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.
| Structural group | Origin |
|---|---|
| Nodular / "chicken-wire" | Sabkha (supratidal) displacive growth — anhydrite/gypsum nucleates and grows WITHIN a host carbonate/siliciclastic mud from capillary-concentrated brine drawn up by evaporative pumping; growing nodules push the host mud aside and eventually coalesce into an interlocking polygonal mosaic separated by thin mud stringers (resembling chicken wire in cross-section). |
| Bedded / laminated | Subaqueous precipitation from a standing evaporating brine body (restricted lagoon, salina or deep evaporite basin) — as the water body progressively concentrates past gypsum/anhydrite saturation, mineral settles from the water column (or crystallizes at the brine floor) in thin, laterally continuous, commonly varve-like laminae that record seasonal or annual concentration cycles. |
| Enterolithic (contorted) | Volume-change buckling — the gypsum↔anhydrite transformation (hydration/dehydration) changes mineral volume within a confining host sediment; the resulting stress folds the evaporite layer into tight, contorted, intestine-like folds without any tectonic deformation of the enclosing beds. |
All three groups share the same ultimate driver — evaporative concentration of restricted marine or marginal-marine brine past gypsum (CaSO4·2H2O) and then anhydrite (CaSO4) saturation — but differ in WHERE that concentration happened (within a damp sabkha mud versus within a standing brine column) and, for the enterolithic group, in a secondary post-depositional volume-change overprint rather than a distinct depositional setting. Recognizing the structural group in core or outcrop is diagnostic of the wider depositional system it belongs to: nodular/chicken-wire fabric predicts an associated dolomitized supratidal-flat facies and an intermittently exposed, arid coastal setting, whereas thick, laterally persistent bedded/laminated gypsum predicts a deeper, longer-lived, more strongly restricted evaporite basin capable of sustaining a much larger standing brine column — two genuinely different basin histories inferred from fabric alone, before any other supporting evidence is examined.
