Stratigraphy Puzzles: Understanding Rock Layering Concepts

Week 14: Stratigraphy Puzzles Stratigraphy is the study of how sedimentary rocks are layered. For instance, you might wonder if the many layers of the Grand Canyon have dinosaur fossils. By studying these layers, stratigraphers determined that the rocks were too old and that many of them formed in marine environments. So no dinosaurs in the Grand Canyon. Directions: While reading this PowerPoint, jot down the correct answers for questions 1-22 and then enter your answers into the Canvas quiz.

1- The Law of Superposition This law states that sedimentary rocks are laid down over time with the lowest layer being the oldest and the top layer being the youngest. This cliff is located in the Virgin River Valley near Zion National Park. The top layers are part of the Chinle Formation which is about 230 million years old and contain some of the oldest dinosaur fossils. The bottom layers form the Moenkopi Formation which is about 240m.y. old and pre- date dinosaurs so they only contain reptile and amphibian fossils. Question 1 – Which layer is youngest – the Chinle or the Moenkopi? youngest oldest

1- The Law of Superposition Physical anthropologists use the principles of stratigraphy (layering of sedimentary rocks) to understand the age of hominid fossils. The Olduvai Gorge is a famous location that the Leakey family discovered in Tanzania. Question 2: Which bed is the oldest? Bed I, II, III or IV Research Gate Dec. 2012

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2- Law of Original Horizontality Most sedimentary rocks are laid down in horizontal layers by water in rivers, lakes and the oceans. Over time these originally flat layers may folded and eroded. With folding and erosion, older layers can be brought up to the surface. Question 3 - In the top diagram, which is the oldest layer? A, B, C, D, or E Question 4- In the bottom diagram, how did the older layer D get to the surface? a) Layer D was laid down last b) Layer D was folded up and then overlying layers were eroded down c) Layer D was folded down and then covered by other eroding layers d) Layer D was laid down first D E d e D E Diagram 1 Diagram 2 Diagram 3 D E

2- Law of Original Horizontality Because of regional compression, many sedimentary rocks are folded and no longer horizontal. The red sandstones in this picture are also from the Moenkopi Formation and this hillside is near Dinosaur National Monument. The red color is from the oxidation of the original iron minerals to hematite . Question 5 – Which layer is youngest A, B, C, D or E? Question 6 – What causes the bright red color of the Moenkopi sandstones? a) volcanic ash layers b) organic material c) oxidation of iron d) reduction of iron A B C D E

3- The Principle of Lateral Continuity – states that sedimentary rocks form in continuous lateral beds that often extend for miles. Anything that interrupts the horizontal beds like a river or a canyon happened after the beds formed. The meanders in the San Juan River shown below cut through the horizontal layers that are laterally continuous throughout this landscape. You can visit this overlook at Goosenecks State Park in southwestern Utah. Question 7 - What came first here? The river or the horizontal sedimentary beds.

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3- The Principle of Lateral Continuity This diagram shows how the sedimentary rock beds used to extended across the valley but were fold and then eroded away. The beds are identical on either side of the valley which is an example of lateral continuity. Question 8 – What was the order of events for this diagram from oldest to youngest? a) K, J, L b) J, K, L c) L, J, K d) K, L, J K = depositions of 4 horizontal layers J= folding of the rocks L = erosion of the valley

4- The Principle of Cross Cutting Relationships If magma or a fault cuts across rock, then the rock had to exist first. This is called the Principle of Cross Cutting. In the picture, dark basalt cuts across lighter granite. So the granite had to exist first before the basalt lava could cut through it. Question 9: Which oldest A or B? A B

4- The Principle of Cross Cutting Relationships Two normal faults have dropped a triangular block or rock down in this photo. So the horizontal sedimentary layers had to pre-exist before the fault broke through the rocks. Question 10- What is the sequence from oldest to youngest here? a) A, B, C, D, F, E b) F, E, D, C, B , A c) A, B, C, D, E, F d) F, A, B, C, D, E A B C D E F = fault

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4- The Principle of Cross Cutting Relationships In this diagram, there are three sedimentary layers: C, B, and A. These three sedimentary layers are then intruded by a magma D and then split by the normal fault B. Question 11: What is the sequence from oldest to youngest? a) A, B, D, C, E b) A, B, C, D, E c) C, B, A, E, D d) C, B, A, D, E

