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.
Turbidity currents are triggered when a mass of sediment on a slope is suddenly destabilized and remobilized into a dense, sediment-laden gravity flow. Common triggers include slope oversteepening by rapid sediment accumulation (especially at a delta front or submarine-canyon head), seismic shaking, storm-wave loading of shelf-edge sediment, and a relative sea-level fall that exposes and destabilizes previously stored shelf-edge sediment. They occur wherever a significant slope break exists below sea level: continental slopes and their submarine canyons, delta fronts, and active fault scarps — feeding sediment down-slope onto submarine fans and into deep basin floors.
The depositional model is the Bouma sequence, the idealized vertical succession left by a single waning turbidity-current event as its competence and capacity drop through time: Ta, a massive or normally graded division deposited from rapid, near-instantaneous suspension fallout of the coarsest load; Tb, planar (parallel) laminated sand deposited under upper-flow-regime plane-bed conditions as flow decelerates; Tc, ripple cross-laminated (and commonly convolute) fine sand/silt deposited as flow drops into the lower flow regime; Td, parallel-laminated silt/mud deposited from the final, very dilute stages of the flow; and Te, structureless pelagic/hemipelagic mud representing the background "fair-weather" deposition between turbidity-current events. Not every division is preserved in every bed — a proximal, high-energy flow may deposit only Ta–Tc before waning completely, while a distal, thin flow may leave only Td–Te.
