18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).
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.
Relative sea-level (base-level/accommodation) change controls whether successive depositional environments stack landward, seaward, or directly on top of one another at a fixed location, and this stacking pattern is read directly from the vertical grain-size trend in a measured section.
During a relative sea-level rise (transgression), accommodation is created faster than sediment can fill it, so the shoreline retreats landward and each environment steps landward over the one deposited before it — a retrogradational stacking pattern in which progressively DEEPER-water facies are deposited directly on top of shallower ones at a fixed location, giving an overall fining- and deepening-upward vertical succession (e.g. shoreface sand overlain by offshore mud), commonly capped by a condensed section at maximum flooding. During a relative sea-level fall or stillstand with ample sediment supply (regression), the shoreline advances basinward and each environment steps seaward over the one before it — a progradational pattern of coarsening- and shallowing-upward successions (e.g. offshore mud passing up into deltaic/shoreface sand), which can be capped by a subaerial-exposure surface if base level falls below the depositional surface entirely. A relatively stable base level with sediment supply balancing the (small) accommodation created produces aggradation — a vertically stacked, laterally persistent facies belt showing little coarsening or fining trend at all.
