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.
Dunham (1962) classifies limestones by depositional texture — whether the rock was mud-supported or grain-supported at deposition, and whether the original components were bound in place — rather than by grain composition.
| Scheme | Class | Definition |
|---|---|---|
| Dunham (1962) | Mudstone | <10% grains in lime-mud matrix; mud-supported |
| Wackestone | ≥10% grains, still mud-supported | |
| Packstone | Grain-supported framework, mud fills the pores | |
| Grainstone | Grain-supported, mud-free | |
| Boundstone | Components bound in place by an organism at deposition | |
| Embry & Klovan (1972) addition | Bafflestone | Delicate branching organisms baffle sediment in place |
| Bindstone | Encrusting organisms (e.g. algal mats) bind sediment as deposited | |
| Framestone | Rigid organisms (e.g. corals) build a load-bearing wave-resistant framework | |
| Floatstone | >10% coarse (>2 mm) allochems floating in finer matrix | |
| Rudstone | Coarse (>2 mm) allochems, grain-supported |
Folk's (1959, 1962) classification is compositional/genetic: it names the rock from its allochem type plus its matrix/cement — [allochem]+"sparite" for grain-supported rock with sparry calcite cement (Dunham's grainstone equivalent) or [allochem]+"micrite" for allochems in a lime-mud matrix (wacke/mudstone equivalent). Allochem prefixes: intraclasts (reworked penecontemporaneous mud clasts), ooids ("oosparite"/"oomicrite"), fossils/bioclasts ("biosparite"/"biomicrite") and pellets ("pelsparite"/"pelmicrite"); pure lime mud is micrite, and organically bound rock is biolithite (Folk's Boundstone equivalent). Folk additionally records grain type, which carries facies information (e.g. oosparite implies a high-energy shoal edge).
