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.
Bedded chert forms as a primary, laterally continuous biogenic ooze in deep-marine settings of high biosiliceous productivity (upwelling zones, oceanic ridges/abyssal plains) — the silica source is biogenic opal-A from the tests of radiolaria and the frustules of diatoms, which accumulate on the seafloor below the carbonate compensation depth (where calcareous plankton dissolves but opaline silica survives) or beneath a highly productive upwelling cell. Burial diagenesis progressively converts opal-A → opal-CT → microcrystalline quartz (chalcedony/quartz), lithifying the ooze into rhythmically bedded chert, often interbedded with shale.
Nodular chert instead forms as a secondary, diagenetic replacement product within a host carbonate (typically limestone or dolomite), growing as discrete nodules or lenses along bedding planes or around burrows/fossils. Its silica source is not biogenic accumulation in place but dissolved silica remobilised during diagenesis — commonly derived from the dissolution of dispersed biogenic silica (sponge spicules, radiolaria) originally scattered through the host carbonate, which reprecipitates locally as silica replaces the carbonate matrix (a diffusion-driven, localised replacement rather than a basin-wide primary deposit).
