Discussion

Match the features in Column 1 to the most appropriate description in Column 2.

Column 1	Column 2
P. Ash layer	i. Part of a turbidite sequence
Q. Graded bedding	ii. Subaerial episode in the sedimentary record
R. Hiatus	iii. Dating of sedimentary rocks
S. Soil profile	iv. Missing faunal zone

Answer: A

The correct matching is P-iii; Q-i; R-iv; S-ii.

An ash layer contains volcanic crystals such as zircon and sanidine that can be dated radiometrically, so it is used for dating sedimentary rocks (iii).
Graded bedding is the Ta division at the base of a Bouma sequence, so it is part of a turbidite (i).
A hiatus is a gap in the record, recognised biostratigraphically as a missing faunal zone (iv).
A soil profile (palaeosol) forms only at the land surface, so it records a subaerial episode (ii).

Option B makes graded bedding the dating tool and ash the turbidite, and option C swaps the hiatus and the soil profile.

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