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 facies is a body of rock distinguished from adjacent rock by its lithology, sedimentary structures, fossil content, or all of these — a descriptive package of characteristics that reflects the depositional environment and process that formed it (e.g. a "cross-bedded, medium-grained, trough-cross-stratified sandstone facies" implies a fluvial or aeolian dune environment).
Walther's Law (Walther, 1894) states that the vertical succession of facies observed at one location, in a conformable sequence with no depositional breaks, records the SAME lateral succession of environments that existed side-by-side at the time of deposition — i.e., only facies that were once laterally adjacent to one another can occur in vertical superposition. As shorelines transgress (move landward) or regress (move basinward) through time, environments migrate laterally, so a vertical section at any one point progressively "samples" the environments that pass over it — a shoreface sand overlain by offshore mud (recording transgression, shoreline retreating landward, deeper-water facies migrating over shallower) or offshore mud overlain by shoreface sand (recording regression, shoreline prograding basinward). Walther's Law is the fundamental basis for interpreting a single vertical borehole or outcrop log as a record of laterally shifting depositional environments through time.
