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.
| Type | Geometry | Recognition |
|---|---|---|
| Angular unconformity | Older strata tilted/folded and eroded before younger strata are deposited non-parallel on top | Obvious angular discordance in bedding attitude across the surface |
| Disconformity | Beds above and below are parallel; an erosional surface separates them | Often subtle — relief/channeling on the surface, or a missing biozone/age interval revealed by biostratigraphy or radiometric dating |
| Nonconformity | Sedimentary strata deposited directly on eroded igneous or metamorphic basement | Contrasting rock type (layered sediment on crystalline basement) makes the contact itself obvious |
| Paraconformity | Beds parallel, NO visible erosion surface or relief at all | The subtlest type — detected only indirectly, via a missing biozone, dated ash bed, or geochemical (chemostratigraphic) gap |
| Diastem | A very short, local, sub-biozone-resolution depositional pause or omission surface within an otherwise continuous succession | Rarely resolvable by dating; often marked by a firm-ground/hard-ground ichnofacies (Question 12) or a thin lag |
These five types form a continuum of decreasing GEOMETRIC obviousness even as the underlying time gap can be just as large: an angular unconformity and a nonconformity announce themselves in the field through an obvious change in bedding attitude or rock type, whereas a disconformity, and especially a paraconformity, can look like an entirely unremarkable bedding plane and require independent chronological control (biostratigraphy, chemostratigraphy, radiometric dating of an interbedded ash) to even recognize that time is missing at all. A diastem, by contrast, represents genuinely little geological time (commonly less than the resolving power of biostratigraphy) and is best thought of as a normal, ubiquitous feature of any depositional system with intermittent, rather than perfectly continuous, sediment supply, distinct from the larger, basin-significant gaps the other four types record.
