18-Geol-A3 Sedimentation and Stratigraphy · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A3, Sedimentation & Stratigraphy, 2018-Dec. Closed book, no calculator, 3 hours. Part 1: Sedimentology (Questions 1–12, 5 marks each) instructs "Answer questions 1 and 2, and any other 6 questions from the remaining 10 questions (i.e., questions 3–12)" for 40 marks total. Part 2: Stratigraphy and Sedimentary Basin Analysis (Questions 13–19, 5 marks each) instructs "Answer five of the following seven questions" for 25 marks total (65/65 maximum).
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, carbonate platforms, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis, grain-size analysis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin classification, 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.
Three sedimentary structures routinely used to measure paleocurrent direction are: cross-bedding (the dip-azimuth of foreset laminae records the migration direction of the bedform, e.g. a dune or ripple); ripple-crest orientation and asymmetry (the steep-lee-side dip direction gives current sense directly); and clast imbrication in conglomerates (elongate/flat clasts dip upstream, so the up-dip direction of the imbricated a-b plane points opposite to flow). Sole marks (flute casts, groove casts) at the base of turbidite beds are a further common indicator.
Measured azimuths are plotted as a rose diagram (a circular histogram of directional frequency). Three characteristic patterns and the processes they represent: (1) a strongly unimodal rose (one narrow, well-defined peak) indicates a unidirectional flow, typical of a fluvial channel or a turbidity current confined to one basin axis; (2) a bimodal–bipolar rose (two peaks roughly 180° apart) indicates a reversing, bidirectional flow, the classic signature of a tidal channel (flood vs. ebb currents); (3) a polymodal (scattered) rose (many low, near-equal peaks around the full circle) indicates a non-directional or highly variable process, such as wind-reworked aeolian dune sets or wave-dominated shoreface deposits where wave approach direction varies with storm events.
