18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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.
Physical (mechanical) weathering breaks parent rock into progressively smaller fragments without changing its mineral composition, which increases the surface area later available for chemical attack. Frost wedging (freeze–thaw) forces water into cracks and pore spaces; as the water freezes it expands roughly 9% in volume, prying the crack open a small amount with every cycle until the rock splits into angular fragments — the dominant process in cold and seasonally-freezing climates. Exfoliation (unloading) occurs when erosion progressively removes overlying rock/ice from a deeply buried pluton, releasing the confining pressure the rock formed under; the outer rock expands slightly and fractures in curved sheets parallel to the exposed surface, producing the onion-skin sheeting seen on granitic domes.
Chemical weathering instead alters the mineralogy itself, converting unstable primary silicates into stable secondary minerals plus a dissolved (solute) load, and in doing so weakens the rock's internal cohesion. Hydrolysis of feldspar (2KAlSi3O8 + 2CO2 + 11H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-) replaces a hard, interlocking silicate framework with soft, expansive clay minerals (kaolinite) plus dissolved potassium, bicarbonate and silica, so a granite's feldspar crystals crumble to clay while the rock's grain-to-grain bonding is destroyed from within. Oxidation of iron-bearing minerals (pyrite, biotite, olivine) converts Fe2+ to Fe3+ oxide/hydroxide (hematite, goethite); the reaction is accompanied by a volume increase around each altered grain, which generates internal stress that helps pry the rock apart (granular disintegration) in addition to producing the characteristic reddish-brown staining of a weathering profile.
Physical and chemical weathering are mutually reinforcing rather than independent: physical breakdown exposes fresh mineral surface area that accelerates chemical attack, while chemical weathering along grain boundaries weakens the rock fabric and makes it more susceptible to further mechanical disintegration.
