What is the last possible date of faulting?

• A. Jurassic
• B. Cretaceous
• C. Triassic
• D. Permian

Answer: (C)

In structural dating, a fault must be younger than the rocks it disrupts. The last possible date for faulting is controlled by the youngest rock unit that shows evidence of being offset by the fault. Here, the fault affects rocks dating to the Triassic, and there’s no clear indication of movement continuing into younger (Jurassic or Cretaceous) layers. If faulting had occurred after Triassic time, we would expect to see offset in the younger rocks as well. Since the youngest units affected by the fault are Triassic in age, the latest time the fault could have formed is by the end of the Triassic. Therefore, Triassic is the last possible date for faulting.

Choosing earlier periods (Permian) would push the faulting date back beyond the observed offset, which doesn’t fit the stratigraphic record. Choosing later periods (Jurassic or Cretaceous) would require evidence of faulting in the younger rocks that isn’t indicated.