Question 11 of 21: Question 4, Part 2: Unconformity Types

Question 4, Part 2: Unconformity Types (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Three cross-section sketches, each showing an arrowed erosional contact: (a) flat sedimentary layers directly on top of an eroded igneous (v-patterned) body; (b) flat sedimentary layers on an erosional surface cut into another, texturally different but still flat-lying sedimentary sequence; (c) flat sedimentary layers on an erosional surface cut across tilted/deformed older sedimentary layers.

Find. The specific unconformity name illustrated by each of (a), (b), (c).

Approach. Classify by (i) whether the rock below the erosion surface is igneous/ metamorphic or sedimentary, and (ii) whether the beds above and below the surface are parallel or at an angle to one another.

(a) Nonconformity. The erosional surface separates sedimentary rock above from IGNEOUS (or metamorphic) crystalline rock below – by definition a nonconformity, regardless of whether the beds above are flat or tilted.

(b) Disconformity. Both units are sedimentary and their bedding is essentially PARALLEL across the erosional surface (no angular discordance) – a disconformity represents a gap in time (missing section) without any tilting between depositional episodes.

(c) Angular unconformity. The lower sedimentary sequence was tilted (and eroded flat) BEFORE the upper, flat-lying sequence was deposited on top – bedding planes meet at a visible angle across the erosion surface, the classic Huttonian "unconformity" relationship.