18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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 correct answer is (a) — dominated by viscous forces, hydraulically smooth. Every turbulent boundary flow develops a thin viscous sublayer immediately above the bed, within which momentum transfer is governed by molecular viscosity rather than turbulent eddies. When the bed's grains (mud to fine sand) are small enough to lie entirely within this sublayer, the roughness elements are effectively "drowned" beneath the viscous film and cannot disturb the overlying turbulent flow, so the boundary behaves as hydraulically smooth regardless of the bed's true grain-scale texture. Once grain size exceeds the viscous sublayer thickness (coarser sand/gravel beds), the grains project through it and generate turbulent wakes directly, and the boundary becomes hydraulically rough — ruling out options (b)–(d), all of which pair the wrong force/roughness combination with the stated fine-grained condition.
The correct answer is (d) — a relatively quiet, sublittoral zone. In Seilacher's ichnofacies scheme, the Cruziana ichnofacies is characteristic of moderate-energy, sublittoral (offshore shelf) settings below fair-weather wave base but above storm wave base, dominated by horizontal grazing and shallow dwelling traces (e.g. Cruziana, Rhizocorallium, Asteriacites) made by mobile deposit-feeders and infaunal organisms in a stable, muddy-to-sandy substrate. It is distinguished from the shallower, higher-energy Skolithos ichnofacies (vertical dwelling burrows in a mobile, sandy-shore substrate — option a), the hard-substrate Trypanites ichnofacies of a rocky/bored coast (option b), and the deep-marine, turbidite-associated Nereites ichnofacies (option c).
