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.
| Property | Glacial (till) | Eolian (dune) | Beach |
|---|---|---|---|
| Sorting | Very poor (clay to boulders in one deposit) | Excellent (narrow, fine–medium sand range) | Good–excellent (wave winnowed) |
| Rounding | Angular to subangular | Well rounded, frosted surfaces | Subrounded to well rounded |
| Fabric | Unstratified, matrix-supported diamict | Large-scale, high-angle (≈30°) cross-bedding | Low-angle, seaward-dipping planar lamination |
| Mineralogical maturity | Immature (unstable minerals, rock flour preserved) | Very mature (quartz-dominated; unstable grains abraded away) | Moderately mature (composition reflects local hinterland; heavy-mineral placers common) |
These contrasts are caused entirely by the transport medium and its selectivity. Glacial ice is an extremely high-viscosity solid medium with essentially no hydraulic sorting capacity and negligible clast-clast abrasion in transport, so till carries every grain size the ice eroded, angular and mineralogically unweathered/unaltered, dumped en masse on melting. Wind is a low-density fluid that can only entrain and carry sand- and finer-size grains (coarser clasts are left behind as lag), and repeated saltation impacts abrade grains intensely against each other, so eolian sand is both extremely well sorted (a narrow size window the wind can move) and well rounded, while mechanically weak minerals (feldspar, mica) are progressively destroyed by the abrasion, leaving a quartz-mature residue. Beach sediment is sorted and rounded by the oscillatory, high-energy swash/backwash of breaking waves in the surf zone, which winnows fines seaward/offshore and rounds grains by surf-zone abrasion, but transport distances are typically much shorter than for wind-blown sand and the sediment supply is local, so beach sand retains more of the hinterland's original mineralogical variety (moderate rather than extreme maturity) and often concentrates dense, resistant heavy minerals into visible placer laminae by wave-energy density sorting.
