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.
Folk's textural-maturity sequence tracks three parameters — matrix content, sorting and rounding — through progressively higher cumulative transport/reworking energy: (1) Immature — abundant clay matrix (>5%) still present, poorly sorted, angular grains: little winnowing or abrasion has occurred (e.g. a fresh debris-flow or short-transport fluvial deposit). (2) Submature — matrix has been winnowed out (clean sand, <5% mud) but sorting is still only moderate and grains remain sub-angular: enough energy to remove mud in suspension, not yet enough to fully homogenise grain size or round the grains. (3) Mature — matrix-free, well-sorted, but grains are still sub-angular to sub-rounded: sustained reworking (e.g. prolonged fluvial/beach transport) has narrowed the grain-size distribution but abrasion has not yet fully rounded the more resistant/finer grains. (4) Supermature — matrix-free, well-sorted AND well-rounded: very high cumulative kinetic energy and/or very long transport distance/time (classic aeolian dune sand, or a multiply-recycled sand), enough to abrade even durable quartz grains into rounded form.
Physically, winnowing (matrix removal) requires the least sustained energy and happens first; sorting narrows next as repeated selective entrainment segregates grain sizes by settling velocity; rounding requires the most cumulative mechanical work (grain-to-grain and grain-to-bed collisions over a long transport path) and so is the last parameter to mature.
