04-BS-14 · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams May 2017 — 04-BS-14, Geology. Closed-book, 3 hours; candidates may use only a Casio or Sharp-approved calculator. Three questions constitute a complete exam paper (Questions 1–3 mandatory). On Question 3, per the exam notes only the first two (2) answers as they appear in the answer book are normally marked; all eight (65–72) are answered here as a complete study resource. Total marks for the exam = 100.
Reference texts: Marshak, Earth: Portrait of a Planet (relative dating, unconformities, plate tectonics, glacial and fluvial landforms, mass wasting, mineralogy, rock textures, Bowen's Reaction Series, volcanoes); Goodman, engineering-geology mapping methods; Freeze & Cherry, Groundwater (permeability/porosity context for weathering and drainage).
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. A block diagram of layered sedimentary rock above a large igneous body, with six labelled features (a)–(f). The six leader lines: (a) goes to the volcano summit; (b) points down onto the dark sheet on the land surface; (c) ends in a thin dark sheet lying along a bedding plane; (d) ends on the vertical feeder that rises to the dark surface sheet; (e) ends in the lens-shaped body under the arched layers; and (f) ends in the large grey body at depth.
Find. The correct geologic name for each labelled feature.
| Label | Feature |
|---|---|
| (a) | Volcano / volcanic cone (erupting) |
| (b) | Lava flow (fissure eruption / flood basalt, fed by the dike beneath it) |
| (c) | Sill (concordant/conformable horizontal igneous intrusion) |
| (d) | Dike (discordant, cross-cutting vertical igneous intrusion) |
| (e) | Laccolith (concordant, lens-shaped intrusion that domes up the overlying layers) |
| (f) | Batholith / pluton (deep, irregular parent intrusion feeding the volcano, dike, sill and laccolith) |
Given. Three cross-sections, each an erosional (wavy) surface separating an older lower unit from a younger, horizontally-bedded upper sedimentary unit: (a) lower unit is igneous (v-pattern); (b) lower unit is horizontally-bedded sedimentary rock of a different texture than the upper unit; (c) lower unit is tilted/folded sedimentary rock.
Find. The specific unconformity type in each panel.
Approach. The three unconformity types are distinguished by the attitude and rock type of the beds immediately below the erosional surface: igneous/metamorphic basement below = nonconformity; parallel sedimentary beds below (no angular discordance) = disconformity; tilted/folded beds below cut off at an angle = angular unconformity.
| Panel | Unconformity type | Diagnostic |
|---|---|---|
| a | Nonconformity | Sedimentary rock overlies eroded igneous (v-pattern) rock — a fundamentally different rock class below the surface. |
| b | Disconformity | Both units are horizontally-bedded sedimentary rock with no angular discordance across the erosional surface, only a texture/unit change and a time gap. |
| c | Angular unconformity | Tilted/folded older beds are truncated at an angle by horizontal younger beds — the classic Hutton's-unconformity geometry. |
Given. Three photographs: (a) well-rounded pebble-to-cobble clasts in a finer matrix; (b) numerous broken and whole marine mollusk-fragment debris cemented together; (c) angular pebble-to-cobble clasts in a finer matrix.
Find. Rock name and origin/formation history for each.
| Photo | Rock | Origin |
|---|---|---|
| a | Conglomerate | Coarse clasts that were transported far enough (typically by high-energy streams or along a beach) to become well rounded through abrasion, then deposited with finer sand/mud matrix and lithified by compaction and cementation (silica, calcite, or iron oxide cement). |
| b | Coquina (fossiliferous/bioclastic limestone) | Formed in a shallow, high-energy marine or beach environment where wave and current action repeatedly breaks and winnows the hard skeletal parts of marine organisms (molluscs, brachiopods), concentrating the fragments, which are then cemented by precipitating calcite — a biochemical/bioclastic sedimentary rock. |
| c | Breccia | Angular clasts indicate minimal transport distance/time (talus/scree accumulation at a cliff base, fault gouge, or rapid mass-wasting/debris deposits) that preserved the original fracture-sharp edges, subsequently buried and lithified with matrix and cement. |
Given. A cross-section with five flat-lying sedimentary layers t, z, c, p, x (bottom to top); a wide diagonal band m that crosses all five layers, from the top edge of x to the base of t; and a narrower diagonal dike h (marked with "+", indicating igneous rock) that crosses all five layers and m. In the figure the contacts of h run straight through m, while the contacts of m stop at the edges of h. The layer boundaries sit at the same level on both sides of m, so m has no offset and is an intrusion, not a fault. No erosion surface is drawn anywhere in the section.
Find. The chronological sequence of events that produced the present configuration.
Approach. Apply the law of superposition to the undisturbed layers, then the principle of cross-cutting relationships to m and h (an intrusion is younger than everything it cuts). Where two intrusions cross, the one whose contacts continue unbroken is the younger.
[Figure not reproduced: Stratigraphic cross-section redrawn from the source: m cuts all five layers t–x, and dike h cuts all five layers and m. See the official exam paper.]
| Order | Event |
|---|---|
| 1 (oldest) | Deposition of layer t |
| 2 | Deposition of layer z |
| 3 | Deposition of layer c |
| 4 | Deposition of layer p |
| 5 | Deposition of layer x |
| 6 | Intrusion of body m (cuts t, z, c, p and x) |
| 7 (youngest) | Intrusion of dike h (cuts everything, including m) |
