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.
Dunham's (1964) scheme classifies carbonate rocks by depositional texture — whether the rock was mud-supported or grain-supported at deposition, and whether original components were bound together in place — rather than by composition. Embry & Klovan (1972) kept Dunham's mud/grain-supported logic but split Boundstone into three organism-specific classes and added two coarse-clast classes (>2 mm allochems) that the original scheme had no place for.
| Class | Fabric | Criterion |
|---|---|---|
| Mudstone | Mud-supported | <10% grains |
| Wackestone | Mud-supported | ≥10% grains |
| Packstone | Grain-supported | Contains mud matrix |
| Grainstone | Grain-supported | No mud (sparry cement only) |
| Floatstone (E&K add) | Matrix-supported | >10% grains >2 mm "floating" in finer matrix |
| Rudstone (E&K add) | Grain-supported | >10% grains >2 mm, clast-to-clast contact |
| Bafflestone (E&K split) | Original components bound in place | Erect, stalk-like organisms baffle/trap sediment (e.g. crinoid thickets) |
| Bindstone (E&K split) | Original components bound in place | Encrusting/laminar organisms bind sediment (e.g. algal mats) |
| Framestone (E&K split) | Original components bound in place | Rigid, wave-resistant frame-building organisms (e.g. corals) |
The Embry & Klovan additions solve two practical problems the original scheme could not: Floatstone/Rudstone give coarse, reef-flank talus and storm-lag deposits (rich in >2 mm allochems) a proper home distinguished purely by whether the coarse clasts touch each other, while splitting Boundstone by baffling/binding/framing organism behaviour lets a field or core description carry genuine paleoecological and reservoir-architecture information (a framestone builds a rigid, high-relief, high-primary-porosity reef core, whereas a bindstone forms a low-relief, laminated, lower-porosity algal-mat crust) that a single "Boundstone" label would otherwise erase.
