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.
Diagenesis is the sum of all physical, chemical and biological changes a sediment undergoes after deposition and before metamorphism (below greenschist-facies temperature/pressure) — the processes that convert a loose sediment into a lithified sedimentary rock and continue to modify it thereafter. In siliciclastic rocks, three stages are recognized, defined by the sediment's burial history and the pore fluid it is in contact with at each stage.
| Stage | Setting / pore fluid | Example process | Example product |
|---|---|---|---|
| Eogenesis | Near-surface, shortly after deposition; meteoric or marine pore water | Early calcite cementation of a shallow, meteoric-water-saturated sand | Poikilotopic calcite cement occluding primary intergranular pores |
| Mesogenesis | Progressive burial; isolated connate/formation water | Mechanical compaction and pressure solution at grain contacts as overburden increases | Stylolites (irregular, interpenetrating dissolution seams) and reduced primary porosity |
| Telogenesis | Uplift and erosion returning the unit to the near-surface, meteoric environment | Dissolution of earlier unstable grains/cement (e.g. feldspar or calcite cement) by undersaturated meteoric water | Secondary (mouldic/vuggy) porosity |
The staging matters because each pore-fluid environment favours a different reaction direction: eogenesis and telogenesis both involve meteoric/marine water at shallow depth but differ in the sediment's burial history (never buried versus previously buried and re-exposed), while mesogenesis is isolated from the surface and dominated by mechanical and thermally-driven diagenetic reactions instead.
