Reading a Roadcut: Trilobites, a Dyke, and Three Dates
A new roadcut exposes the following sequence, listed from bottom to top.
- Steeply folded, dark shale and sandstone containing fragmented trilobites.
- A wavy erosion surface cut into the folded beds, overlain by boulders of that same sandstone.
- Flat-lying conglomerate and sandstone, with a thin volcanic ash bed near the base.
- A vertical basaltic dyke that cuts layers 1 and 3 but stops abruptly at the base of layer 5.
- A horizontal basalt lava flow forming the top of the cut.
Laboratory results: zircon from the ash bed gives 65 ± 1 Ma; the dyke gives 40 ± 1 Ma; the lava flow gives 12 ± 0.5 Ma. A visiting student says the trilobites prove the shale is about 500 million years old and asks why the ash is so much younger. The roadcut is the only exposure for 30 km in any direction.
1. Place the events in order from oldest to youngest and justify each step with a named principle.
Read the worked answer
Order: (a) deposition of the shale/sandstone of layer 1; (b) folding of those beds; (c) uplift and erosion producing the wavy surface of layer 2; (d) deposition of layer 3, including the ash; (e) intrusion of the dyke; (f) eruption of the lava flow of layer 5.
Justification, step by step:
- Layers 1 and 3 are sedimentary, so original horizontality and superposition place 1 below 3. The folding must postdate lithification - soft sediment deforms, it does not fold brittlely on this scale.
- The erosion surface is an angular unconformity: the lower beds were tilted and beveled before the flat-lying conglomerate was deposited on top. The sandstone boulders in layer 3 are inclusions derived from layer 1, which independently requires layer 1 to be older.
- The dyke cross-cuts layers 1 and 3 (cross-cutting relationships: a feature that cuts another is younger), but it is truncated at the base of layer 5, so the flow must postdate the dyke.
- The flow sits on top, consistent with superposition.
Dates test the order: ash 65 Ma < dyke 40 Ma < flow 12 Ma. The order derived from geometry matches the radiometric order exactly, which is the strongest kind of confirmation available at a single outcrop.
2. Assess whether the three dates are mutually consistent, and say precisely what would follow if the dyke had instead given 200 ± 2 Ma.
Read the worked answer
Consistency check: the ash (65 Ma) is in layer 3, which the dyke cuts, so the dyke must be younger than 65 Ma - 40 Ma satisfies this with 25 Myr to spare. The flow (12 Ma) overlies the dyke's truncation surface, so it must be younger than 40 Ma - satisfied with 28 Myr to spare. The uncertainties (±1, ±1, ±0.5 Myr) are far smaller than the gaps, so no overlap issue arises. The dates are not just consistent; they are comfortably so.
If the dyke gave 200 Ma: that contradicts the cross-cutting relationship, because a 200 Ma dyke cannot cut a 65 Ma conglomerate. A date never overrides field geometry; when they conflict, one of the two is wrong. The plausible explanations are:
- Inherited or excess argon/zircon xenocrysts - the dyke picked up older crustal material, so the analysed grains record the source age, not the intrusion age.
- The contact was misread - perhaps the dyke does not actually cut layer 3 but is truncated by it and only appears to pass through in the weathered face.
- Analytical problems (unreset system, poor plateau, contamination).
Correct response: re-examine the contact, re-sample away from the chilled margin, and date a phase with a higher closure temperature or use several systems (e.g. Ar–Ar on hornblende plus U–Pb on zircon) before publishing an age that contradicts the map.
3. Explain why the shale itself cannot simply be dated, and state exactly what the trilobites do and do not establish.
Read the worked answer
Why direct dating fails: a shale is a mechanical mixture of detrital grains that were already old when they were deposited. A radiometric date on those grains returns the age of the source rock, not the depositional age - a maximum age at best. The authigenic material that could date deposition (clay minerals, glauconite, carbonate) is readily reset by burial heating and weathering, so it is unreliable. The workable approach is to date a primary volcanic ash interbedded with the sediment: its zircon crystallises at eruption and is deposited essentially instantaneously, so its age is the depositional age.
What the trilobites establish: trilobites are an exclusively Palaeozoic group, so the shale is constrained to roughly 541–252 Ma. That is a broad biostratigraphic bracket, not a date.
What they do not establish: 'about 500 million years' is an over-reading - 500 Ma is within the range but so is 300 Ma. The ash gives 65 Ma for layer 3, which sits above the unconformity, so the shale is older than 65 Ma - entirely consistent with the trilobite bracket, and adding nothing to it. The student's error is treating a fossil range as a point estimate.
Also worth stating: because this is the only exposure for 30 km, all regional correlation rests on one outcrop, which is a reason for caution in any larger interpretation, and a reason to look for the ash horizon elsewhere.