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.
A rimmed shelf has a shallow, low-energy lagoon behind a high-energy reef/shoal rim at the shelf-slope break; the rim absorbs incoming wave energy, so it accumulates coarse, well-washed grainstone/boundstone, while the protected lagoon deposits low-energy lime mud (wackestone/mudstone) with restricted, sometimes evaporitic, fauna. A carbonate ramp has no abrupt rim — energy decreases gradually and continuously offshore-to-onshore (or the reverse, onshore-to-offshore on a homoclinal ramp), so grain size and sediment texture grade smoothly from high-energy grainstone/packstone in the shoreface to low-energy lime mud in the deeper ramp, with no sharp facies break. An epeiric (or epicontinental) platform is an extremely broad, very shallow, low-relief sea flooding a continental interior; tidal/wave energy is damped by the huge fetch-limited, shallow-water friction, producing widespread low-energy mudstone/wackestone punctuated by localised high-energy shoal/bank complexes where local shoaling concentrates tidal currents. An isolated platform (carbonate bank, e.g. an atoll) sits surrounded by deep water on all sides, so its margins face open-ocean swell on every side, producing a symmetric high-energy reef/grainstone rim encircling a low-energy central lagoon — a rimmed-shelf energy pattern repeated all around the platform rather than facing only one open-ocean direction.
