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 depositional sequence is a relatively conformable succession of genetically related strata bounded above and below by unconformities (and their correlative conformities) — the fundamental building block of sequence stratigraphy, generated by one full cycle of relative sea-level change (rise and fall).
A fully-developed sequence is subdivided into systems tracts, each an assemblage of contemporaneous depositional systems formed during a specific segment of the relative sea-level cycle: (1) Lowstand Systems Tract (LST) — deposited during and just after maximum sea-level fall/lowstand, directly above the sequence boundary; includes basin-floor fans, slope fans, and a prograding lowstand wedge as sea level begins to slowly rise again. (2) Transgressive Systems Tract (TST) — deposited during rapid relative sea-level rise, when the rate of accommodation creation outpaces sediment supply; a retrogradational (backstepping) parasequence set, capped at its top by the maximum flooding surface (MFS), typically the most condensed, organic-rich interval of the sequence. (3) Highstand Systems Tract (HST) — deposited as the rate of sea-level rise slows and then stalls near its highstand; a progradational-to-aggradational parasequence set building out over the condensed MFS. Where included as a fourth division, a Falling-Stage Systems Tract (FSST) is deposited during active relative sea-level FALL (forced regression), producing a sharp-based, seaward-stepping (forestepping) unit distinct from the normal-regressive HST above and the following LST below.