5- The Principle of Inclusions The Principle of Inclusions features older rocks trapped inside younger rocks. At the right a chunk of dark pyroxenite is trapped inside a lighter lava rock called trachyte. Question – 12 Which is older A or B? A B

5- The Principle of Inclusions The originally horizontal sedimentary layers (A-J)have been tipped up on an angle. Then the granite (K) broke its way through the sedimentary layers and gobbled up pieces of the sedimentary rocks as inclusions. The step (L) was erosion and it leveled everything off like an eraser. Finally sedimentation resumed to produce layers M-P. Question 13 Which is the oldest layer here? A, K or P Question 14 Are the granite inclusions in layer M younger or older than layer M? A B C D E F G H I J K L M N O P inclusions

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6- Unconformities An unconformity in rock layers means that erosion has taken place sometime in the past. Erosion surfaces are usually rough appearing. There are four main types of unconformities: a) Disconformity is when erosion has occurred between 2 sedimentary rock beds. b) Nonconformity is when erosion occurred on an igneous (granite) or metamorphic (marble) layer and then covered by sedimentary rock. c) An angular unconformity occurs between a tilted bed and a flat overlying bed of sedimentary rock. d) A paraconformity occurs between 2 very flat lying sedimentary layers. It is really hard to pick out.

6- Unconformities Angular unconformities are very common in sedimentary rocks. Question 15 – What type of rocks are laid down in water? a) Igneous b) Sedimentary c) Metamorphic Question 16 – What might cause rocks to deform? a) Magma intrusion b) Compression from plate tectonics c) Both of the above Question 17- What type of conformity is shown in step 4 of the diagram? a) Nonconformity b) Paraconformity c) Disconformity d) Angular unconformity

Question 18 In this diagram, N, A, T and S are sedimentary layers. G D is a magma intrusion that cuts across the 3 lower sedimentary layers. J U is an erosion surface that partly destroys Layer T and then layer S covered everything. Question: What is the sequence here from oldest to youngest? a) N, A, T, G D , J U , S b) N, A, T, J U , G D , S c) G D , N, A, T, J U , S d) S, J U , G D , T, A, N

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Question 19 In this diagram, there are five sedimentary layers, one erosion layer and one fault. List the sequence from oldest to youngest: a) T, R, C U , G F , M, V, J b) T, R, G F , C U , M, V, J c) J, V, M, C U ,G F , R, T d) J, V, M, G F , C U , R, T

Question 20 In this diagram, there are 8 sedimentary layers, 2 erosion surfaces, one igneous (magma) intrusion and one tilting event labeled as M. List the sequence from oldest to youngest. a) C, Q, E, T, Y, M, G U , H, N, R D , S U , V b) Y, T, E, Q, C, M, R D , GU, H, N, SU, V c) Y, T, E, Q , C, M, G U , H, N, R D , S U , V d) V, S U ,R D , N, H, G U , M, C, Q, E, T, Y

Question 21 In this diagram, there are six sedimentary layers, three erosion layers, one fault and one magma intrusion. Remember this fault could have made a sudden jump in one day but erosion cold continue for thousands of years. What is the sequence from oldest to youngest? a) D, S, H U , K, B U , N, A, X U , L U , G, M D , P F b) G, L U , N, B U , X U , A, P F , K, M D , H U , S, D c) G, L U , N, B U , A, P F , X U , K, M D. H U , S, D d) G, L U , N, B U , A, X U , P F , K, M D , H U , S, D X U

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Question #22 – worth 12 points. This question comes from Richard Harwood’s lab page with his permission 11/16/20. But if you need help on the puzzle don’t e - mail him because he isn’t a Chaffey tutor  ! http ://profharwood.x10host.com/GEOL101/Labs/Dating/index.htm Instructions: 1) Go to website and do the first puzzle called “Relative Dating #1”. You will need to enter the answers “tilting” twice and “erosion” five times in 1 -20. Erosion is represented on the diagram as a roughly drawn line instead of a ruler straight line. The word “folding” is not used in this puzzle. You can repeat it multiple times. 2) Once you get all of the 20 steps correct, record your answers on paper and then put them into the Canvas quiz for matching. 3) The second puzzle entitled “Relative Dating #2” is optional and can be used as extra credit to replace any one lab exercise or discussion. If you choose to do the second puzzle, just e-mail me your responses.

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