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.
Physical (mechanical) weathering disaggregates rock into smaller fragments by purely mechanical stress, with no change in mineral composition — driven by processes such as frost wedging (water freezing/expanding in fractures), thermal expansion/contraction (insolation weathering), salt-crystal growth, root-wedging and exfoliation (pressure-release sheeting). Its products are angular, mineralogically unaltered rock and mineral fragments of the same composition as the parent rock, ranging from boulders down to sand-sized grains — the direct source of lithic (rock-fragment) grains and of unaltered feldspar/mafic grains in immature sandstones.
Chemical weathering instead alters mineral composition through reactions with water, dissolved CO2 and oxygen — principally hydrolysis (breakdown of silicates, especially feldspar, into clay), oxidation (Fe2+ → Fe3+ oxides/hydroxides) and dissolution (carbonates and evaporites dissolving outright in acidic meteoric water). Its products are neoformed clay minerals (kaolinite, illite, smectite), iron/aluminum oxide-hydroxide residues (going to the extreme of laterite/bauxite under intense tropical leaching), and a dissolved (solute) load (Ca2+, Mg2+, Na+, K+, HCO3-, dissolved silica) exported in solution — the feedstock for chemical/biochemical sediments (carbonates, evaporites, chert) precipitated elsewhere.
In practice both operate together and reinforce each other — mechanical fracturing increases surface area for chemical attack, and chemical alteration weakens grain boundaries, promoting further mechanical breakdown — but their relative dominance (set by climate) controls whether a source terrane sheds mostly coarse lithic/feldspathic debris (cold/arid, mechanically dominated) or mostly clay + quartz + dissolved ions (hot/humid, chemically dominated).
