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18-Geol-A3 Sedimentation and Stratigraphy · December 2018

Question 2 of 19: Sandstone Classification Scheme (QFL)

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Notes on this paper

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 2: Sandstone Classification Scheme (QFL) (5 marks)

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 from a thin section by point-counting framework grains into three end members — Quartz (mono- + polycrystalline + chert), Feldspar (K-feldspar + plagioclase) and Lithic fragments (volcanic/metamorphic/sedimentary rock clasts) — recast to 100% and plotted on a QFL ternary diagram (Folk scheme). Quartz arenite (Q > 90%) is the most compositionally mature end member, reflecting intense weathering and/or recycling; arkose (Q < 75%, F > L) signals a granitic/gneissic source eroded rapidly with little chemical weathering; litharenite (Q < 75%, L > F) points to a sedimentary/volcanic/low-grade metamorphic source; the intermediate 75–90% Q band splits into subarkose and sublitharenite.

Q F L Quartz Arenite Subarkose Sublitharenite Arkose Litharenite
Fig. Q2 — QFL ternary classification of sandstones (Folk scheme): boundaries at Q = 90% (quartz-arenite field), Q = 75% (arkose/litharenite vs. subarkose/sublitharenite), and the F = L median splitting feldspathic from lithic compositions.

Orthogonal to composition, the Dott/Pettijohn textural axis splits every compositional field by matrix content: arenite (<15% matrix, clean, grain-supported) vs. wacke (15–75% matrix, poorly sorted, e.g. greywacke), giving full names such as "well-sorted quartz arenite" or "feldspathic greywacke."