Question 4 of 7: Geologic Map Interpretation

Question 4: Geologic Map Interpretation (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.

(a) Stratigraphic column, youngest to oldest. Superposition/cross-cutting age is fixed by the labelled geologic periods, independent of the map graphic:

1. Quaternary — Qab (basalt), youngest. A young volcanic flow, < 2.6 Ma.
2. Cretaceous — Kj (granite). ~145–66 Ma; intrudes everything older.
3. Jurassic — Js (granodiorite). ~201–145 Ma.
4. Cambrian — Cb, Cbw, Ce, Cj, Cjm, Cn, Co, Csj. ~541–485 Ma. All eight units share the same period label, so their internal order cannot be fixed by name alone; it must be read from the map by applying the law of superposition (oldest bed at the base of an undisturbed sequence) and any cross-cutting/contact relationships shown between them — this is the part that needs the actual map.
5. Proterozoic — Ps (gneiss), oldest. > 541 Ma; the metamorphic basement on which the Cambrian sequence was deposited.

So the column is, from top: Qab > Kj > Js > {Cb, Cbw, Ce, Cj, Cjm, Cn, Co, Csj — internal order from the map} > Ps.

(b) Without the map graphic the specific symbol-to-number pairing cannot be confirmed, but the standard point symbols used on a geologic map of this kind — and that a five-symbol legend of this sort would draw from — are: strike-and-dip of bedding (a short tick mark with a longer perpendicular line and dip angle), an anticline axis (a line with outward-pointing arrows), a syncline axis (a line with inward-pointing arrows), a fault (a line with ball-and-bar or arrow ticks showing the sense/direction of relative movement — the "#4" symbol the question calls out for extra description, consistent with a fault needing its motion sense drawn explicitly), and an unconformity (a wavy line marking a buried erosion surface). Match each numbered circle to the matching symbol drawn on the original map.

(c) (i) The Jimly (Kj, Cretaceous granite) and Sallymander (Js, Jurassic granodiorite) Formations are both coarse-grained, intrusive igneous (plutonic) rocks of different ages — i.e. they represent igneous intrusions (a batholith/stock complex) emplaced in at least two separate magmatic events, cutting across the older Cambrian and Proterozoic country rock. (ii) Batholith-type granite bodies are typically discordant (they cut across host-rock layering rather than following it); absent the map's contact trace, the geologically expected answer is False — verify against the drawn contact. (iii) Whether the country rock dips toward or away from the intrusion depends on the specific fold/contact geometry drawn on the map and cannot be determined from the legend alone — Check against the map. (iv) The Sambushmaster (Ps) Formation is a gneiss, a metamorphic rock that forms by recrystallization of a pre-existing protolith largely in place (without the transport and deposition that forms a sedimentary rock), so it was indeed likely derived in-situ: True.

(d) A geologic cross-section from A to A′ is built by: (1) drawing a topographic profile along the A–A′ line; (2) projecting each formation contact crossed by the line down from the surface, using the contact's strike-and-dip symbols to set its angle into the page; (3) applying the stratigraphic order from part (a) and any faults/folds from part (b) to connect the same unit across the section at depth, honouring cross-cutting relationships (the Kj/Js intrusions cut across the Cambrian sequence and Ps basement); (4) labelling each unit with its map symbol (Cb, Cbw, …, Kj, Js, Ps, Qab).