Combine Relative Ages with Dating Uncertainty

Bracket a layer's age while keeping assumptions and uncertainty visible.

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Use stratigraphic order to set older and younger bounds. Keep age ranges rather than replacing them with an unsupported exact date. Check that dated material belongs to the depositional event rather than being older, reworked material.

Worked example

A bed above a primary ash dated 80 million years and below one dated 75 million years is bracketed between those ages under undisturbed-deposition assumptions.

Invented Age Bounds

An undisturbed bed lies between two primary ash deposits. The lower ash dates to 102 plus or minus 1 million years; the upper to 99 plus or minus 1 million years. For this exercise, treat these ranges as hard bounds and assume crystallization dates ash deposition. Real uncertainty requires fuller analysis.

1. Which ash layer is older? The upper ash The lower ash They must be identical

2. Using the stated ranges as hard bounds, what broad interval can contain the middle bed's age? Exactly 100.5 million years Between 110 and 120 million years Between 98 and 103 million years

3. Is an exact age of 100.5 million years established for the middle bed? No, a midpoint is not an independent date Yes, all middle beds form at midpoints Yes, uncertainty vanishes when averaged

4. Which discovery would challenge the stated dating interpretation? The diagram uses two colors The lower ash contains older crystals reworked from another deposit The middle bed has a label

5. What should accompany the reported bracket? Only one rounded midpoint A claim of no uncertainty The dating ranges and depositional assumptions

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