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.
Herringbone cross-stratification consists of stacked cross-bed sets in which successive sets dip in opposite (commonly near-180°) directions, producing a "fishbone" appearance in vertical section. It forms where a bedform migrates first one way, then is reactivated and migrates the opposite way, before the next set buries it — the signature of a strongly bidirectional, reversing current. It is diagnostic of tidal channels and tidal sand flats, where the ebb and flood tidal currents are both competent enough to build and preserve opposed cross-bed sets.
Flaser bedding is current-rippled sand in which thin, discontinuous mud drapes are preserved selectively in the ripple troughs, while the sand-rich ripple crests remain largely mud-free; sand dominates the bed overall. It forms where current/sand supply is generally strong but is punctuated by brief slack-water intervals long enough to deposit a thin mud drape before the next current resumes and reworks the crests. Lenticular bedding is the mud-dominated opposite: isolated, discontinuous lenses of ripple-form sand are enclosed within a dominantly muddy background, indicating that mud deposition (during slack water) dominates and sand is supplied only occasionally, during comparatively brief higher-energy pulses. Both, together with the intermediate wavy bedding (roughly equal sand and mud, continuous rippled sand layers with continuous mud drapes), form a continuum first described by Reineck & Wunderlich that tracks a gradient of tidal current energy and mud availability, and all are characteristic of tide-influenced environments — tidal flats, tidal channel margins and estuarine mixed flats — ranging from sand-dominated, higher-energy settings (flaser) through mixed (wavy) to mud-dominated, lower-energy settings (lenticular).
