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 Bouma Sequence (Bouma, 1962) is the idealized vertical succession of sedimentary structures deposited by a single waning turbidity-current flow, recording progressively decreasing flow competence and velocity upward through one event bed. A complete sequence has five divisions, base to top:
| Division | Structure | Flow regime |
|---|---|---|
| Ta | Massive or normally-graded sand/gravel, sharp erosional/loaded base | High-velocity, high-concentration sediment fallout — too rapid for traction structures |
| Tb | Planar (parallel) lamination in sand | Upper-flow-regime plane bed |
| Tc | Ripple cross-lamination and/or convolute (soft-sediment-deformed) lamination in fine sand | Lower-flow-regime traction, ripple migration |
| Td | Parallel, faint lamination in silt | Weak/waning traction current, low-energy suspension settling |
| Te | Structureless to laminated mud/clay (pelitic) | Quiescent, purely suspension settling of the finest tail; commonly bioturbated once deposition ceases |
Individual event beds rarely preserve all five divisions — a thinner, more distal or less energetic flow may begin at Tb, Tc or Td (a "truncated" Bouma sequence), while a proximal, high-concentration flow may deposit a thick amalgamated Ta with the upper divisions eroded by the next flow. The Bouma Sequence is the diagnostic depositional model for turbidites, deep-marine/lacustrine gravity-flow deposits typically built into submarine-fan systems (channel, levee, lobe) at the base of a slope, and represents a single depositional EVENT (not a facies succession through time, unlike the Walther's-Law progressions of Question 14).
