A Trilobite in a Cretaceous Sequence
Three road cuts expose the same rock sequence. At Cut 1, a basal conglomerate contains rounded granite pebbles and a broken trilobite; above it, shale contains ammonites and an ash bed. A lab dates zircons from the ash at 78.2 ±0.4 Ma. At Cut 2, the same shale contains a bone fragment and a Cretaceous pollen assemblage. At Cut 3, a basalt dike cuts the shale but not the overlying sandstone. A colleague argues that the trilobite proves the shale is Paleozoic and that the ash date must be wrong because the zircon grains could be detrital. The granite pebbles give a whole-rock Rb-Sr age of about 1.1 Ga. No index fossil from the shale has yet been identified to species level.
1. Reconstruct the order of geologic events and evaluate the colleague's trilobite argument.
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The order is: deposition of the conglomerate (containing older granite and trilobite clasts), then deposition of the shale with ammonites and pollen, then eruption and deposition of the ash at 78.2 Ma, then lithification, then intrusion of the basalt dike, then erosion that produced the road cuts. Superposition places the conglomerate below the shale; cross-cutting relations place the dike after the shale but before the overlying sandstone if the dike does not cut that sandstone. The trilobite does not date the shale. It is in a conglomerate, it is broken and rounded, and it is associated with 1.1 Ga granite pebbles, so it is a reworked clast derived from older rock. A trilobite in a conglomerate tells you that Paleozoic rock was eroded nearby; it does not tell you when the conglomerate was deposited. The Cretaceous pollen and ammonites in the shale are consistent with the 78.2 Ma ash age.
2. The colleague claims the zircon date is unreliable because the zircons may be detrital. How would you test that claim, and what evidence would settle it?
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The detrital-zircon objection is testable, not something to dismiss by assertion. An ash-bed zircon population should be dominated by euhedral, glassy, volcanic grains and should yield a single concordant age population; detrital grains would be rounded, varied in age, and possibly older. The lab should date many grains individually and report a concordia age and mean square weighted deviation. Field evidence also matters: an ash bed should be laterally continuous, fine-grained, and stratigraphically consistent. Here the 78.2 ±0.4 Ma result agrees with the Cretaceous pollen and ammonites, which is independent support. If the zircons were detrital, the ash would only give a maximum age, and the shale could be younger, not older. The colleague's claim does not rescue a Paleozoic age for the shale; it would at most weaken the exact numerical age.