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.
A formation is the fundamental mappable lithostratigraphic unit — a body of rock with distinctive, consistent lithologic character, unifiable and mappable at the Earth's surface or in the subsurface.
Vertical (stratigraphic) boundaries between formations: (1) a sharp/abrupt but conformable contact — an abrupt lithology change with continuous deposition (e.g. an abrupt flooding-surface change from sandstone to shale); (2) a gradational (transitional) contact — the lithology changes progressively over a measurable interval as depositional conditions slowly evolve (e.g. sandstone gradually becoming more shaly upward); (3) an unconformable (erosional) contact — a surface of erosion and/or non-deposition separating the formations, representing missing time (angular unconformity, disconformity, or nonconformity depending on the underlying rock's structural/rock-type relationship).
Lateral boundaries between formations: (1) an interfingering (intertonguing) contact — two formations physically interleave as wedge-shaped tongues at their mutual lateral boundary, recording repeated oscillation of the environmental boundary through time (e.g. fluvial sandstone tongues interfingering with marine shale near a fluctuating shoreline); (2) a gradational facies-change contact — a continuous lateral transition in lithology reflecting a smoothly changing depositional environment along a single time-line (e.g. reef facies grading laterally into fore-reef then basinal facies); (3) a time-transgressive (diachronous) boundary — the same lithologic contact migrates in age laterally as an environment (e.g. a shoreline) sweeps across the basin through time, so the formation boundary is not a single time-plane.
