18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).
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.
(1) Cross-bedding/cross-stratification — the dip direction of foreset laminae points directly downcurrent, the single most widely used paleocurrent tool. (2) Flute casts (sole marks on the base of a turbidite or channel-fill bed) — scoured by a discrete, unidirectional erosive flow, with the elongate axis parallel to flow and the blunt, deeper end pointing upcurrent. (3) Clast imbrication in conglomerates — elongate/platy clasts settle tilted with their upstream face dipping gently upcurrent (like overlapping roof tiles leaning into the flow), giving a direct current-direction reading from clast fabric alone. (Ripple cross-lamination and groove casts are equally valid alternatives.)
(1) Unimodal, unidirectional — a narrow, single-peaked spread indicates one dominant, laterally confined unidirectional flow, typical of a fluvial channel or a delta-front distributary where flow direction is fixed by channel geometry. (2) Bimodal–bipolar — two modes roughly 180° apart indicates a regularly REVERSING flow, the classic signature of a tidal environment (flood-tide and ebb-tide currents running opposite directions through the same channel or sand-wave field). (3) Polymodal/scattered — many modes with no single dominant direction indicates reworking by multiple, independently-oriented current or wave systems, typical of a shallow marine shelf reworked by storms and longshore currents from varying directions (or, in a non-marine setting, an aeolian dune field with a shifting wind regime).
