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.
Where a seismic reflector (representing a bedding surface) meets a sequence boundary, its termination style records whether the contact is concordant (the reflector runs parallel through the boundary with no discernible truncation or onlap) or discordant (the reflector is cut off at an angle to the boundary). Two independent surfaces must be examined: the UPPER boundary of the underlying Sequence A, and the LOWER boundary of the overlying Sequence B.
At the upper (top) boundary of Sequence A, three configurations are possible: erosional truncation — A's reflectors are cut off abruptly at a high angle against an unconformable, erosionally-irregular boundary (discordant, erosional); toplap — A's gently-dipping clinoform reflectors progressively flatten and terminate just below a relatively planar, NON-erosional boundary, recording sediment bypass/non-deposition rather than erosion (discordant, non-erosional); and concordance — A's reflectors run parallel and continuous right up to the boundary (no discordance at all, a conformable top).
At the lower (base) boundary of Sequence B, the mirror-image three configurations occur: onlap — B's initially near-horizontal reflectors progressively terminate updip against a boundary that had original relief/slope, each successively higher reflector extending further onto the paleo-high as accommodation increases (discordant); downlap — B's inclined (clinoform, prograding) reflectors terminate downdip onto a relatively flat boundary below them (discordant); and concordance — B's reflectors are parallel to and continuous with the boundary (no discordance, a conformable base). A real sequence boundary/unconformity is typically expressed as truncation updip (near basin margin, subaerial exposure and erosion dominate) grading basinward into a correlative conformity where the same time-surface shows onlap or even concordance — the same physical surface can therefore show different termination styles at different points along its length.
