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18-Geol-A3 Sedimentation and Stratigraphy · May 2016

Question 11 of 19: Sandstone Classification Scheme

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2016-May. Closed book, no calculator, 3 hours. Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes) instructs "Answer eight questions of your choice" (8 × 5 = 40 marks) and Part 2 (Questions 13–19, Stratigraphy and Sedimentary Basin Analysis) instructs "Answer five of the following seven questions" (5 × 5 = 25 marks), for a maximum attainable grade of 65/65.

Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, sequence stratigraphy, biostratigraphic correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone/carbonate classification, diagenesis, provenance); Selley & Sonnenberg, Elements of Petroleum Geology (turbidite reservoirs, trace-fossil facies context).

Question 11: Sandstone Classification Scheme (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.

Q F L Quartz Arenite (>95% Q) 95% Q line 75% Q line Subarkose Sublitharenite Arkose Litharenite
Folk/Dott QFL ternary classification of sandstones (arenite, <15% matrix, row shown). Position along Q-F-L sets the compositional class; a separate matrix axis (not shown) reclassifies the same QFL fields as "wacke" equivalents once matrix exceeds 15%.

Sandstones are classified along two independent axes. The first is matrix content (the Dott/Pettijohn division): a sandstone with <15% clay/silt matrix is an arenite; one with 15–75% matrix is a wacke (above 75% matrix the rock is no longer a sandstone but a mudrock). The second axis is framework composition, plotted on a Q-F-L ternary diagram with quartz (Q, including polycrystalline quartz and chert) at one apex, feldspar (F) at a second, and lithic/rock fragments (L) at the third. Within the ternary, a sandstone with >95% quartz is a quartz arenite; between 75% and 95% quartz it is a subarkose (feldspar > lithics) or sublitharenite (lithics > feldspar); below 75% quartz, with feldspar dominant over lithics, it is an arkose, and with lithics dominant over feldspar it is a litharenite. Applying the same QFL position but with 15–75% matrix gives the wacke equivalents — feldspathic wacke and lithic wacke — so the full scheme is effectively two parallel QFL triangles, one for arenites and one for wackes, selected by the matrix content.