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.
Tidal deposits form in the intertidal-to-subtidal zone under the twice-daily reversing flow of flood (landward) and ebb (seaward) tidal currents, and are diagnosed by structures that record that current reversal: herringbone cross-stratification (two sets of cross-beds dipping in opposite directions, one from each tidal phase), reactivation surfaces (a cross-bed foreset partially eroded and reactivated by the reversing current), tidal bundles (rhythmic thick/thin foreset couplets recording the neap-spring tidal cycle) and alternating sand/mud (flaser, wavy and lenticular bedding) recording alternating higher-energy sand transport and slack-water mud drape deposition.
Secondary porosity is pore space created after deposition by diagenetic modification, as opposed to primary (depositional) porosity — the original intergranular pore space present at deposition, subsequently reduced by compaction and cementation. The principal mechanism is dissolution of unstable framework grains (feldspar, lithic rock fragments) or of earlier cements (carbonate cement) by acidic pore fluids generated during burial (organic-acid- and CO2-charged waters from maturing organic matter, or meteoric water during uplift/unconformity exposure), which can create oversized, irregular "moldic" or "vuggy" pores that partially or wholly reverse the porosity lost to compaction/cementation; fracturing (tectonic or diagenetic) is a second important secondary-porosity mechanism. Recognizing secondary porosity matters because it can locally restore excellent reservoir/aquifer quality at burial depths where primary porosity alone would have been destroyed, but the resulting pore network (isolated dissolution vugs, poorly connected) can have much lower permeability than its porosity value alone would suggest, which is an important distinction for reservoir/aquifer engineering.
A carbonate reef model is an idealized facies model describing the predictable lateral zonation of a wave-resistant, organically-constructed carbonate buildup, from the open ocean toward the protected lagoon: a fore-reef talus slope of reef-derived debris shed basinward; the reef crest/reef flat, the shallowest, highest-wave-energy zone built by the most robust, wave-resistant frame-building organisms (corals, coralline/calcareous algae); and a back-reef/lagoon zone of low-energy, often muddy carbonate sediment with patch reefs, behind the main framework. The model is used predictively: because each zone has a characteristic, recognizable facies (framework limestone at the crest, poorly-sorted talus breccia fore-reef, muddy peloidal/skeletal wackestone back-reef), identifying one facies in outcrop or core lets a geologist predict the position and character of the others.
Pelagic sediments are fine-grained deposits that accumulate slowly on the deep ocean floor, far from terrigenous clastic input, by the settling of biogenic and fine detrital particles through the water column. Two broad classes are recognized by what dominates the settling flux: biogenic oozes, dominated by the microscopic skeletal remains of planktonic organisms — calcareous ooze (coccolithophore plates and foraminiferal tests, e.g. chalk) above the carbonate compensation depth, and siliceous ooze (diatom or radiolarian tests) where either productivity is very high or the seafloor lies below the CCD so calcareous material dissolves; and pelagic (red/brown) clay, the very slowly accumulating fine terrigenous and volcanic dust fraction that dominates where biogenic productivity is too low, or the water too deep, for oozes to accumulate. Examples: the White Cliffs of Dover chalk (calcareous ooze), deep Pacific radiolarian ooze, and abyssal red clay.
Aeolian (wind-blown) deposits comprise well-sorted, well-rounded, frosted sand in dune bedforms (with large-scale, high-angle cross-stratification, grain-flow/grain-fall laminae) and airborne silt deposited as blanket-like loess far from its source. Engineering characteristics differ sharply between the two: dune sand is typically loose, uniformly-graded and highly permeable, prone to liquefaction under cyclic/seismic loading and to wind erosion once vegetation cover is disturbed, and provides poor bearing capacity until compacted. Loess is deposited with an open, metastable, weakly cemented (commonly carbonate- or clay-bonded) grain fabric that gives it deceptively high strength and stands in near-vertical natural cuts when dry, but is collapsible: on wetting, the cementing bonds fail and the fabric collapses suddenly to a much denser, lower-volume state, producing large, often differential settlement under foundations — a major geotechnical hazard requiring pre-wetting, compaction or deep foundations wherever structures are sited on loess.
