18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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 (1962) scheme classifies limestones by depositional texture — whether the rock is mud- or grain-supported, and whether original components were bound together in place — rather than by composition.
| Class | Fabric | Criterion |
|---|---|---|
| Mudstone | Mud-supported | <10% grains |
| Wackestone | Mud-supported | ≥10% grains |
| Packstone | Grain-supported | Contains mud matrix filling intergranular pores |
| Grainstone | Grain-supported | Mud-free (sparry cement or open pores 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) | Bound in place | Erect, stalk-like organisms baffle sediment (e.g. crinoid thickets) |
| Bindstone (E&K split) | Bound in place | Encrusting/laminar organisms bind sediment (e.g. algal/stromatolitic mats) |
| Framestone (E&K split) | Bound in place | Rigid, wave-resistant frame-building organisms (e.g. corals) |
Embry & Klovan (1972) extended Dunham's original five classes in two directions: they split the single Boundstone class into three organism-specific classes (Bafflestone, Bindstone, Framestone) that carry paleoecological and reservoir-architecture information a single "Boundstone" label would otherwise erase, and they added Floatstone and Rudstone to give coarse (>2 mm) allochthonous carbonate debris — reef-flank talus, storm lags — a proper texture-based home that the original scheme, built around sand-size grains, had no place for.
