18-Geol-A3 Sedimentation and Stratigraphy · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A3, Sedimentation & Stratigraphy, 2018-Dec. Closed book, no calculator, 3 hours. Part 1: Sedimentology (Questions 1–12, 5 marks each) instructs "Answer questions 1 and 2, and any other 6 questions from the remaining 10 questions (i.e., questions 3–12)" for 40 marks total. Part 2: Stratigraphy and Sedimentary Basin Analysis (Questions 13–19, 5 marks each) instructs "Answer five of the following seven questions" for 25 marks total (65/65 maximum).
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, carbonate platforms, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis, grain-size analysis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin classification, reservoir facies).
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.
| Tectonic regime | Basins (letter) |
|---|---|
| Extensional | a) Rift basin, b) Passive margin basin, d) Aulacogen, f) Intracratonic basin |
| Transform | h) Pull-apart basin, i) Wrench basin |
| Convergent | c) Foreland basin, e) Retro-arc basin, g) Fore-arc basin, j) Mariana Trench (subduction trench) |
Extensional: a rift basin is the direct fault-bounded product of continental lithospheric stretching; a passive-margin basin is its thermal-subsidence successor once seafloor spreading begins; an aulacogen is a failed third arm of a triple-junction rift, left as a linear extensional trough intersecting a passive margin at a high angle; an intracratonic basin is a broad, low-relief thermal-sag basin within old, often previously-rifted, continental crust — all four originate from lithospheric extension/thermal relaxation. Transform: a pull-apart basin forms at a releasing bend/step-over along a strike-slip fault; a wrench basin is the broader term for a basin generated by strike-slip (wrench) faulting — both are driven by transform/strike-slip motion, not extension or convergence. Convergent: a foreland basin forms by flexural lithospheric loading on the continental side of a collision/thrust belt; a fore-arc basin sits between the trench and the volcanic arc on the oceanic side of a subduction zone; a retro-arc basin forms behind (continent-side of) the volcanic arc under compressional loading; the Mariana Trench is itself the subduction trench of a convergent (oceanic-oceanic) plate boundary, the deepest expression of convergent-margin tectonics.
