Correlating Outcrops with an Ash Layer and a Fault
Three outcrops are exposed along a road cut. Outcrop A, from bottom to top: sandstone with shell fossils, shale with graptolites, a 1.2 m volcanic ash layer, and limestone with trilobites. Outcrop B: shale with the same graptolites, the same ash layer, and limestone with the same trilobites. Outcrop C: sandstone, shale, the same ash, and limestone, but a fault offsets the ash layer by 3 m; the fault does not cut the overlying limestone. A basalt dike cuts the ash and the limestone in Outcrop C but is not present in A or B. Zircons from the ash in A give an age of 452 ± 3 million years. The students must build a sequence of events and decide whether the fault is older or younger than the ash.
1. Correlate the three outcrops and explain which layer is the best time marker. Why?
Read the worked answer
The three outcrops share the same vertical order: sandstone with shells, shale with graptolites, ash, then limestone with trilobites. The ash is the best time marker because it is a single, distinctive, laterally extensive layer that can be radiometrically dated; the 452 ± 3 Ma zircon age applies to the same eruption in all three outcrops. Fossils are also useful for correlation, but they give relative age and can have longer time ranges. The ash is essentially isochronous: it was deposited rapidly over a wide area. The fault in C offsets the ash but not the overlying limestone, so the ash also serves as a strain marker.
2. Determine the relative age of the fault in Outcrop C. Justify using cross-cutting relationships and the age of the ash.
Read the worked answer
The fault cuts and offsets the ash layer by 3 m, so the fault must be younger than the ash. The fault does not cut the overlying limestone, so the fault must be older than that limestone. Using superposition, the limestone was deposited after the faulting, burying the offset surface. Therefore the sequence is: deposition of sandstone, shale, and ash; faulting; deposition of limestone. The basalt dike cuts both the ash and the limestone, so the dike is younger than all of them. The absolute age of the fault is bracketed between 452 ± 3 Ma and the age of the limestone, which is not given.
3. A student says the 452 ± 3 Ma date proves the fault moved at that time. Evaluate this claim.
Read the worked answer
No. The 452 ± 3 Ma date is the age of zircon crystallization in the ash, not the age of faulting. The fault could have moved any time after the ash was deposited and before the limestone was deposited. Without an age for the limestone or a direct date on the fault, the fault's absolute age is unknown within that bracket. The date does prove the ash is older than the fault, because the fault offsets it. To date the fault, one could date a cross-cutting igneous dike, date fault gouge with a suitable method, or find a datable layer that seals the fault.