Skolithos Ichnofacies: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, May 2016, Q11

Question 11 of 20: Skolithos Ichnofacies

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 11: Skolithos Ichnofacies (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: (b) Skolithos. The Skolithos ichnofacies is characterized by simple to complex, densely packed, near-vertical, unlined to well-lined dwelling burrows (e.g., Skolithos, Ophiomorpha, Diplocraterion) made by suspension-feeding organisms that need a stable vertical retreat in a constantly shifting, well-sorted, high-energy sandy substrate – classically a foreshore/shoreface beach or high-energy tidal-channel sand. Cruziana (a) is a lower-energy, offshore/shelf ichnofacies made by mobile deposit-feeding/grazing trilobite-like organisms in firm but muddier substrates. Zoophycos (c) indicates quiet, deeper, lower-oxygen offshore-to-slope conditions with systematic deposit-feeding spreiten burrows. Trypanites (d) is a hardground/rockground boring ichnofacies, formed in cemented or lithified substrates, not loose sand.

Topic. Seilacherian ichnofacies and depositional-energy interpretation.

Key relations. Skolithos = high-energy shifting sand (vertical dwelling burrows); Cruziana = moderate-energy offshore mud/sand; Zoophycos = quiet, deep, low-oxygen; Trypanites = hard/lithified substrate.

Why this works. Trace-fossil assemblages reflect the behavioral strategy an organism needs to survive in a given substrate consistency and energy level – a shifting high-energy sand favors vertical, rapidly re-excavatable dwelling structures, which is exactly the Skolithos morphology.

Common pitfall. Confusing ichnofacies (a recurring behavioral/environmental association, independent of the exact taxon present) with a body-fossil biozone – the same ichnogenus can occur in more than one ichnofacies, and it is the overall assemblage and substrate that define the ichnofacies, not one trace alone.

Source. Reading (ed.), Sedimentary Environments, 3rd ed., ch.4 (trace fossils and ichnofacies); Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.5.