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.
Sorting describes the spread (uniformity) of grain sizes present in a sediment — quantitatively, the standard deviation of the grain-size (phi) distribution about its mean. A well-sorted sediment has grains clustered tightly around one size; a poorly-sorted sediment spans a wide range of sizes (fine mud through coarse gravel) in the same deposit.
Sorting is significant because it is a direct proxy for the selectivity and steadiness of the transporting medium. A medium that can continuously winnow and rework grains (persistent wind, oscillating wave swash, a steady unidirectional current) removes both the finest (mud, carried away in suspension) and, over time, segregates the remaining sizes by settling velocity, producing excellent sorting. A medium with no selective capability — one that simply dumps its entire sediment load at once, or that has enormous, near-instantaneous competence — deposits everything it was carrying together, producing poor sorting.
Well-sorted example: aeolian dune sand (e.g. an erg/desert dune field) — wind can only entrain a narrow range of grain sizes by saltation, so dune sand is famously well- to very-well-sorted fine-to-medium sand with essentially no mud or gravel. Poorly-sorted example: glacial till (or, similarly, a debris-flow/alluvial-fan deposit) — ice has no hydraulic selectivity at all and simply bulldozes and then dumps everything from clay to boulders together upon melting, producing a chaotic, unsorted diamicton.
