18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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.
The correct answer is (b) Foreland basin. A foreland basin forms adjacent to an actively growing fold-and-thrust belt at a convergent margin (continent–continent collision or continent–arc/subduction-related thrusting): the flexural load of the thickening, advancing thrust sheets depresses the adjacent lithosphere, creating accommodation that fills with synorogenic clastic sediment. By contrast, a passive margin basin (a) forms on a rifted, tectonically quiet continental margin long after rifting ceased (a divergent-margin product), an aulacogen (c) is a failed rift arm of a triple-junction system that never became a spreading ridge (also a divergent/extensional feature), and an intracratonic basin (d) is a broad, slowly-subsiding sag basin in a stable craton interior, unrelated to any active plate margin.
The correct answer is (c) — faults dying out upward and terminating within the sedimentary fill. A genuine syndepositional (growth) fault remains active throughout deposition of the section it influences: its displacement is greatest in the oldest, most deeply buried beds and decreases progressively upward, but the fault plane itself continues to propagate close to the contemporary depositional surface for as long as movement continues, so its influence persists up to (or very near) the youngest beds of the syntectonic section rather than being buried and sealed by a thickness of later, undisturbed strata. A fault that instead dies out completely and is overlain by demonstrably undeformed younger beds records movement that STOPPED before deposition of the rest of the sequence — i.e. an episode that was not syndepositional through the full thickness of fill in question. Options (a), (b) and (d) are, by contrast, the textbook diagnostic consequences of active syndepositional tectonics: differential subsidence directly creates the accommodation-space contrast that produces (b) lateral thickness variation (thickening on the downthrown/subsiding side, an "expansion index" > 1) and (d) lateral facies changes (different parts of the basin subside into different depositional environments at the same time).
