If you drilled a vertical borehole through nearly horizontal beds offset by a thrust fault, what stratigraphic pattern would you expect with depth?

• A. Repetition of beds with depth
• B. A single continuous layer without interruptions
• C. Gradual thinning of beds with depth
• D. No recognizable stratification

Answer: (A)

When beds are nearly horizontal and a thrust fault offsets them, the fault places slices of the same stratigraphic sequence on top of one another. Drilling a vertical borehole through this stack will therefore intersect the same bed multiple times as you pass from one fault block to another, causing repetition of the same beds with depth. In other words, the borehole log shows the same lithologies appearing more than once as you go deeper because the fault has displaced blocks that contain the same layers. This contrasts with a single continuous layer, which would require no faulting to explain, or with gradual thinning or no recognizable stratification, which don’t fit a scenario where faulting repositions identical beds at different depths.