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.
A transgression is a landward migration of the shoreline through time, caused by relative sea-level rise (a rise in eustatic sea level, an increase in basin subsidence, a fall in sediment supply, or some combination of the three) such that the shoreline moves inland and progressively younger, deeper-water facies come to overlie progressively older, shallower-water facies at any fixed location. A regression is the opposite: a seaward migration of the shoreline caused by relative sea-level fall or by sediment supply outpacing the rate of accommodation creation, so that shallower-water facies prograde out over deeper-water facies through time.
What actually controls transgression and regression is not eustatic sea level alone but relative sea level — the position of the sea surface relative to the subsiding (or, less commonly, uplifting) basin floor — because it is relative sea level that sets the accommodation space (the volume available for sediment to fill between the basin floor and sea level). Subsidence (thermal cooling of oceanic/continental lithosphere, sediment/tectonic loading and flexure, or fault-controlled extension) continuously creates new accommodation even when eustatic sea level is static, which is why passive continental margins can record long-term transgressive trends through the Phanerozoic even during eustatic highstands and lowstands alike. The interplay is captured by the simple balance between the rate of accommodation creation (eustasy + subsidence) and the rate of sediment supply: when accommodation creation outpaces supply, the shoreline transgresses (a "starved" or underfilled basin, however much sediment is actually arriving); when supply outpaces accommodation creation, the shoreline regresses (a "filled" or overfilled basin, even during continued subsidence and eustatic rise).
Because depositional environments migrate with the shoreline, transgression and regression produce distinctive, predictable vertical facies successions that follow Walther's Law (laterally adjacent facies belts, when they migrate over one another through time, are stacked vertically in the same order they occur laterally). A transgressive succession fines and deepens upward — e.g. fluvial/alluvial sands overlain by estuarine or shoreface sands, overlain in turn by offshore muds, commonly resting on a thin, fossil-rich transgressive lag above a ravinement surface at its base, where wave reworking during landward shoreline migration erodes the underlying deposits and concentrates coarse material and skeletal debris. A regressive (progradational) succession coarsens and shallows upward — offshore mud passing up into shoreface/deltaic sand, capped by fluvial or coastal-plain deposits — and is the classic "coarsening-upward" log signature engineers and geologists use to identify prograding shoreline, deltaic or fan systems. Two kinds of regression are distinguished: a normal regression occurs while relative sea level is still rising (or static) but sediment supply outpaces accommodation creation, and grades continuously from marine to nonmarine with no erosional break; a forced regression is driven by a fall in relative sea level, which forces the shoreline seaward regardless of supply, commonly producing a sharp-based, downstepping shoreface sand on a regressive surface of marine erosion and a subaerial unconformity (river incision) on the exposed shelf landward of it.
Recognizing transgressive versus regressive packages matters directly to engineering practice: a coarsening-upward regressive sand body (better sorted, better cemented near its top) commonly makes a competent, laterally continuous foundation or aquifer unit, while a fining-upward transgressive lag sitting on an erosion surface is thin, discontinuous and often a poor, unpredictable bearing stratum; ravinement/transgressive surfaces are also classic loci for reworked, coarse, permeable material that can form preferential seepage paths beneath dams and levees. Because transgressive-regressive cycles repeat (driven by orbital-scale eustatic cycles superimposed on longer basin subsidence trends), a single borehole log through a passive-margin or foreland-basin succession typically shows several stacked coarsening-upward/fining-upward cycles, and correlating these cycles between boreholes is one of the principal tools of subsurface stratigraphic correlation for site investigation.
