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.
Sandstones are classified petrographically from a thin section using the QFL (quartz–feldspar–lithic fragments) framework-grain method. The procedure is: (1) cut a thin section and, under the polarizing microscope, point-count (typically 300 points on a grid) every framework grain, sorting each into one of three end members — total quartz (monocrystalline + polycrystalline + chert), total feldspar (K-feldspar + plagioclase) and total lithic (rock) fragments (volcanic, metamorphic and sedimentary clasts); (2) recast the three counts to 100% and plot the point on a QFL ternary diagram; (3) separately estimate the matrix content (detrital clay/silt <30 μm filling the pore space between grains) as a percentage of the whole rock — this sets the textural class (below); (4) name the rock from its position on the ternary diagram, qualified by the textural prefix.
The QFL diagram (Dott/Pettijohn) divides into five compositional classes. Near the Q apex (Q ≥ 95%) is quartz arenite — texturally and mineralogically the most mature sand, reflecting intense weathering and/or prolonged transport/recycling that destroys unstable grains, typical of stable cratonic interiors and mature passive-margin beach/aeolian settings. Arkose (F > 25%, F > L) is feldspar-rich, signalling a granitic/gneissic source terrane eroded rapidly under arid or cold climate with little time for feldspar to hydrolyse to clay — classic of fault-bounded rift and fan-delta settings adjacent to basement uplifts. Litharenite (L > 25%, L > F) is rich in unstable rock fragments, pointing to a fine-grained sedimentary, volcanic or low-grade metamorphic source (e.g., a fold-thrust belt or volcanic arc) eroded and deposited quickly, commonly in foreland or forearc basins. Between these end members lie two transitional classes (Q 75–95%): subarkose (F > L), a moderately mature, quartz-dominated sand still carrying some feldspar — typically a granitic source with more weathering or transport than an arkose; and sublitharenite (L > F), a quartz-dominated sand with a minor rock-fragment fraction, common in recycled-orogen and cratonic fluvial/deltaic settings.
Orthogonal to composition, the textural (Dott/Pettijohn) classification splits each compositional field by matrix content: arenites (<15% matrix, clean, grain-supported, well winnowed — high-energy, well-sorted depositional processes such as beach, aeolian dune or fluvial channel-bar) versus wackes (15–75% matrix, poorly sorted, e.g. greywacke — deposited rapidly with little winnowing, characteristic of turbidity currents and glacial/debris-flow settings). Above 75% matrix the rock grades into a mudrock. Combining the two axes gives the full name, e.g. "well-sorted quartz arenite" versus "feldspathic greywacke," and together they encode both provenance/weathering history (compositional maturity) and depositional energy/sorting process (textural maturity).
