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 conglomerate is a coarse clastic sedimentary rock composed mainly of rounded gravel-sized (>2 mm) clasts cemented in a finer sand/mud matrix. A diamictite is a poorly-sorted rock containing a wide range of clast sizes (clay to boulder) scattered through a muddy matrix, with no hydraulic sorting — typically of glacial (tillite), debris-flow, or mass-wasting origin.
Classification by fabric support: an orthoconglomerate (clast-supported) has gravel clasts touching and load-bearing, with matrix only filling the intervening pores — it records a hydraulically-sorted, current-winnowed deposit (fluvial channel-lag, beach gravel). A paraconglomerate (matrix-supported) has clasts "floating," isolated from one another, in an abundant fine matrix — it records a single non-selective, high-strength mass-transport event (debris flow, till) with no winnowing; diamictite is by definition matrix-supported/paraconglomeratic.
Classification by clast stability: extraformational (polymictic) conglomerates contain durable, chemically/mechanically stable clast types (quartzite, chert, granite) sourced from outside the depositional basin and surviving long transport, recording a mature, distant or recycled source. Intraformational (oligomictic) conglomerates contain soft, mechanically unstable "rip-up" clasts (mud/shale intraclasts) eroded from penecontemporaneous sediment within the same basin — they cannot survive long transport, so they indicate erosion and immediate local re-deposition (e.g. storm-scoured tidal-flat muds).
