18-Geol-A3 Sedimentation and Stratigraphy · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2013-May. Open book, 3 hours. All twelve questions are of equal value (12 marks each, plus 4 bonus marks for neatness) and the exam instructs "answers to eight (8) questions constitute a full examination paper".
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, stratigraphic principles throughout); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary rock classification, carbonate and chemical/biochemical rocks, diagenesis).
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.
| Term | Definition |
|---|---|
| Allochems | The discrete, transported or in-situ-formed carbonate grains (ooids, peloids, skeletal/bioclastic fragments, intraclasts) that make up the framework of a limestone, analogous to the detrital grains of a siliciclastic sandstone; bound together by a micrite matrix and/or sparry calcite cement, allochem type and abundance are the basis of the Folk carbonate classification. |
| Carbonate compensation depth (CCD) | The ocean depth below which the rate of calcium carbonate dissolution (driven by increasing pressure and CO₂ solubility, and decreasing temperature, with depth) exceeds the rate of carbonate supply from surface productivity, so no calcareous sediment accumulates and only siliceous/clay pelagic sediment survives to the seafloor; typically found at roughly 4,000–5,000 m depth in the modern ocean (shallower in the Pacific, deeper in parts of the Atlantic). |
| Competency | The maximum particle size (diameter or mass) that a current of given velocity is capable of entraining and transporting, for both fluvial (water) and aeolian (wind) processes; competency increases sharply (roughly with the square, and for initiation of motion closer to a higher power, of flow velocity), which is why a modest increase in flood or wind velocity can suddenly mobilize much coarser material than the same current normally carries. |
| Debris flow | A rapid, gravity-driven mass movement of a poorly sorted, matrix-supported mixture of sediment (mud to boulders) and water (typically 20–60% water by volume) that behaves as a viscous, non-Newtonian slurry rather than as a stream; characteristic of steep, sediment-charged channels (alluvial fans, volcanic slopes) and deposits as a chaotic, ungraded, matrix-supported bed with a steep, lobate flow front, distinguishing it from the better-sorted, clast-supported deposits of normal stream flow. |
| Diastrophism | The general term for all large-scale deformation of the Earth's crust by tectonic processes — folding, faulting, uplift and subsidence — that produces the mountain-building (orogenic) and basin-forming (epeirogenic) movements ultimately responsible for creating the relief, subsidence and accommodation that drive erosion, sediment supply and deposition (Question 1). |
| Flute marks | Small, asymmetric, spoon-shaped scour structures cut into a cohesive mud substrate by turbulent, eddying flow (typically the head of a turbidity current) and then filled and preserved by the overlying coarser bed; the steep, deep end points upstream (where the scouring eddy formed) and the shallow, tapering end points downstream, making flute casts (seen in positive relief on the sole of the overlying bed) one of the most reliable sole marks for determining paleocurrent (paleoflow) direction, especially in turbidite successions. |
| Loess | A wind-blown (aeolian), unstratified, typically buff-to-yellow, well-sorted silt-grade deposit accumulated downwind of glacial outwash plains, deserts or other unvegetated silt sources; characteristically forms steep, stable vertical faces because of weak carbonate/clay cementation and a distinctive vertical jointing (pseudo-columnar) fabric, but is notoriously prone to catastrophic collapse (hydrocompaction/liquefaction-like failure) on wetting, making it a well-known engineering hazard for foundations and slopes in loess terrain. |
| Lysocline | The depth zone in the ocean, shallower than and distinct from the CCD, where the RATE of calcium carbonate dissolution increases sharply as water becomes progressively undersaturated with respect to calcite with depth; below the lysocline, carbonate tests show increasing dissolution/etching and preferential loss of more soluble forms (e.g. aragonite, high-Mg calcite) before disappearing entirely at the CCD. |
| Milankovitch cycle | Cyclic, predictable variation in the Earth's orbital geometry — eccentricity (~100 and ~405 kyr), axial obliquity/tilt (~41 kyr) and precession of the equinoxes (~19–23 kyr) — that modulates the seasonal and latitudinal distribution of incoming solar radiation and thereby paces climate cycles (glacial-interglacial cycles, monsoon strength); recorded in the stratigraphic record as rhythmic, repetitive bedding cyclicity (e.g. limestone-marl couplets, varved sequences) used for high-resolution "cyclostratigraphic" dating and correlation. |
| Oolith (ooid) | A small (typically 0.25–2 mm), spherical to sub-spherical carbonate (or, more rarely, siliciclastic/ironstone) grain with a concentric (and/or radial) internal lamination built up around a nucleus (a bioclast fragment or peloid) by repeated agitation-driven precipitation in warm, shallow, high-energy, CO₂-degassing water (Question 6); abundant ooid accumulations (oolite/oolitic grainstone) indicate a high-energy shoal environment and, because of their good sorting and high intergranular porosity prior to cementation, are a classic reservoir/aquifer facies. |
| Orogenic | Relating to orogeny — the tectonic process of mountain building by crustal shortening, thickening and uplift, typically at a convergent plate margin (continent-continent collision or subduction-related arc/fold-thrust belt formation); orogenic belts are the principal source terrain supplying siliciclastic sediment to adjacent foreland and passive-margin basins. |
| Pelagic sediments | Fine-grained sediment deposited slowly from suspension in the open ocean, far from direct terrigenous (land-derived) input, dominated either by the calcareous or siliceous skeletal remains of planktonic microorganisms (foraminifera, coccolithophores, radiolaria, diatoms — biogenic ooze, where productivity exceeds dissolution above the CCD/lysocline) or, in the deepest basins below the CCD, by wind-blown and current-transported clay (red/brown pelagic clay); characterized by very low accumulation rates, fine lamination or bioturbated homogeneity, and the absence of coarse terrigenous material. |
