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.
A conformable boundary represents essentially continuous, uninterrupted deposition — no significant break in the rock or time record, with parallel bedding across the contact and no evidence of subaerial exposure or erosion. An unconformable boundary instead represents a significant gap (hiatus) in the sedimentary/time record, produced either by a pause in deposition (non-depositional hiatus) or by active erosion that removed previously deposited strata.
| Type | Defining relationship |
|---|---|
| Angular unconformity | Older strata tilted/folded then eroded (planed off) before younger strata were deposited essentially horizontally on top; bedding sets meet at a discordant (angular) contact |
| Nonconformity | Younger sedimentary strata deposited on older, eroded igneous or metamorphic basement rock — a fundamental rock-type change across the surface |
| Disconformity | Two parallel-bedded sequences separated by an erosional surface showing subtle relief, channeling, or a paleosol |
| Paraconformity | Bedding parallel across the contact with no obvious erosional relief at all; hiatus recognized only indirectly, typically by biostratigraphic or radiometric dating |
