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.
As sediment is deposited (or a volcanic rock cools through the Curie temperature), fine magnetic mineral grains (magnetite, hematite) align with the Earth's ambient magnetic field at that instant and lock in a record of its polarity (normal, matching today's field, or reversed) — this remanent magnetization is preserved essentially permanently once the rock lithifies, forming the basis of magnetostratigraphy.
Because the Earth's magnetic field has flipped polarity repeatedly and essentially randomly (but globally synchronously) through geologic time, a stratigraphic section can be sampled at closely-spaced intervals and its sequence of normal/reversed magnetozones plotted as a local magnetic polarity column. This local pattern of reversals is then matched (correlated) against the globally-calibrated Geomagnetic Polarity Time Scale (GPTS), itself built and dated from marine magnetic anomaly stripes and radiometrically-dated volcanic sequences, allowing the local section to be assigned precise numerical ages.
This application is especially valuable in two situations where biostratigraphy alone is weak: (1) non-marine/terrestrial sequences that lack the abundant, well-zoned marine fossils needed for fine biostratigraphic resolution, and (2) correlating and dating sequences with SPARSE OR NO FOSSILS at all (since the reversal signal is a purely physical, not biological, property preserved in essentially any fine-grained or volcanic rock). Magnetostratigraphy also provides an entirely independent cross-check on biostratigraphic and radiometric age models, and (because reversals are globally instantaneous events) offers correlation precision that facies-dependent biostratigraphic datums cannot match.
