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.
Clastic (allochemical) chemical and biochemical rocks are built of discrete, transported or reworked grains — texturally analogous to a siliciclastic sandstone even though the grains themselves are chemically/biochemically precipitated — and are cemented by a chemical (sparry calcite) or fine (micrite) matrix. The archetypal example is an oolitic grainstone (Question 8: ooliths formed by agitation-driven CaCO₃ precipitation, then transported and deposited as current/wave-worked shoal sand) and, more broadly, any bioclastic grainstone/packstone built of transported, current-sorted skeletal fragments (bioclast hash, crinoid ossicle grainstone). Engineering features (oolitic grainstone): good original sorting and grain-supported fabric give high primary porosity before cementation, similar to a clean quartz arenite, making oolitic grainstones a classic reservoir/aquifer facies; however, once cemented by sparry calcite (common, given the high original permeability that let cementing fluids circulate freely), strength and permeability become highly dependent on the degree and pattern of cementation, and, being calcite, the rock remains susceptible to later karst dissolution (Question 4) wherever exposed to undersaturated groundwater.
Non-clastic (crystalline) chemical and biochemical rocks instead form as an interlocking crystalline mosaic precipitated directly in place, with no discrete transported grains — texturally more like an igneous rock than a sandstone. Examples include evaporites (rock salt/halite, gypsum/anhydrite, precipitated by evaporative concentration of a restricted brine body), crystalline (non-bioclastic) limestone/dolomite formed by direct chemical precipitation or by pervasive replacement (dolomitization) of a precursor limestone, and biochemical examples such as chert (microcrystalline silica, precipitated inorganically or via diagenetic replacement/recrystallization of originally biogenic opaline silica from siliceous plankton). Engineering features (evaporite — gypsum/anhydrite): evaporites are highly soluble (far more so than limestone) and karst-like dissolution can proceed rapidly, producing sinkholes, cavities and differential settlement wherever evaporite beds are present near the water table or along a flow path — a well-documented hazard for dams, highways and foundations sited over gypsum or salt formations; anhydrite additionally undergoes a volume-increasing hydration to gypsum (anhydrite + 2H₂O → gypsum) when exposed to fresh groundwater, which can generate significant swelling pressure against confined structures (tunnel linings, foundations), a distinct and serious geotechnical hazard specific to this rock type.
The clastic/non-clastic distinction maps onto a broader engineering contrast: clastic (allochemical) rocks inherit a grain-supported, porous fabric whose engineering behaviour (before and after cementation) parallels an ordinary sandstone/carbonate grainstone, while non-clastic (crystalline) rocks are typically low-porosity, interlocking, and their principal engineering risk is chemical (solubility, hydration/swelling) rather than purely textural/mechanical — a distinction that directly determines which category of site investigation (porosity/permeability testing versus solubility/karst-hazard and swelling-potential testing) is appropriate for a given rock type.
