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.
Forced regression belongs to the Falling-Stage Systems Tract (FSST) — the systems tract that develops while relative sea level is actively FALLING, positioned between the underlying highstand systems tract and the overlying lowstand systems tract in a full sequence-stratigraphic succession.
A forced regression develops whenever relative sea level falls at the shoreline: the fall removes accommodation, so the shoreline is pushed (forced) basinward by the falling base level itself, whatever the sediment supply. This is different from a normal regression, where sea level is stable or slowly rising and the shoreline advances only because sediment supply outpaces the creation of accommodation. The conditions are therefore: (1) a genuine relative sea-level fall — a eustatic fall and/or tectonic uplift (or slowing subsidence) that together lower base level at the shoreline; and (2) enough sediment supply to build shoreline deposits during the fall (supply controls how far and how thickly the shoreline progrades, not whether the regression is forced — with little supply the shoreline still retreats basinward but leaves mostly an erosional ravinement/regressive surface). The result is successive shoreline positions that step basinward and DOWNWARD in elevation (a falling shoreline trajectory: forestepping, offlapping, downstepping clinoform sets). The FSST sits on the basal surface of forced regression and is capped updip by the subaerial unconformity (sequence boundary) cut by fluvial incision across the newly exposed shelf.
This distinguishes forced regression sharply from a normal regression (lowstand or highstand systems tract): in a normal regression, sea level is stable or slowly rising and the shoreline progrades purely because sediment supply exceeds the (slow) rate of accommodation creation, producing aggradational-to-progradational (not purely offlapping/forestepping) parasequence stacking.
