(c) In the second cross section of Exercise 17.5, Strophomena is found in E, Pla
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(c) In the second cross section of Exercise 17.5, Strophomena is found in E, Platystrophia and Petrocrania in F, and Petrocra nia, Chonetes, and Neospirifer in C O When were Units E, A, F, and I tilted? Explain. (i) What is the extent of the gap in the geologic record represented by the contact between C and the tilted rocks beneath it? Explain. (ii) When in geologic time did the fault cutting Units K, C, I, F, A, and E occur? Explain. EXERCISE 17.5 Applying Physical Principles of Relative Age Dating (continued) Name: Course: Section Date: Geologic cross section 2. Le 0Explanation / Answer
c) i. The fossil Strophonema is found in E, Platystrophia and Petrocania in F and there is no fossil record from A and I. So from the given age ranges of fossils it has been found that Strophonema extends in Ordovician time and Platystrophia and Petrocania extend from Ordovician to Permian. Again the fossil record in C unit(deposited later) indiactes the age of C ranges from Silurian to Permian. Therefore it can be inferred that the units E,A,F and I were tilted at Ordovician time.
ii. The common fossil found between F and C is Petrocania. So it is not used as an indicator of age. The Platystrophia extends in Silurian age and Neospirifer in C is indicating of age Pennsylvanian to Permian. Therefore there is a gap in geologic record from Devonian to Mississippian.
iii. The age of fault that cuts across the units K,C,I,F, A and E is post Permian because the units are deposited from Ordovician to Permian. From the given figure it has been found that the faulting occurred after deposition of K unit, but as there is no record of age of that unit hence the exact age cannot be determined.
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