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 facies is a body of rock with a distinctive set of characteristics — lithology, sedimentary structures, texture, fossil content and geometry — that together reflect (are diagnostic of) a particular depositional environment or process, and that distinguish it from adjacent rock bodies formed under different conditions.
Walther's Law (1894) states that, in a conformable, unbroken vertical succession with no hiatus, the sequence of facies observed stacked VERTICALLY through time must be the same sequence of facies that existed LATERALLY, side by side, in space at any one time — because a facies can only come to vertically overlie its neighbour if that neighbouring environment migrates (via transgression or regression) into the same location. Equivalently: only facies that were lateral neighbours at the time of deposition can occur in vertical superposition in a conformable sequence.
This is a powerful and heavily-used tool because it lets a geologist reconstruct the full lateral (2-D map-view) arrangement of depositional environments in an ancient basin from a single vertical (1-D) core or outcrop section, and conversely predict the vertical facies sequence expected at a new location from a known lateral facies belt. It is the basis for interpreting classic transgressive (fining/deepening-upward, e.g. offshore mud over shoreface sand as the sea transgresses landward, each facies migrating up-succession as the shoreline retreats) and regressive (coarsening/shallowing-upward) facies successions, and underlies sequence-stratigraphic interpretation of parasequences (Question 17) and systems tracts (Question 18) generally.
