18-Geol-A3 Sedimentation and Stratigraphy · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2014-May. Open book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, stratigraphic principles, glacial and aeolian systems throughout); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary rock classification, carbonate petrology, diagenesis and porosity); Allen & Allen, Basin Analysis (isostasy, subsidence and accommodation space).
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.
The Froude number is the dimensionless ratio of inertial to gravitational forces in an open-channel flow:
$$Fr = \dfrac{v}{\sqrt{g\,d}}$$
where v is mean flow velocity, g is gravitational acceleration and d is flow depth. Fr < 1 is lower flow regime (subcritical, tranquil); Fr > 1 is upper flow regime (supercritical, rapid); Fr = 1 is the critical transition at which a hydraulic jump forms. Its significance in sedimentary geology is that Froude number, together with grain size and the particle Reynolds number, is the primary control on bedform type (ripples and dunes in lower regime; plane bed and antidunes in upper regime), so a preserved cross-stratification style in the rock record can be read directly as a paleo-flow-regime indicator.
An oolite is a carbonate (or, rarely, siliceous/ferruginous) sedimentary rock composed dominantly of ooids — small (typically 0.25–2 mm), sub-spherical grains built of concentric (and/or radial) laminae of aragonite or calcite precipitated around a nucleus (a skeletal fragment, peloid or quartz silt grain) as the grain is repeatedly rolled by wave and current agitation in warm, shallow, CaCO3-supersaturated water. Their concentric cortex forms because each grain spends alternating time in the agitated, CO2-degassing surf zone (where a lamina precipitates) and briefly at rest (where the lamina stabilizes), so ooids are a direct indicator of persistently high-energy, shallow, warm marine conditions — classic modern examples are the Bahama Banks and Persian Gulf shoals.
Allochems are the discrete, transportable carbonate particles that make up allochemical (clastic) limestone: ooids, peloids (structureless carbonate pellets, largely fecal in origin), intraclasts (fragments reworked from a penecontemporaneous, weakly lithified carbonate substrate) and bioclasts (skeletal debris — mollusc valves, echinoderm ossicles, foraminiferal tests, coral and algal fragments). Their significance in the geological record is twofold: as the carbonate analogue of siliciclastic sand grains they carry a direct depositional-energy and paleoenvironmental signal (allochem type and sorting distinguish a high-energy ooid shoal from a quiet lagoon dominated by peloids and micrite), and bioclastic allochems in particular carry biostratigraphic and paleoecological information through the identity of the contributing organisms, linking carbonate petrology directly to Question 4/5's stratigraphic and depositional tools.
An esker is a long, narrow, sinuous ridge of stratified sand and gravel deposited by a subglacial or englacial meltwater stream flowing within an ice tunnel beneath (or within) a retreating glacier or ice sheet; when the enclosing ice melts, the channel-fill sediment is let down onto or into the underlying till, leaving a positive-relief ridge that can run for kilometres to tens of kilometres, tracking the former meltwater conduit largely independent of present-day surface topography. Internally an esker shows the coarse, well-sorted, cross-bedded and imbricated gravels typical of a high-energy, confined fluvial channel. Economically, eskers are prized as local sources of sand and gravel aggregate for concrete and road construction (especially valuable in glaciated regions where clean, well-sorted aggregate is otherwise scarce), and their coarse, permeable fill can also form productive shallow groundwater aquifers.
Sensitivity, St, is the ratio of a clay's undisturbed (intact) undrained shear strength to its fully remolded strength at the same water content: St = su(undisturbed)/su(remolded). A "quick" clay is one at the extreme high end of this scale (Canadian Foundation Engineering Manual: St > 16, "very sensitive"; some clays measure in the hundreds) where remolding by even a small strain collapses the material from a stiff solid to a near-liquid slurry with essentially no residual strength. Quick clays of eastern Canada (the Champlain Sea / Leda clay of the St. Lawrence – Ottawa valleys), and their counterparts in Sweden and Norway, share the same origin: a fine-grained marine clay deposited in a glaciomarine or postglacial marine embayment, whose original flocculated, open, "house-of-cards" fabric was held together by a salt-water pore-fluid ionic bond; subsequent postglacial isostatic rebound uplifted the clay above sea level, and slow groundwater flushing over millennia progressively leached the original salt from the pore water, destroying the ionic bonding while leaving the open, metastable fabric intact and the strength largely dependent on that now-weakened bonding. Engineering significance: because the fabric is metastable, a quick clay slope can fail progressively and retrogressively — a small initial disturbance (erosion at the toe, a cut, vibration, rainfall-driven pore pressure rise) remolds a limited zone to a liquid, removes lateral support, and the failure surface retrogresses far upslope, producing catastrophic long-runout flow slides (e.g. the 1971 Saint-Jean-Vianney and 1978 Rissa, Norway landslides); quick clay terrain therefore requires specialized site investigation (in-situ vane shear/sensitivity testing, pore-water salinity profiling) before any grading, loading or excavation.
