Roadcut Stratigraphy, an Ash Date, and a Dinosaur Bone
During a field lab at a roadcut, a high-school class records four sedimentary units from bottom to top: (A) cross-bedded sandstone with shell fragments, (B) dark shale containing ammonites, (C) a 12 cm volcanic ash layer, and (D) mudstone with a hadrosaur limb bone. A normal fault offsets units A, B, and the ash layer by about 3 m but does not cut unit D; the fault surface is weathered and truncated at the base of D. The ash yields a U-Pb zircon date of 74.2 +/- 0.6 Ma. The bone is too altered for direct radiometric dating. A student argues that the bone must be exactly 74.2 million years old because it lies above the ash. The teacher wants students to reconstruct the sequence of events and to evaluate that claim using stratigraphic principles.
1. Place the units and events in relative order from oldest to youngest, and justify each step with a stratigraphic principle.
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The oldest-to-youngest sequence is: A, B, ash layer C, faulting, erosion or nondeposition, D, then the bone within D.
- A is below B and the ash, so superposition makes A older than B and C.
- B is below the ash, so B is older than the ash.
- The fault cuts A, B, and the ash. The principle of cross-cutting relationships says the fault is younger than the units it cuts.
- The fault does not cut D, and the fault surface is truncated at the base of D. Therefore D was deposited after faulting, and the fault is older than D.
- The bone is contained in D, so the bone is younger than the fault and is the same age as deposition of D, unless it was reworked.
The weathered fault surface also suggests exposure and erosion before D was deposited. The ash date applies directly to the ash and only indirectly to the bone.
2. Evaluate the student's claim that the bone is exactly 74.2 Ma. What age range is justified, and what assumptions matter?
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The claim is too precise and not justified. The ash is below the bone, so the bone is younger than 74.2 +/- 0.6 Ma, but the data do not show how much younger. There could be a long interval of erosion or nondeposition between the ash and unit D. The bone could be 70 Ma, 65 Ma, or another younger age; the ash date is a maximum age for the bone, not its actual age.
The ash date also depends on assumptions: the zircons are volcanic rather than reworked detrital grains, the U-Pb system has not been disturbed, and the dated ash is the same layer sampled everywhere. If the zircon grains were reworked from older rock, 74.2 Ma would be a maximum age for the ash itself.
A defensible statement is: 'The bone is younger than about 74.2 Ma, based on superposition above the dated ash, but its exact age is unknown because the bone cannot be dated directly and the time gap above the ash is unconstrained.' Additional biochronology or a dated ash above D would narrow the range.
3. A second student says the fault must be younger than unit D because faults are always younger than the rocks they cut. Identify the reasoning error and describe one observation that would test whether faulting continued after D was deposited.
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The reasoning error is misapplying the cross-cutting principle. A fault is younger than the rocks it cuts, not younger than every rock in the outcrop. Here the fault cuts A, B, and the ash but stops at the base of D. The principle therefore says the fault is younger than the ash and older than D, not younger than D. The student has reversed the logic.
To test whether faulting continued after D was deposited, look for deformation within D: offset layers, drag folds, slickensides, fault gouge, or a scarp cutting D. If D is undeformed and lies flat across the fault trace, faulting probably ended before D was deposited. If D is offset or contains fault rock, faulting continued after deposition. A dated dike or fault gouge could also constrain the timing, and a regional map might show whether the fault trace continues into younger units.