24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2013-May. 3 hours duration; closed book, no calculator permitted. Candidates answer any 10 of the 15 questions (10 marks each, 100 marks total) and are asked to illustrate answers with drawings wherever possible.
Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (texture classification, evaporites, clay minerals, sediment gravity flows, storm/shelf processes, bedforms, stable isotopes, geological time scale); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (carbonate fabric, dolomitization, reef facies); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (depositional systems, transgression/regression, sequence stratigraphy); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rock maturation, petroleum systems); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).
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.
Both are sediment gravity flows – masses of sediment-water mixture moving downslope under their own weight – and both are commonly triggered by the same events (slope oversteepening, rapid deposition/loading, or seismic shaking causing failure of an unstable subaqueous or subaerial slope). They differ fundamentally in flow rheology and sediment concentration, which in turn produces very different deposits.
| Debris flow | Turbidity current | |
|---|---|---|
| Rheology | Laminar, cohesive, non-Newtonian (plastic) – a high-concentration, mud-rich matrix supports clasts by matrix strength and buoyancy | Turbulent, dilute, essentially Newtonian – grains are kept in suspension by fluid turbulence alone |
| Deposition | "Freezes" en masse once driving stress drops below the matrix's yield strength – abrupt, whole-flow deposition | Progressive settling as turbulence wanes, largest/densest grains first |
| Deposit texture | Poorly sorted, matrix-supported diamict; ungraded or crudely graded; sharp, non-erosive to loading-structured base | Normally graded throughout (Bouma Ta–Te: graded sand → planar-laminated sand → ripple cross-laminated sand → parallel-laminated mud → pelagic mud); sharp, erosive base with flute/groove casts |
| Typical setting | Steep submarine-canyon walls, fan-delta slopes, base-of-slope aprons | Submarine-fan channels and lobes fed by a canyon, often initiated BY a debris flow or slump that dilutes downslope into a turbidity current |
The two are, in fact, end-members of a continuum: as a cohesive debris flow travels and entrains water it can progressively dilute and become turbulent, transforming into a turbidity current down-flow – which is why the two deposit types are so often interbedded in the same submarine-fan succession.
