A Roadcut With a Fault, a Dike, and a Conglomerate
A roadcut exposes six units. From bottom to top: unit 1, limestone with coral fossils; unit 2, shale with trilobites; unit 3, cross-bedded sandstone; unit 4, shale with ammonites; unit 5, conglomerate containing pebbles of units 1–4; unit 6, flat-lying siltstone with no fossils. A fault offsets units 1–4 but does not offset unit 5. A basalt dike cuts units 1–5 but stops at the base of unit 6. The class chart gives fossil ranges: trilobites 521–252 million years ago, ammonites 201–66 million years ago, and coral 535–0 million years ago. A student argues that because the dike does not cut unit 6, unit 6 must be younger than the dike, and that the fault must be younger than unit 5 because unit 5 is not broken. Another student says the conglomerate proves unit 5 is the oldest unit. The teacher asks for an event sequence and for an evaluation of both claims.
1. List the geologic events from oldest to youngest, including the fault and dike. Justify each placement with a principle.
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Oldest to youngest: units 1, 2, 3, 4 (superposition, bottom to top), then the fault, then unit 5, then the dike, then unit 6. The fault cuts units 1–4, so it is younger than unit 4; unit 5 is a conglomerate containing pebbles of 1–4 and is not offset, so it was deposited after faulting. The dike cuts units 1–5, so it is younger than unit 5. The dike stops at the base of unit 6, so unit 6 is likely younger than the dike if unit 6 was deposited on top after the dike; however, the absence of cutting alone does not prove that, because an intrusion can stop at a boundary. The fossil ranges support the sedimentary order: trilobites in unit 2 (521–252 Ma) are older than ammonites in unit 4 (201–66 Ma).
2. A student claims the fault must be younger than unit 5 because unit 5 is not offset. Evaluate this claim.
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The claim is wrong. A fault that offsets units 1–4 but not unit 5 is older than unit 5, not younger. The likely history is that faulting broke units 1–4, erosion produced pebbles, and unit 5 was deposited across the fault, burying it. Unit 5 is not offset because it did not exist when the fault moved. To say the fault is younger than unit 5, the fault would have to cut unit 5.
3. Which unit's fossils best constrain the maximum age of the fault? Explain your reasoning.
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Unit 4 gives the best maximum age for the fault. The fault cuts unit 4, so the fault must be younger than unit 4. Unit 4 contains ammonites, whose chart range is 201–66 Ma, so the fault is younger than 201 Ma. Unit 2's trilobites only limit the fault to younger than 252 Ma, which is less precise. Unit 5 is younger than the fault, so it gives a minimum age, not a maximum.