of M, F, P, X, Erosion of X, Deposition of W, P, Intrusion by dike H Figure 6 M F W H P X P

(Appendix 1) Age range Age range Age range 14) Using fossils as a guide and indicator of different periods by assigning ages, you can correlate different sets of rocks across distances to develop a detailed understanding about the history of a region. Using your skills gained in this lab, which is the oldest outcrop in Figure 8? Which is the youngest outcrop? Explain. The oldest outcrop is A and the youngest outcrop is C. Because for one the assemblage of outcrop A is at the bottom of outcrop B, that means that outcrop B has younger sediment or rock than A. Vice versa for B and C. RELATIVE AGE DATING GEOL 1122: Lab #7 (Part I) 11) Write the order of events for this cross-section. Include any tilting, faulting, or erosion. Start with the oldest rock. 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 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. 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 , and so are key indicators of specific periods in earth’s history. An age range is the part of geologic time during which certain fossil species are known to have existed (Figure 7). As is shown in Figure 7, fossils V, W, X, Y, and Z each existed during a specific time, and the arrow next to the species indicates the extent of the age range for when that organism existed in earth history. The age range is always described starting with the oldest age first and then finishing with the end of the species range. So for fossil V, it’s age range is Silurian-Triassic. For fossil W, it is Triassic-Cretaceous instead. For fossil X, it’s Ordovician- Jurassic, and then Jurassic- Quaternary for fossil Y, and finally, for fossil Z it’s age range is Cambrian-Devonian. T Figure 7: An example of age ranges. Quaternary T ertiary Fossil Y Rock A Rock B Cretaceous Jurassic Triassic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian Fossil W Fossil V Fossil X Fossil Z Rock C Rock D Figure 8: An example of biostratigraphy. Based on the age ranges in Figure 6, Rock A is Ordovician-Devonian, as it is the only time during which both species lived. Rock B is Triassic following the same logic that the rock had to have formed when both species co-existed in order for them to end up in the same rock. Figure 8 Use Figure 7 to answer the following questions: 12) If a rock has fossils W and X, what is the age of the rock? Triassic-Jurassic 13) If a rock has fossils Y and W, what is the age of the rock? Jurassic-Cretaceous Comprehension Check #4: a) What is the age of Rock C Silurian-Devonian b) What is the age of Rock D? Jurassic

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