18-Geol-A3 Sedimentation and Stratigraphy · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2018-May. Closed book, no calculator, 3 hours, 80 marks total. Part 1 (Questions 1–10, Sedimentology and Sedimentary Processes, 50 marks) instructs "Questions 1 and 2 must be answered (10 points each); answer any FIVE of Questions 3–10 (6 points each)." Part 2 (Questions 11–19, Stratigraphy, 30 marks) instructs "Answer any FIVE of Questions 11 to 19 (6 points each)."
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin settings, 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 (1964) classifies carbonates by depositional texture — whether the rock was mud- or grain-supported at deposition and whether original components were bound together in place — rather than by grain composition:
| Dunham class | Description |
|---|---|
| Mudstone | <10% grains ("allochems") in a lime-mud (micrite) matrix; mud-supported |
| Wackestone | ≥10% grains, still mud-supported (grains float in micrite) |
| Packstone | Grain-supported framework, but with mud matrix filling the pores |
| Grainstone | Grain-supported, mud-free (pores are cement-filled or open) |
| Boundstone | Original components bound together during deposition by an organism (e.g. reef framework) |
Embry & Klovan (1972) retained Dunham's mud/grain-supported logic but resolved Boundstone into three organism-specific classes and added two classes for coarse (>2 mm) allochems that the original scheme lacked: Bafflestone (delicate branching/stalked organisms baffle and trap sediment in place), Bindstone (encrusting organisms, e.g. algal mats, bind sediment as it is deposited), Framestone (rigid, load-bearing organisms such as corals build a true wave-resistant framework), Floatstone (>10% coarse >2 mm allochems "floating" in a finer matrix — the coarse-grain analogue of a wackestone) and Rudstone (coarse >2 mm allochems that are grain-supported — the coarse analogue of a grainstone).
Folk's (1959, 1962) classification is compositional/genetic rather than purely textural: it names the rock from its allochem type plus its matrix/cement type, e.g. [allochem prefix]+"sparite" (grain-supported, sparry calcite cement, energy sufficient to winnow mud — Folk's equivalent of a grainstone) or [allochem prefix]+"micrite" (allochems in a lime-mud matrix, low-energy — equivalent of a wacke/mudstone). Allochem prefixes are intraclasts (reworked penecontemporaneous carbonate mud clasts), ooids (concentrically coated grains — "oosparite"/"oomicrite"), fossils/bioclasts ("biosparite"/"biomicrite") and pellets (structureless fecal/micritic grains — "pelsparite"/"pelmicrite"); pure lime mud with no allochems is micrite, and Folk further recognises biolithite for organically bound rock (his equivalent of Boundstone).
The two schemes are complementary: Dunham/Embry&Klovan is preferred in the field and in core description (fast, purely textural, no allochem identification needed), while Folk is preferred at thin-section scale because it additionally records grain type, which carries facies/paleoenvironmental information (e.g. an oosparite implies a shoal/high-energy platform edge; a pelmicrite implies a restricted lagoon).
