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.
A paleosol is an ancient (buried, lithified or preserved) soil horizon — a surface that was subaerially exposed long enough for pedogenic (soil-forming) processes to modify the parent sediment/rock in place, before being buried by younger deposits and thus removed from further modern soil development.
In the field, a paleosol is recognized from a combination of features rarely all produced by normal sedimentation: (1) a vertical soil profile with differentiated horizons (an organic-rich A horizon grading down into a B horizon of illuviated clay/oxide accumulation, over a C horizon of weathered parent material); (2) root traces/rhizoliths — vertical, downward-tapering, often carbonate- or iron-lined tubes; (3) pedogenic nodules, especially calcareous caliche/calcrete nodules or ferruginous concretions formed by in-situ precipitation; (4) mottled or gleyed colouration (red/orange Fe-oxide mottles alternating with grey/green reduced zones) recording fluctuating water table and redox conditions; (5) destruction of primary sedimentary structures (bedding, lamination) by pedoturbation (root/burrow mixing, shrink-swell clay churning); and (6) commonly a sharp but non-erosional (or gently erosional) contact with the overlying fresh sediment, capping a zone that becomes progressively more altered downward.
Paleosols are important stratigraphic markers: they record subaerial exposure surfaces (useful sequence-boundary indicators, allied with the unconformities of Question 15), and their degree of development (a thick, strongly horizonated, mature profile vs. a weakly developed "Entisol"- or "Inceptisol"-type profile) constrains the duration of the exposure/non-deposition interval they represent.
