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.
Paleocurrent data are collected in outcrop or core by measuring the orientation of directional sedimentary structures with a compass and clinometer: the dip azimuth of cross-bed foresets (which points downcurrent), the trend of asymmetric ripple crests, the elongation and blunt-to-tapered orientation of flute and groove casts on the base of turbidite beds, and the imbrication (upstream-dipping) orientation of elongate clasts. Dozens to hundreds of individual readings from a single bed set or stratigraphic interval are plotted on a rose diagram (an azimuthal histogram), and the SHAPE of that rose diagram is itself diagnostic of the depositional process.
Three characteristic patterns recur. A unimodal rose diagram, with one dominant, tightly clustered mode, indicates a single, persistently unidirectional flow — a fluvial channel, a longshore drift cell, or a turbidity current. A bimodal-bipolar pattern, with two modes opposed at roughly 180°, indicates a current that regularly reverses direction — the classic signature of a tide-dominated setting where ebb and flood currents both leave a strong, opposed imprint. A polymodal (scattered) pattern, with several unrelated modes or no clear preferred direction, indicates a setting without one dominant unidirectional process — a storm- or wave-dominated shoreface reworked from multiple directions, an eolian dune field with a variable wind regime, or a bioturbated interval where physical current structures have been partly destroyed.
