18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).
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.
All five grain types carry some provenance signal, but they differ sharply in how DIAGNOSTIC of a specific source lithology each one is, because they differ in how directly they preserve the texture/mineralogy of the parent rock.
| Rank | Grain type | What it indicates |
|---|---|---|
| 1 (most) | (a) Lithic fragments | Literal pieces of the parent rock — a volcanic, schist, chert or limestone lithic clast identifies the exact source-rock category unambiguously |
| 2 | (e) Polycrystalline quartz grains | Multiple sutured quartz crystals in one grain indicate a metamorphic (or coarse plutonic/vein) source, as opposed to a single crystal |
| 3 (tie) | (b) Feldspathic grains | Indicate a granitic/gneissic source and/or an arid climate or proximal, rapid-erosion setting that limited chemical breakdown of feldspar |
| 3 (tie) | (c) Micas | Indicate a metamorphic or coarse granitic source; mechanically fragile, so their survival also implies rapid burial/short transport |
| 5 (least) | (d) Monocrystalline quartz grains | Chemically and mechanically the most stable common grain of all — occurs in virtually every igneous, metamorphic AND recycled-sedimentary source, so a single quartz crystal alone carries almost no source-lithology information |
The practical implication is that a petrographer reconstructing provenance leans most heavily on the lithic-fragment and polycrystalline-quartz populations (options a and e), uses feldspar and mica (b, c) as secondary, climate/maturity-sensitive corroborating evidence, and treats abundant monocrystalline quartz (d) chiefly as a MATURITY indicator (Question 4) rather than a lithology indicator, since a quartz-arenite's high monocrystalline-quartz content says "intensely reworked/recycled" far more clearly than it says "derived from any one rock type."
