04-BS-14 · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2015 — 04-BS-14, Geology. Closed-book, 3 hours. Five questions constitute a complete paper: Questions 1-4 are mandatory and one of Questions 5-7 must be chosen; every question (5, 6, and 7) is answered here as a complete study resource. Marks for each sub-part are printed as given in the source; totals are transcribed as printed, not forced to a uniform 20.
Reference texts: Marshak, Earth: Portrait of a Planet (structural geology, relative dating, weathering, glacial and fluvial landforms, mineralogy/igneous classification); Goodman, engineering-geology mapping methods (strike and dip, cross-cutting relationships); Freeze & Cherry, Groundwater (water table, zones of aeration/saturation, well hydraulics).
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.
An unconformity is a surface representing a gap in the geologic record (missing time due to non-deposition and/or erosion). The three panels differ in what lies immediately below the surface marked by the arrow:
| Panel | What the cross section shows | Unconformity type |
|---|---|---|
| a) | An arrow-marked, irregular (erosional) surface separates an igneous body (v-pattern, a buried volcanic high) below from flat-lying sedimentary strata above. | Nonconformity — sedimentary rock deposited directly on eroded igneous (crystalline) rock. |
| b) | The arrow marks a gently wavy but essentially parallel surface between two sets of flat-lying, non-tilted sedimentary strata (no angular discordance). | Disconformity — a time gap between parallel sedimentary sequences, recognized from the irregular (erosional) surface and/or a missing fossil/stratigraphic interval rather than from any change in dip. |
| c) | The arrow marks the surface separating older, tilted/dipping sedimentary strata below from younger, differently oriented strata above, truncating the lower beds at an angle. | Angular unconformity — the lower sequence was tilted (and eroded) before the upper sequence was deposited on top. |
By the law of superposition, in an undisturbed layered sequence the oldest bed is at the bottom: reading the section from the base upward gives the deposition order l (oldest) → z → c → p → x (youngest of the layered units). Both dike m and dike n cut across all five sedimentary layers, so by the law of cross-cutting relationships both dikes are younger than every layer, including x. Where the two dikes cross, dike n's texture continues unbroken through the intersection while dike m's texture is interrupted at n's margins — i.e. n cuts across m without itself being offset. By cross-cutting relationships, the feature that cuts another (and is not itself offset) is the younger one.
| Feature | Answer | Reasoning |
|---|---|---|
| a) Youngest feature | Dike n | n cross-cuts dike m (and all layers) without being offset by anything — it is the last event recorded in the section. |
| b) Oldest feature | Layer l | The lowermost layer in the undisturbed sequence, older than every overlying layer and both cross-cutting dikes by superposition and cross-cutting relationships respectively. |
