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 conformable contact represents continuous, uninterrupted deposition between Unit A (below) and Unit B (above) — no significant time gap, no subaerial exposure/erosion, and bedding of A and B is parallel and depositionally continuous across the contact. Conformable contacts are of two kinds: abrupt (sharp) contacts, where lithology changes suddenly at a single bedding plane with no erosion or gap (e.g., a quick change in sediment supply); and gradational contacts, either progressive (A passes gradually into B through a zone of continuously changing grain size or composition, e.g. sandstone fining up into shale) or intercalated (beds of A and B interfinger/alternate through a transition zone before B takes over). An unconformable contact (unconformity) instead represents a significant gap in the rock/time record — a period of non-deposition and/or erosion separates the time A finished forming from the time B began.
Four principal unconformity types are recognized, distinguished by the structural relationship of A to B: an angular unconformity — Unit A was deposited, then tilted/deformed and eroded to a flat (or otherwise non-parallel) surface, before horizontally-bedded Unit B was deposited on top, so A's bedding is measurably ANGULAR (discordant) to B's; a disconformity — A and B remain essentially PARALLEL (both flat-lying or both dipping identically), but an erosional surface (commonly showing relief, channelling or a paleokarst) separates them, representing a period of exposure/erosion without any intervening tectonic tilting; a nonconformity — Unit B (sedimentary) is deposited directly on top of Unit A that is IGNEOUS or METAMORPHIC basement rock, recording deep erosion down to crystalline rock before B's deposition; and a paraconformity (a cryptic, non-depositional break) — A and B are parallel with NO visible erosional relief or obvious physical break at all, and the time gap is detected only indirectly, via a biostratigraphic hiatus (a "missing" biozone) or radiometric dating.
