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.
Structures. Convolute bedding/lamination, load casts and their complementary flame structures, ball-and-pillow (pseudonodule) structures, dish-and-pillar (fluid-escape) structures, clastic dykes and sand volcanoes (liquefaction injection features), and slump folds/slump sheets are all classed as soft-sediment deformation (SSD) structures.
Causes. All arise from deformation of still water-saturated, poorly consolidated sediment, driven by one of three mechanisms: (1) gravitational (density) instability — a denser sand deposited over a less-dense, water-saturated mud is a Rayleigh-Taylor-unstable configuration that founders into load casts and complementary flame structures; (2) seismic shaking or wave loading triggers momentary liquefaction of loosely packed, saturated sand, and the escaping pore water carves dish-and-pillar structures or erupts as sand volcanoes/clastic dykes; (3) downslope gravitational failure of a water-saturated sediment package on even a gentle depositional slope produces coherent slump folds and slump sheets bound by a basal detachment (glide plane).
Distinguishing from tectonic structures. Three criteria, used together: (i) Confinement — SSD structures are confined to ONE bed or a thin package, sharply truncated by an overlying UNDEFORMED bed that simply buried and sealed them, and rest on an undeformed bed below; a tectonic fold or fault instead cuts through and offsets many beds and typically propagates up to the modern erosion surface or an angular unconformity. (ii) Orientation consistency — SSD slump-fold axes/vergence are controlled by the LOCAL paleoslope dip direction and so vary unsystematically from outcrop to outcrop, whereas tectonic folds/faults across a region share one consistent orientation and vergence tied to the regional stress field. (iii) Fabric — SSD structures deform primary depositional lamination in a ductile, fluid-lubricated way (dish structures, water-escape pipes, liquefied "clotted" textures) with no accompanying tectonic strain fabric, while genuinely tectonic small folds/faults are commonly accompanied by axial-planar cleavage, slickensided fracture surfaces, or other strain indicators, and can usually be correlated with the region's independently known deformation history.
