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.
The correct answer is (d) lithic (rock) fragments.
A lithic grain is a fragment of the actual source rock itself (a mini rock-in-a-rock, e.g. a piece of volcanic, metamorphic or fine-grained sedimentary rock), so its internal texture and mineralogy directly and unambiguously fingerprint the parent lithology it was eroded from. Quartz grains (a), by contrast, are compositionally and texturally similar whether they were sourced from a granite, a gneiss, or a recycled older sandstone — quartz's extreme chemical/mechanical durability erases most of the provenance signal by the time it is deposited. Feldspars (b) carry some provenance information (K-feldspar vs. plagioclase ratios can hint at source-rock type) but are far less diagnostic than a whole lithic fragment and, being chemically unstable, are also easily destroyed by weathering before that signal is preserved. Matrix (c) is largely diagenetic/depositional detrital mud, not a discrete provenance-bearing grain, and micas (e), while sometimes source-diagnostic (e.g. biotite vs. muscovite ratios), are a minor accessory constituent, not a principal framework indicator.
