04-BS-14 · December 2019

Question 3 of 4: Short Answer (30 marks)

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Question 3: Short Answer (30 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.

32(a) — Figure Q3-1: igneous intrusive structures (10 marks, part)

Given. A block diagram of flat-lying layered strata over a large, coarse-textured igneous mass. Four white boxes sit above the block, and their leader lines are traced from left to right. Box 1 has two leaders. One ends on the domed roof of a flat-floored, lens-shaped body just beneath the surface, fed from below by a vertical conduit. The other ends on the ground surface that this body arches up. Box 2's leader ends on a thin sheet that runs parallel to the bedding, branching sideways from a second vertical conduit. Box 3's leader ends on a thin sheet that cuts steeply across the layers, from the deep mass up to the surface. Box 4's leader ends on the right-hand part of the block, where the large mass itself is exposed at the surface in rugged, coarse-textured ground. A small volcanic cone sits on the surface at the back of the block.

Find. The names of the four intrusive/extrusive structures indicated.

Box 1 — Laccolith. A concordant intrusion fed from below whose viscous magma was too stiff to spread far, so it inflated a lens-shaped blister with a flat floor and an arched roof, bowing the overlying strata upward.

Box 2 — Sill. A tabular concordant intrusion emplaced parallel to the bedding, injected where magma pressure exceeded the weight of the overburden and a weak bedding plane offered the path of least resistance.

Box 3 — Dike. A tabular discordant intrusion that cuts steeply across the layering, carrying magma from the deep pluton up toward the surface.

Box 4 — Batholith. The large, discordant plutonic mass, exposed where deep erosion has stripped away its cover. By convention an exposed pluton larger than 100 km² is a batholith; a smaller one is a stock. The Coast Plutonic Complex of British Columbia is the Canadian type example.

The key rests on geometry. Box 1 is concordant with a domed roof and a flat floor, so it is a laccolith. Box 2 is concordant and tabular, so it is a sill. Box 3 is discordant and tabular, so it is a dike. Box 4 is the large deep mass itself, so it is the batholith.

32(b) — Figure Q3-2: groundwater features (10 marks, part)

Given. A block diagram of a wooded hillside above a stream valley, read from the printed figure of page 7 (reproduced below). A house near the hilltop has a shallow well that ends on top of a small, dark, lens-shaped layer buried in the hill. A second, deeper well beside it is labelled "Unsuccessful well". A lighter upper zone is separated from a darker lower zone by a line that rises gently from the stream toward the hill. Six boxes are to be filled in:

• Box A (left, over the valley): its arrow points to the hillside where the lens crops out.
• Box B (upper centre): its leader runs to the top of the saturated layer resting on the lens, where the shallow well ends.
• Box C (lower centre): its leader runs up to the underside of the lens.
• Box D (bottom centre): its arrow points up to the line between the light and dark zones.
• Box E (upper right, captioned "(Zone)"): its arrow points into the light zone above that line.
• Box F (lower right, captioned "(Zone)"): its arrow points into the dark zone below it.

Find. The name of the groundwater feature at each box.

This is the standard perched water table figure (Tarbuck & Lutgens). The lens is a small low-permeability layer that holds up a local body of saturated ground above the main water table.

A — Spring. Water perched on the lens moves sideways to the lens edge. It discharges where the lens meets the hillside, so a spring issues from the slope.

B — Perched water table. This is the top of the local saturated zone on the lens. It is separated from the main water table by unsaturated ground. The house's shallow well draws from this small, easily depleted body.

C — Aquitard (impermeable lens). This clay or shale lens has too low a permeability to let infiltrating water pass down. It holds the water up and creates the perched zone.

D — Main (regional) water table. This is the surface where pore-water pressure equals atmospheric pressure, and it is the top of the zone of saturation. It follows the topography in subdued form and meets the land surface at the stream, which it feeds as base flow.

E — Zone of aeration (unsaturated or vadose zone). Pores here hold both air and water. The unsuccessful well was drilled past the lens and ends in this zone, above the main water table, so it yields no water.

F — Zone of saturation. All pores are water-filled. A well must reach below the main water table, into this zone, to give a reliable supply.

33 — Drainage patterns (5 marks)

Given. Five block diagrams of drainage networks, read from the printed figure. (a) A tree-like network of branching tributaries on flat-lying beds. (b) Straight channel segments that meet and bend at right angles, following a blocky grid of fractures. (c) Long, parallel main streams in valleys between two ridges annotated "Ridges of resistant rock", fed by short tributaries. (d) Streams running outward in all directions from a central peak. (e) A disordered network of short, irregular streams linking scattered lakes and ponds on a low surface.

