Reading the Rocks at a Faulted Coastline
Geologists map a coastal cliff exposing three rock units. Unit X is a layered sandstone containing marine fossils. Unit Y is a basalt intrusion that cuts across Unit X. Unit Z is a loose gravel layer resting on an eroded, uneven surface carved into both X and Y. A fault offsets Unit X by 4 m but does not cut Unit Z. Fossil evidence in X indicates an age of about 90 million years; radiometric dating of Y gives 60 million years. A road is planned along the cliff, and engineers ask which unit is most stable for anchoring and what the relative timing of events was.
1. Order the geological events from oldest to youngest and justify each step using principles of relative dating.
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Oldest to youngest: (1) Deposition of sandstone X; (2) intrusion of basalt Y, which cross-cuts X, so Y is younger than X (cross-cutting relationships); (3) faulting that offsets X but not Z, so the fault is younger than X and Y but older than Z; (4) erosion forming the uneven surface; (5) deposition of gravel Z on top (superposition). The radiometric dates agree: X at 90 Ma is older than Y at 60 Ma. Z and the fault postdate Y, so they are younger than 60 Ma. Note the fault does not cut Y in this description, so it postdates X; if it also offset Y, it would postdate Y.
2. Which unit is the least suitable for anchoring a road and why?
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Unit Z, the loose gravel, is least suitable. It is unconsolidated, so it has low shear strength and is prone to sliding, especially on a steep coastal cliff. Units X and Y are solid rock, though the fault zone in X is a weakness. Anchors should be placed in competent bedrock (X or Y) away from the fault, not in the gravel.
3. A resident claims the basalt is older because it is 'below' the sandstone in one outcrop. Evaluate this claim.
Read the worked answer
The claim is incorrect. The basalt is an intrusion that cuts across the sandstone, and cross-cutting relationships show the intrusion is younger than the rock it cuts, regardless of local elevation. The radiometric dates confirm Y (60 Ma) is younger than X (90 Ma). Local position 'below' can be misleading where intrusions rise from depth.