A Dike, an Ash, and a Borrowed Date
A roadcut exposes a 60 m section, bottom to top: (1) 1.7 Ga gneiss; (2) 12 m of cross-bedded sandstone; (3) 18 m of dark shale containing ammonites of index Zone B; (4) a 15 cm volcanic ash bed; (5) 15 m of fossiliferous limestone; (6) an erosional surface with 2 m of river gravel and cobbles of gneiss, sandstone, and limestone. Zircon from the ash gives 92.4 ± 0.8 Ma. A basalt dike cuts units 1 through 5 but stops at the base of the gravel and yields 55 ± 3 Ma. A colleague writes in a draft report: "The 55 Ma dike date shows that the limestone is 55 million years old."
1. Determine the age bracket for the ammonites and for the limestone, explaining precisely what each radiometric date does and does not constrain.
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Ammonites: older than 92.4 ± 0.8 Ma, with no lower limit from this section.
The ash lies above the shale. Because the section is upright (younger upward), the shale and its ammonites are older than the ash. The ash date therefore gives a maximum age of 92.4 Ma for the top of the shale.
The dike does not date the shale. It cuts units 1–5, so it is younger than all of them; 55 ± 3 Ma is a minimum age for the shale and limestone, not their age.
Combined: the limestone is between 92.4 and 55 Ma; the ammonites are older than 92.4 Ma. Zone B, as an index fossil, should indicate an age consistent with that bracket, but the section alone cannot pin the shale's lower limit. A date on the sandstone or on another ash below the shale would.
2. Place the events in order and identify each unconformity. What do the cobbles in the river gravel indicate about the history after limestone deposition?
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Event order (oldest to youngest):
- Gneiss formed and was deformed (1.7 Ga).
- Long erosion, then sandstone deposited on gneiss.
- Shale with ammonites deposited.
- Ash bed erupted and deposited at 92.4 Ma.
- Limestone deposited with forams.
- Basalt dike intruded through units 1–5 at 55 Ma.
- Uplift, erosion, and deposition of river gravel with mixed cobbles.
Unconformities:
- Gneiss–sandstone: nonconformity (igneous/metamorphic basement overlain by sediment).
- Limestone–gravel: erosional unconformity. If the lower units are tilted, it is an angular unconformity.
Clast meaning: cobbles of gneiss, sandstone, and limestone in the gravel show that all three older units were exposed, lithified, and eroded before the gravel was deposited. The dike is not reported as a clast. The dike clearly predates gravel deposition because it stops at the gravel base, so intrusion occurred before the erosion surface was buried.
3. Critique the colleague's sentence and describe two independent methods that could date the limestone directly.
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The claim is wrong because it misreads what a crosscutting date means.
A dike that cuts a rock must be younger than that rock. So 55 ± 3 Ma is a minimum age for the limestone, not its age. The limestone could be 60 Ma, 80 Ma, or 92 Ma. Its actual age is constrained to between the 92.4 Ma ash below and the 55 Ma dike, but that is a 37-million-year window.
The date does tell us something useful: intrusion and lithification both happened before 55 Ma, and the dike itself is older than the river gravel because it stops at the gravel base.
How to date the limestone directly:
- Find zircons in an ash or bentonite layer within the limestone and date them by U–Pb.
- Use strontium-isotope stratigraphy on the forams, or biostratigraphy tied to a calibrated zone.
The error to avoid is treating any crosscutting date as the age of the host rock.