Biostratigraphic Unit Scope: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, May 2016, Q17

Question 17 of 20: Biostratigraphic Unit Scope

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2016-May. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes – 10 marks each; answer any eight of the twelve, 80 marks total) and Part 2 (Questions 13–20, Stratigraphy and Sedimentary Basin Analysis – 10 marks each; answer any five of the eight, 50 marks total) – a 130-mark maximum (80 for Part 1 + 50 for Part 2).

Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (grain texture, sediment transport, bedforms, carbonate/evaporite systems, sequence stratigraphy, unconformities, stratigraphic principles); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, trace fossils, coastal processes); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, basin classification, reservoir quality); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).

Question 17: Biostratigraphic Unit Scope (10 marks)

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.

Answer: (d) all of the above. Biostratigraphic units (biozones) are defined strictly by their fossil content and are, by definition, independent of lithostratigraphic unit boundaries (member, formation, group). Because fossil ranges, assemblages or abundance peaks bear no necessary relationship to lithologic contacts, a biozone's physical extent can fall anywhere relative to the lithostratigraphic framework: it may occupy only part of a single member (a), coincide closely with one complete formation (b), or cut across and span two or more members or formations (c) – all three relationships are commonly observed, so the correct, most complete answer is that a biostratigraphic unit may encompass any of them, i.e., (d).

Topic. Independence of biostratigraphic units from lithostratigraphic boundaries.

Key relations. Biozone boundaries are controlled by fossil occurrence/abundance; lithostratigraphic boundaries are controlled by rock type; the two classification systems are defined independently and need not coincide.

Why this works. Organisms respond to environmental and evolutionary controls (habitat, migration, extinction) that are largely decoupled from the sedimentological controls (sediment supply, energy, provenance) that define a lithologic contact, so there is no a priori reason for a fossil range or assemblage boundary to align with a bed, member or formation contact.

Common pitfall. Assuming a named biozone must correspond to a named formation because they are often mapped and discussed together in the same basin – the coincidence, where it occurs, is a local, empirical observation, not a rule of stratigraphic classification.

Source. Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.15; International Stratigraphic Guide (ISSC).