Reading a Road Cut: Relative Age Without a Picture
A geologist maps a cliff exposed by highway construction. From bottom to top, the cliff contains horizontal sedimentary layers A, B, C, and D. Layer A is conglomerate, B is shale, C is sandstone, and D is limestone. A vertical igneous dike cuts through layers A, B, and C but stops at the base of layer D. A fault offsets layers A and B, but the fault does not cut layer C. The boundary between C and D is an uneven surface with cobbles of sandstone embedded in the lowest part of D. The geologist also finds a piece of the dike as a loose block inside layer D, but the dike itself does not enter D. A student lists the events as: A, B, C, D, fault, dike. The geologist says the student's list cannot be correct because of cross-cutting relationships and the unconformity. The geologist asks the student to reconstruct the sequence and to explain which evidence is most reliable when the loose block is considered.
1. Order the events from oldest to youngest: deposition of A, B, C, D; faulting; intrusion of the dike; erosion forming the C-D boundary. Justify each placement with a principle.
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Oldest to youngest: deposition of A, deposition of B, faulting, deposition of C, intrusion of the dike, erosion forming the C-D boundary, deposition of D.
A and B are oldest because they are at the bottom and superposition says lower sedimentary layers are older, assuming the layers have not been overturned. The fault must be after B because it offsets B, but before C because it does not cut C; if the fault were younger than C, it would offset C as well. C is therefore after the fault. The dike cuts A, B, and C, so it is younger than C by the cross-cutting principle. The dike does not enter D, so it is older than D. The uneven C-D boundary with cobbles of sandstone in the base of D is an unconformity: erosion removed some material and created a surface before D was deposited. The dike must be older than that erosion surface because the dike stops at the base of D and does not cut D; the erosion surface represents a gap after the dike and before D. D is youngest because it lies above the unconformity and is not cut by the dike or fault.
2. The loose block of dike inside layer D: a classmate says this proves the dike is younger than D. Evaluate that reasoning. What could explain the block without making the dike younger than D?
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The classmate's reasoning is backwards. A loose block of dike rock inside layer D means a piece of the dike was already present, then weathered out, transported, and deposited with the sediment that became layer D. That makes the dike older than D, not younger. If the dike were younger than D, the dike would cut across D in place, and we would see a cross-cutting relationship, not just a loose block.
The block could be a reworked clast: erosion of the dike exposed at the surface produced a fragment that was incorporated into the youngest sediment. This is consistent with the dike stopping at the base of D: the top of the dike may have been eroded before D was deposited. The most reliable evidence is the in-place cross-cutting relationship, not the loose block. The loose block is useful supporting evidence, but it must be interpreted as a clast, not as an intrusion into D.
3. Explain why the fault cannot be younger than layer C, using the evidence given. Then state one additional observation that would test whether the dike is older or younger than the fault.
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The fault offsets layers A and B but does not cut layer C. By the cross-cutting principle, a fault must be younger than the layers it offsets, so the fault is younger than A and B. If the fault were younger than C, it would also cut C, because a fault that forms after C would break the already-deposited C layer. Since C is unbroken by the fault, C must have been deposited after the faulting event, burying the fault surface. Therefore the fault cannot be younger than C; it is older than C.
To test the dike-fault relationship, look at the intersection of the dike and the fault. If the dike cuts across and offsets the fault, the dike is younger than the fault. If the fault offsets the dike, the fault is younger. If they do not intersect, the relative age may remain unresolved from that exposure alone, and another outcrop would be needed.