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.
The Froude number is the ratio of inertial to gravitational forces in an open-channel flow, and it is what physically distinguishes the two flow regimes recognized in sedimentology:
$$Fr = \dfrac{v}{\sqrt{g\,d}}$$
where v is mean flow velocity, g is gravitational acceleration and d is flow depth. Lower flow regime corresponds to Fr < 1 (subcritical, tranquil flow, where a surface disturbance can propagate upstream); upper flow regime corresponds to Fr > 1 (supercritical, rapid flow, where disturbances are swept downstream and cannot propagate against the current); Fr = 1 is the critical condition at which a hydraulic jump forms as flow transitions between the two regimes.
Ripples (wavelength < ~0.6 m, height a few cm) form first as velocity rises above the threshold of grain motion in fine sand; they migrate downstream by avalanching down the lee slope and produce small-scale, asymmetric cross-lamination. As velocity increases further, dunes (larger, wavelength decimetres to metres) develop the same lee-side avalanche mechanism at a larger scale, producing medium- to large-scale trough and planar cross-bedding — the single most diagnostic sedimentary structure of unidirectional subcritical flow in sand-grade sediment. The lower-regime (lower-stage) plane bed belongs at the LOW-velocity end of this sequence, not above dunes: in coarse sand (coarser than about 0.7 mm) ripples never form, and at velocities just above the threshold of motion grains roll and slide as a thin bedload sheet over a flat bed, producing planar (horizontal) lamination of rather poorly sorted coarse sand before dunes appear at higher velocity.
Crossing the transition near Fr ≈ 1 (through a washed-out, unstable zone where dunes are destroyed), flow enters the upper flow regime. An upper-regime plane bed again produces planar lamination, but this one is distinguished from its lower-regime counterpart by associated primary current lineation (parting lineation) — faint grain alignments parallel to flow visible on bedding-parallel partings — and by better sorting/grain alignment from the higher shear. At the highest Froude numbers, standing waves develop on the water surface that are in phase with symmetrical bed waves called antidunes; unlike ripples and dunes, antidunes can migrate upstream (against the flow direction) as the standing wave train migrates, and their preservation potential is low because they typically collapse as flow wanes, but where preserved they produce low-angle, commonly upstream-dipping cross-lamination that is diagnostic of high-energy supercritical flow (e.g. flash floods, proximal alluvial-fan channels, some turbidite tops).
Because each bed form and its internal structure is diagnostic of a specific flow regime, reading cross-bedding, lamination style and their scale from core or outcrop lets an engineer reconstruct the paleoflow energy, direction and likely channel geometry of a sand body — directly informing predictions of grain-size trends, permeability anisotropy (cross-bed foresets create strong directional permeability), and the geometry/continuity of channel-fill sand bodies relevant to foundation design, aggregate resource assessment and groundwater flow modelling.
