18-Geol-A3 Sedimentation and Stratigraphy · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A3, Sedimentation & Stratigraphy, 2018-Dec. Closed book, no calculator, 3 hours. Part 1: Sedimentology (Questions 1–12, 5 marks each) instructs "Answer questions 1 and 2, and any other 6 questions from the remaining 10 questions (i.e., questions 3–12)" for 40 marks total. Part 2: Stratigraphy and Sedimentary Basin Analysis (Questions 13–19, 5 marks each) instructs "Answer five of the following seven questions" for 25 marks total (65/65 maximum).
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, carbonate platforms, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis, grain-size analysis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin classification, reservoir facies).
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 breccia is a coarse clastic rock composed mainly of angular gravel-sized fragments (in contrast to a conglomerate's rounded clasts), indicating little or no abrasive transport before final deposition.
Breccias form by several distinct processes: (1) Sedimentary/talus breccia — angular rockfall/talus debris accumulating at a cliff base with essentially zero transport. (2) Tectonic (fault) breccia — brittle shattering of rock along a fault plane during movement, producing angular clasts cemented in situ by later mineralisation. (3) Solution-collapse breccia — dissolution of an underlying soluble unit (evaporite or carbonate karst) removes support and the overlying strata collapse and shatter in place. (4) Volcanic breccia — explosive fragmentation of lava/pyroclastic material, or autobrecciation of a flow's brittle crust as the still-mobile flow interior continues to move. (5) Impact breccia — hypervelocity meteorite impact shatters target rock instantaneously. In every case the common thread is that the fragmentation mechanism (shattering, collapse, or minimal-transport gravity fall) leaves no time or hydraulic process available to round the clasts, unlike the sustained fluvial/current transport that rounds conglomerate clasts.
