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.
Mineralogical (compositional) maturity measures how close a sandstone's framework-grain composition has come to the chemically/mechanically most stable end member, pure quartz — i.e., how completely unstable grains (feldspar, lithic fragments, mafic minerals) have been eliminated by weathering and abrasion. Textural maturity instead measures the physical state of the clastic fabric independent of composition: matrix content (clay winnowed out or not), sorting (uniformity of grain size) and rounding (angular vs. well-rounded grains), reflecting the cumulative mechanical work (transport distance, reworking energy, time) the sediment has experienced.
The two are controlled by different, though related, factors. Mineralogical maturity is set primarily by climate and weathering intensity (hot, humid climates hydrolyse feldspar rapidly; cold or arid climates preserve it), source-rock composition, and recycling history (multiple erosion–deposition cycles progressively concentrate quartz). Textural maturity is set primarily by depositional energy and duration of reworking (prolonged wave/wind agitation, as on a beach or dune, winnows mud and rounds/sorts grains) and transport distance/mechanism (aeolian and high-energy shoreline transport mature a sand fastest; a short debris-flow or turbidity-current trip leaves it texturally immature regardless of composition).
Because the controlling factors differ, the two maturities need not track together: a texturally mature (well-sorted, well-rounded, mud-free) sand can still be mineralogically immature (arkosic) if it was derived from a granitic source and deposited quickly in an arid setting with little chemical weathering; conversely, a texturally immature glacial or debris-flow deposit can be mineralogically mature if it simply reworks an already quartz-rich source.
