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.
A depositional sequence is a relatively conformable succession of genetically related strata, bounded above and below by unconformities (and their laterally correlative conformities) — it is the fundamental building-block unit of sequence stratigraphy, generated by one cycle of relative sea-level change (Question 18) acting on a basin's sediment supply.
Within a sequence, three (sometimes four) system tracts are recognized, each corresponding to a distinct segment of the relative sea-level curve. The Lowstand Systems Tract (LST) is deposited immediately after the sequence boundary, while relative sea level is at or near its lowest point and beginning to rise slowly; it is characterized by incised-valley fill, and basin-floor or slope fan deposits sourced from sediment bypassing the exposed shelf. The Transgressive Systems Tract (TST) is deposited as the rate of relative sea-level rise accelerates and outpaces sediment supply, producing a retrogradational (landward-stepping) stack of parasequences; it is capped by the maximum flooding surface (MFS), the point of maximum landward shift of facies and minimum sediment supply to the basin (often marked by a condensed section). The Highstand Systems Tract (HST) is deposited above the MFS as the rate of relative sea-level rise decelerates and sediment supply again outpaces the (slowing) creation of accommodation space, producing an aggradational-to-progradational (seaward-building) parasequence stack that continues until the next sequence boundary. Some models additionally recognize a Falling-Stage Systems Tract (FSST) for sediment deposited during active relative sea-level fall (forced regression), positioned between the HST and the next sequence boundary.
