18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).
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.
An ichnofacies is a recurring, characteristic ASSEMBLAGE of trace fossils (burrows, trails, borings, resting/feeding traces) that reflects a particular set of environmental conditions — substrate consistency, water energy, oxygenation, salinity and sedimentation rate — rather than a particular geological TIME period, in contrast to a biozone. Seilacher's archetypal ichnofacies form a well-known onshore-to-offshore/bathymetric series: Skolithos (vertical dwelling burrows, high-energy shifting sand of the shoreface/beach), Cruziana (grazing/crawling traces of a moderate-energy, muddier shelf), Zoophycos (spiral feeding traces of a deeper, quieter, lower-oxygen slope), Nereites (complex grazing trails of deep-marine, sediment-starved turbidite basins), and the diagenetically-early, substrate-controlled Glossifungites (firm-ground, omission-surface borings) and Trypanites (hard-ground borings into lithified substrate).
Importance: because the organisms making these traces respond to the physicochemical CONDITIONS of the substrate and water column at the moment of burrowing, not to a specific time interval, ichnofacies provide direct, in-place (non-transported, unlike most body fossils or clasts) evidence of paleoenvironment and energy level, and they SURVIVE diagenesis and even considerable physical/chemical alteration of the host sediment that can destroy body fossils and primary physical structures. The firm-ground and hard-ground ichnofacies (Glossifungites, Trypanites) are especially valuable because they specifically identify OMISSION SURFACES — horizons of early cementation or non-deposition that commonly mark sequence boundaries or other significant stratigraphic discontinuities (Question 15) — giving the stratigrapher an independent, biologically-anchored tool for recognizing gaps that might otherwise be missed from lithology alone. A second, equally important application is in intervals where storm reworking or intense bioturbation has homogenised or destroyed the primary PHYSICAL sedimentary structures a facies interpretation would normally rely on: the surviving trace-fossil assemblage is then often the only environmental evidence left in the rock, making ichnofacies analysis indispensable precisely where physical sedimentology alone runs out of information.
