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.
Both are terrestrial, gravity-and-discharge-driven, unconfined-to-semi-confined fluvial systems whose channels migrate laterally through time, both interbed channel-fill sand/gravel with finer overbank/interchannel deposits, and both produce composite bodies built by the superposition of many individual channel and flood events rather than a single depositional episode.
A meandering stream is a single, low-gradient, sinuous channel of roughly constant discharge and grain size along a given reach, confined between levees, that migrates primarily by lateral point-bar accretion (gradual, one bend at a time) and produces a classic fining-upward vertical sequence (basal channel-lag gravel/coarse sand, fining up through point-bar cross-bedded sand, capped by fine overbank silt/mud, often with paleosols). Its geometry is elongate and ribbon-like in plan, roughly parallel-sided down the length of the valley, with a near-constant channel width and depth for a given discharge. An alluvial fan, in contrast, is a high-gradient, radiating, half-cone-shaped body deposited where a confined, sediment-charged mountain stream loses confinement and gradient abruptly at a range front; discharge and grain size decrease systematically outward from the fan apex (proximal fan: coarse, poorly sorted debris-flow and channel gravel; medial fan: braided/sheetflood sand and gravel; distal fan: fine sheetflood sand interfingering with basin-floor mud), producing a radial fining trend outward from the apex rather than a simple fining-upward trend at one location, and vertical sections instead often show abrupt, poorly-sorted, matrix-supported debris-flow deposits interbedded with better-sorted stream-flow sheetflood/channel deposits, reflecting the fan's characteristically episodic, flashy discharge regime (arid/semi-arid climates and tectonically active range fronts are the classic settings) rather than the perennial, steady discharge typical of meandering rivers.
A meandering-river sand body is a laterally continuous but internally heterogeneous (fining-upward, cross-bedded) unit whose main hazard is unpredictable channel avulsion/erosion undermining structures built close to the active channel. An alluvial fan is geotechnically more hazardous for a different reason: debris flows are sudden, high-energy, poorly-sorted mass-movement events capable of destroying structures with little warning, fan channels avulse abruptly and unpredictably across the whole fan surface (not just laterally within a belt), and the coarse, poorly-sorted proximal-fan deposits typically have very high but extremely heterogeneous permeability, complicating both foundation design and groundwater/contaminant transport modelling.
