18-Geol-A3 Sedimentation and Stratigraphy · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2016-May. Closed book, no calculator, 3 hours. Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes) instructs "Answer eight questions of your choice" (8 × 5 = 40 marks) and Part 2 (Questions 13–19, Stratigraphy and Sedimentary Basin Analysis) instructs "Answer five of the following seven questions" (5 × 5 = 25 marks), for a maximum attainable grade of 65/65.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, sequence stratigraphy, biostratigraphic correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone/carbonate classification, diagenesis, provenance); Selley & Sonnenberg, Elements of Petroleum Geology (turbidite reservoirs, trace-fossil facies context).
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.
Textural maturity describes how far a sandstone has been carried, by transport and reworking, from a poorly organized initial deposit toward a fully winnowed and abraded one. Folk's four-stage scheme tracks three independent fabric properties in a fixed order: an immature sandstone still contains >5% clay/silt matrix and angular, poorly sorted grains; a submature sandstone has lost its matrix but remains poorly to moderately sorted and angular; a mature sandstone is matrix-free and well sorted, yet its grains are still angular to subangular; and a supermature sandstone is matrix-free, well sorted, AND well rounded. The order matters because matrix removal (winnowing) happens fastest, sorting improves next, and rounding is the slowest fabric change to develop.
Compositional (mineralogical) maturity instead measures the ratio of chemically/mechanically stable grains (quartz) to unstable ones (feldspar, lithic fragments, mica): a compositionally mature sand approaches a pure quartz arenite, while an immature sand retains abundant feldspar and rock fragments.
Both maturities are governed by the same underlying controls but respond at different rates. Transport distance/duration and depositional energy (wave/current reworking) drive textural maturity by repeatedly abrading and winnowing the grains. Climate is the dominant control on compositional maturity: humid climates accelerate chemical weathering, which destroys feldspar and lithic fragments before or during transport, whereas arid or cold climates preserve them. Relief and tectonic setting also matter — high-relief, rapidly uplifting source terranes shed sediment so quickly that unstable grains survive (low compositional but sometimes high textural maturity, e.g. some fan deltas), while a low-relief, tectonically stable craton allows long residence time, multiple recycling events and slow transport that raise both maturities together. Recycling (multiple sedimentary cycles) independently boosts both, since each cycle re-winnows and re-weathers the sediment.
