18-Geol-A3 Sedimentation and Stratigraphy · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2016-May. Closed book, no calculator, 3 hours. Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes) instructs "Answer eight questions of your choice" (8 × 5 = 40 marks) and Part 2 (Questions 13–19, Stratigraphy and Sedimentary Basin Analysis) instructs "Answer five of the following seven questions" (5 × 5 = 25 marks), for a maximum attainable grade of 65/65.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, sequence stratigraphy, biostratigraphic correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone/carbonate classification, diagenesis, provenance); Selley & Sonnenberg, Elements of Petroleum Geology (turbidite reservoirs, trace-fossil facies context).
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 conformable boundary represents essentially continuous, uninterrupted deposition — there is no significant break in the rock or time record, and bedding across the contact is parallel with no evidence of subaerial exposure or erosion. An unconformable boundary instead represents a significant gap (hiatus) in the sedimentary/time record, produced either by a pause in deposition (non-depositional hiatus) or by active erosion that removed previously deposited strata.
Four principal types of unconformable contact are recognized. A disconformity separates two parallel-bedded sequences across an erosional surface — the beds above and below are parallel to each other, but the contact itself may show subtle erosional relief, channeling, or a paleosol, and is often difficult to recognize without careful bed-by-bed mapping or dating. An angular unconformity occurs where the older strata below were tilted or folded, then eroded (planed off), before younger strata were deposited on top essentially horizontally — the two sets of bedding meet at a discordant (angular) contact and are usually the easiest unconformity type to recognize in outcrop. A nonconformity is the contact between younger sedimentary strata and older, eroded igneous or metamorphic basement rock — it records not just a depositional hiatus but a fundamental change in rock type across the surface. A paraconformity is the most subtle type: bedding is parallel across the contact with no obvious erosional relief at all, and the hiatus is recognized only indirectly, typically through biostratigraphic or radiometric dating showing a missing time interval.
