18-Geol-A3 Sedimentation and Stratigraphy · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2018-May. Closed book, no calculator, 3 hours, 80 marks total. Part 1 (Questions 1–10, Sedimentology and Sedimentary Processes, 50 marks) instructs "Questions 1 and 2 must be answered (10 points each); answer any FIVE of Questions 3–10 (6 points each)." Part 2 (Questions 11–19, Stratigraphy, 30 marks) instructs "Answer any FIVE of Questions 11 to 19 (6 points each)."
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin settings, 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.
Sedimentary basins are classified by the plate-tectonic setting that creates the accommodation space (subsidence) in which sediment accumulates. At divergent (extensional) margins, lithospheric stretching produces rift basins (fault-bounded grabens/half-grabens, active continental rifting, e.g. East African Rift) that, once seafloor spreading begins, evolve thermally into passive-margin basins (broad, slowly subsiding thermal-sag basins on the trailing edge of a continent, e.g. the Atlantic margins).
At convergent margins, subduction and collision generate several distinct basin types: forearc basins (between the trench and the volcanic arc, floored by accretionary-wedge/oceanic crust, receiving arc-derived volcaniclastic sediment); foreland basins (flexural basins formed by lithospheric loading/downwarping in front of a thrust belt during continent–continent or arc–continent collision, e.g. the Western Canada Sedimentary Basin in front of the Rocky Mountain fold-thrust belt); and retroarc/back-arc basins (behind the volcanic arc, either extensional (back-arc spreading) or compressional, depending on subduction dynamics).
At transform (strike-slip) margins, releasing bends along a strike-slip fault system create localized, rapidly-subsiding pull-apart basins (e.g. the Ridge Basin, California), typically small but very deep and short-lived. Finally, away from active plate margins, thermal or loading subsidence within stable continental crust produces broad, low-relief intracratonic (sag) basins (e.g. the Michigan Basin), accumulating thin, long-duration, relatively undeformed sedimentary sequences.
