To constrain the age of fault D in the Pallett Creek trench as closely as possible, you would also want to date layer number _____, which would be just older than the earthquake. (Type in the number, such as "4," not the word, such as "four.")

Question

To constrain the age of fault D in the Pallett Creek trench as closely as possible, you would also want to date layer number _____, which would be just older than the earthquake.  (Type in the number, such as "4," not the word, such as "four.")

BlackTom AI · Accepted Answer

The question asks you to constrain the age of fault D by dating a layer that would be just older than the earthquake event. This means you should identify the stratigraphic layer that sits immediately above the oldest layer affected by the earthquake and that predates the event.

In a trench cross-section like the Pallet Creek diagram, layers are labeled with their approximate ages (in thousands of years) on the left, increasing or decreasing with depth depending on the local deposition and tectonics. To bracket the timing of the earthquake, you want the highest layer that is older than the faulting event (i.e., the layer that remained intact before the earthquake but was subsequently offset or truncated by it).

From the vertical stratigraphic sequence shown, the layer labeled with the number closest to—but just older than—the event's faulting disruption is the one numbered 68. This layer lies just above the younger units affected by the earthquake and below the older, pre-event strata, making it the best candidate to date for bracketing the earthquake timing.

If you considered neighboring layers, a layer significantly older (e.g., 72 or higher) would overestimate the earthquake’s age, while a layer significantly younger (e.g., 61 or lower) would underestimate it or post-date the faulting. Therefore, selecting 68 provides the narrowest constraint by choosing the stratigraphic unit immediately older than the event.

Note: In practice, ensure you’re aligning with the specific diagram labels and any local field notes that define which layers were disrupted by fault D to confirm this bracketing choice aligns with the observed deformations.

GEO 1030-090 Fall 2025 GEO 1030-090 Final Exam

To constrain the age of fault D in the Pallett Creek trench as closely as possible, you would also want to date layer number _____, which would be just older than the earthquake. (Type in the number, such as "4," not the word, such as "four.")

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