18-Geol-A3 Sedimentation and Stratigraphy · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2013-May. Open book, 3 hours. All twelve questions are of equal value (12 marks each, plus 4 bonus marks for neatness) and the exam instructs "answers to eight (8) questions constitute a full examination paper".
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, stratigraphic principles throughout); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary rock classification, carbonate and chemical/biochemical rocks, diagenesis).
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.
Terrigenous (siliciclastic) sandstones are first subdivided texturally by the proportion of fine-grained (< 30 μm, clay/silt-size) matrix versus the sand-size framework grains, following the classification scheme popularized by Dott and Pettijohn: an arenite is a "clean" sandstone with less than 15% matrix (framework grains supported directly by other framework grains, with pore space occupied by cement and/or open porosity); a wacke (or greywacke, in older/informal usage) is a "dirty" sandstone with 15–75% matrix (framework grains suspended within, and often not touching each other through, an abundant clay/silt matrix); and beyond 75% matrix the rock is no longer a sandstone at all but a mudstone (or, distinguishing further by fissility, a shale) — a matrix-dominated rock with only scattered, "floating" sand grains, if any.
The amount of matrix is not an arbitrary label but a direct record of depositional process and provenance maturity. Arenites form where transport/reworking is prolonged and energetic enough (beach, shoreface, aeolian, fluvial channel-bar environments with repeated winnowing) to physically remove the fine clay/silt fraction by current/wave action before or during deposition, leaving well-sorted, well-rounded, grain-supported sand — a texturally (and often compositionally) mature deposit. Wackes form where fine material is deposited together with the sand rather than being winnowed out, either because deposition is too rapid for winnowing to act (e.g. sediment-gravity-flow/turbidity-current deposition, where the whole poorly-sorted grain-size population is dumped essentially at once) or because the depositional setting itself has low current energy (e.g. glaciomarine or some deltaic settings); wackes are consequently texturally immature and, being deposited quickly with little reworking, often also compositionally immature (retaining unstable lithic fragments and feldspar rather than only resistant quartz). A mudstone represents deposition from suspension in quiet water with essentially no traction-current sand supply or winnowing at all.
The matrix content directly controls porosity/permeability and strength: clean, well-sorted arenites typically retain high primary intergranular porosity and permeability (good aquifer/reservoir rock, and, once cemented, a comparatively strong, brittle foundation material), whereas matrix-rich wackes have their pore space largely occluded by clay/silt at deposition, giving low permeability even before burial compaction, and behave geotechnically more like a fine-grained, potentially clay-sensitive rock than a "clean" sandstone; mudstones are typically the lowest-permeability, most compressible and, if clay-rich, most swelling-prone of the three, commonly forming the confining/cap units that isolate arenite aquifers or reservoirs.
