18-Geol-A3 Sedimentation and Stratigraphy · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2018-May. Closed book, no calculator, 3 hours, 80 marks total. Part 1 (Questions 1–10, Sedimentology and Sedimentary Processes, 50 marks) instructs "Questions 1 and 2 must be answered (10 points each); answer any FIVE of Questions 3–10 (6 points each)." Part 2 (Questions 11–19, Stratigraphy, 30 marks) instructs "Answer any FIVE of Questions 11 to 19 (6 points each)."
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin settings, reservoir facies).
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 answer is (d) Relatively quiet sublittoral (subtidal) zone. The Cruziana ichnofacies is one of Seilacher's archetypal trace-fossil assemblages, dominated by grazing and shallow-dwelling deposit-feeder/predator traces (Cruziana itself — a trilobite grazing/ploughing trace — along with Rhizocorallium, Asteriacites, Thalassinoides) that require a soft, cohesive, muddy-to-fine-sandy substrate below fair-weather wave base but still within reach of storm reworking — conditions typical of the open, moderate-energy, relatively quiet subtidal shelf.
The other options are wrong for physical reasons: (a) the deep-marine/abyssal zone is instead characterized by the Nereites ichnofacies (complex grazing/farming traces adapted to a food-poor, low-energy, pelagic-mud setting far beyond the reach of Cruziana's shallower-water trace makers); (b) a rocky coast is dominated by the Glossifungites/Trypanites-style firmground/hardground boring and encrusting assemblage, since Cruziana's soft-substrate burrowers cannot excavate consolidated or lithified rock; (c) a high-energy sandy shore (intertidal to shoreface) is instead the domain of the Skolithos ichnofacies — vertical, robust dwelling burrows (Skolithos, Ophiomorpha, Diplocraterion) adapted to a mobile, shifting, high-energy substrate, which is a fundamentally different (and much more vigorously reworked) setting than Cruziana's quiet subtidal mud/sand.