Find. The name of each drainage pattern, and the geology it indicates.

a. Dendritic — irregular, tree-like branching with tributaries joining at acute angles; develops where the substrate is essentially uniform in resistance and structure (flat-lying sedimentary rocks, massive crystalline rock, or thick till). It is the default pattern and its presence argues against strong structural control.

b. Rectangular — straight stream segments meeting at sharp right-angle bends. It reflects bedrock cut by two intersecting sets of joints or faults, which the channels exploit. It is common on jointed granite and faulted terrain of the Canadian Shield.

c. Trellis — long parallel main streams joined by short tributaries at right angles. It develops on tilted or folded sequences of alternating resistant and weak beds. The main streams occupy strike valleys cut in the weak units, between ridges of resistant rock, which is what panel (c) annotates. Folded belts such as the Foothills of the Canadian Rockies produce it.

d. Radial — streams flowing outward in all directions from a central high; develops on an isolated conical or domal topographic high such as a stratovolcano or a structural dome.

e. Deranged — a chaotic network of short, irregular streams wandering between lakes, ponds and swamps, with no consistent direction or branching order. It marks a young landscape whose drainage has not had time to organise. The classic setting is recently glaciated terrain, where irregular till and scoured bedrock hollows trap water. Much of the Canadian Shield, left by the Laurentide Ice Sheet, drains this way.

34 — Chronological sequence of geologic events (15 marks)

Given. The units carry textbook ornaments but no letters, so they are named here by rock type.

• Basement. Wavy (foliated) ornament at the bottom left and bottom right: metamorphic rock such as schist or gneiss.
• Thin fault. A thin black line in the lower left cuts the basement only. It stops at the base of the conglomerate above it.
• Tilted and folded sequence. Resting on the basement: conglomerate (pebble ornament), sandstone (dots), shale (dashes) and limestone (brick). All dip toward the right. On the far right the limestone band bends round in a fold.
• Diagonal intrusion. A broad band edged with short contact-alteration ticks rises from the lower left to the upper right. It cuts the basement, conglomerate, sandstone and shale. The shale band is split across it.
• Angular unconformity. A gently undulating surface bevels the tilted sequence and the top of the diagonal intrusion.
• Upper sequence. Above the unconformity: sandstone at the base, holding a pentagon-shaped block of different, speckled rock that sits on the unconformity surface; then shale; then limestone. The beds are near-horizontal.
• Steep dike. A dark, uniform band cuts steeply through every unit, the diagonal intrusion included, up to the present ground surface. It offsets nothing, so it is a dike, not a fault.
• Ground surface. Two hills separated by a valley, where the upper limestone and part of the shale have been removed.

Find. The geologic events that produced this configuration, oldest to youngest.

Approach. The whole sequence follows from five principles:

• Superposition: in an undisturbed pile, lower beds are older.
• Original horizontality: beds are laid down flat, so tilted or folded beds were deformed later.
• Cross-cutting relationships: anything that cuts a rock is younger than that rock.
• Inclusions: an enclosed fragment is older than the rock that encloses it.
• Unconformities: an erosion surface records an interval of uplift and erosion.

1. Formation of the metamorphic basement. Older rocks were buried and regionally metamorphosed to schist or gneiss. This is the oldest rock in the section, because everything else lies on it or cuts it.
2. Faulting of the basement. The thin fault cuts the basement but ends at the base of the conglomerate. It is younger than the basement and older than every unit above it.
3. Uplift and erosion of the basement. This formed a nonconformity, where sedimentary rock rests directly on metamorphic rock.
4. Deposition of the lower sedimentary sequence. In order: conglomerate (the coarse basal deposit on the erosion surface), then sandstone, then shale, then limestone. All were laid down horizontally.
5. Tilting and folding. Compression tilted the lower sequence and its basement toward the right and folded the limestone on the far right. The upper sequence does not share this deformation, so it came later.
6. Intrusion of the diagonal dike. It cuts the basement, conglomerate, sandstone and shale, and its margins show contact alteration. It is straight and undeformed across the tilted beds, so it was intruded after the tilting. It is truncated at the unconformity, so it is older than the next erosion event.
7. Uplift and erosion. This cut the angular unconformity, which bevels the tilted beds and the diagonal dike alike.
8. Deposition of the upper sequence. In order: basal sandstone, then shale, then limestone. The speckled block in the basal sandstone is an inclusion, a clast eroded from older rock during the previous erosion interval, so it is older than the sandstone that encloses it.
9. Intrusion of the steep dark dike. It cuts every unit, including the diagonal dike and the upper sequence, so it is the youngest rock in the section.
10. Uplift and present-day erosion. This shaped the modern hills and valley. It removed the upper limestone, and part of the shale, from the valley and exposed the steep dike at the surface.

The order of events 5 and 6 comes from the dike's geometry. A dike intruded before the tilting would itself have been rotated, and would be folded with the limestone. This one cuts straight across the tilted beds. Even so, both events fall inside the same interval, after the lower sequence was deposited and before the angular unconformity was cut. That interval is the load-bearing constraint for the marks.