18-Geol-A3 Sedimentation and Stratigraphy · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2014-May. Open book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, stratigraphic principles, glacial and aeolian systems throughout); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary rock classification, carbonate petrology, diagenesis and porosity); Allen & Allen, Basin Analysis (isostasy, subsidence and accommodation space).
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.
All three rock types begin as clastic detritus delivered to a depositional basin by weathering and erosion of a source terrane, but the balance between two components at the moment of deposition — sand-grade framework grains and clay/silt-grade matrix — is set by depositional energy and, critically, by how well the sediment was sorted before final deposition. A mudstone forms where the depositional energy is persistently low (offshore shelf, lagoon, distal floodplain, deep basin plain) so that only silt and clay settle out and virtually no sand-grade framework survives. An arenite forms where a sustained, effective transport/winnowing process (beach and shoreface wave action, river traction, wind, strong tidal currents) has removed essentially all of the fine mud, leaving a clean, sand-grade, grain-supported framework bound later by chemical cement. A wacke forms in between: a rapid, poorly-sorted depositional event (turbidity current, debris-flow-adjacent settling, glacially-influenced or tectonically-active rapid-erosion setting) delivers sand grains and a substantial mud matrix together, too fast for winnowing to separate them, so the sand grains end up "floating" in an abundant primary clay/silt matrix.
The Dott/Pettijohn textural scheme classifies sandstones by their matrix content (grains <0.03 mm, interpreted as primary depositional mud rather than diagenetic cement):
An arenite has <15% matrix (grain-supported, cement-bound framework, well sorted); a wacke has 15–75% matrix (poorly sorted, mud-supported to grain-supported transitional fabric, sand "floating" in abundant clay); a mudstone is >75% matrix (essentially all fine-grained, with only scattered, dispersed sand or silt grains, if any).
Detrital composition tracks provenance and weathering/transport maturity identically across all three, but is read very differently because of the matrix contrast: arenites can be sub-classified by their sand framework composition (quartz arenite: >95% quartz, intense weathering/reworking and chemical maturity; feldspathic arenite/arkose: significant feldspar, short transport from a granitic/gneissic source or arid climate limiting chemical breakdown; lithic arenite: significant unstable rock fragments, immature, proximal to source). Wackes are named the same way by their framework composition (e.g. lithic wacke = "lithic wacke"/greywacke, feldspathic wacke) but the abundant clay matrix itself is diagnostic of depositional process rather than provenance — it signals rapid, unsorted deposition (classically, submarine-fan turbidites) rather than a particular source rock. Mudstones carry the least framework-grain provenance information of the three (too little sand survives) and instead are characterized by their clay mineralogy (illite, kaolinite, smectite, chlorite) and by physical/chemical structures (fissility defining shale, versus blocky/non-fissile mudstone) that record compaction history and depositional water chemistry rather than sand-grain source.
