Question 2 of 4: Relative Time

Question 2: Relative Time (20 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.

21. Chronology of Figure Q21

Reading the section. The printed figure shows the following units and structures:

• J, K, the unlabelled dark beds, F and H — a layered (sedimentary) package whose bedding lines are tilted, dipping toward the left of the section. Along any horizontal line the beds get younger toward the left, so J (lowest, right) is the oldest exposed bed, then K, then the dark beds, with F and H the highest preserved beds. H carries a pebble symbol (a conglomerate); F is plain (e.g. a sandstone).
• B — a flat-lying brick-patterned bed (limestone) whose flat base cuts straight across the tilted beds.
• E — a flat-lying dark bed (e.g. shale) resting on B.
• G — a black, steep, tabular igneous body (a dike) that cuts the tilted beds (between H and F) and the limestone B, but stops at the base of E.
• D — a straight inclined line in the right-hand block that offsets both B and E: the block on the right (above the inclined plane, i.e. the hanging wall) has dropped relative to the left, so D is a normal fault. Both ends of the fault stop at the margin of body I.
• I — a large body with a random-dash (igneous) texture and a dark rim along all its contacts (baked/contact-metamorphic margin): an igneous intrusion (pluton). It cuts J, K, the tilted beds, B, E and fault D, and an offshoot of it rises through B and E to the base of C, where it is cut off flat.
• Ovals at the base of C — pebbles (a basal conglomerate) lying on the flat surface that truncates E and the top of I.
• C and A — flat-lying beds covering the whole section, A on top of C.

Applying original horizontality, superposition, cross-cutting relationships, inclusions and unconformity recognition, the events from oldest to youngest are:

1. Deposition of J, then K, then the overlying beds, then F and H as horizontal layers (original horizontality; superposition places J at the base of the preserved sequence).
2. Tilting (folding) of the J–H package by tectonic stress. Tilting post-dates the youngest tilted bed.
3. Uplift and erosion bevel the tilted beds flat, forming an angular unconformity (the flat base of B truncating the tilted beds).
4. Subsidence and deposition of limestone B on that surface (a marine limestone implies a return to below sea level).
5. Intrusion of dike G, cutting the tilted beds and B (cross-cutting relationships: G is younger than everything it cuts).
6. Erosion of the top of B and G, then deposition of E. G is cut off at the base of E and does not enter it, so G is older than E and the B/E contact is an erosion surface (a disconformity: parallel beds above and below).
7. Normal faulting along D, which offsets B and E (so it is younger than E) and records extension.
8. Intrusion of pluton I, which cuts J, K, the tilted beds, B, E and fault D, and bakes a contact-metamorphic rim into the rocks it touches. Because I cuts the fault, I is younger than D.
9. Uplift and erosion bevel E and the top of the pluton flat. Where C now rests on the igneous rock of I the contact is a nonconformity; where it rests on E it is a disconformity. The pebbles at the base of C are clasts eroded from the underlying rocks, which confirms this erosion surface.
10. Deposition of C, then A (superposition), both undisturbed, so no later tilting, faulting or intrusion affected the section.

What cannot be dated relative to each other: H and F. Both sit at the top of the tilted package on opposite sides of dike G, directly beneath the unconformity. Neither lies on top of the other, they share no contact (G separates them), and no fossil or cross-cutting evidence is given. So it cannot be determined whether H is older than, younger than or the same age as F (they may be a sideways change of one bed from conglomerate to sandstone). Every other pair can be ordered. For example, G is older than E, which is older than D and I, even though G never touches D or I.

22. Three types of unconformities

An unconformity is a surface in the rock record representing a gap in time — a period of non-deposition and/or erosion between the rocks below and above it. The three fundamental types are distinguished by the character of the rocks immediately below and above the surface: