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 delta progrades until channel-mouth friction losses and gradient become unfavourable, at which point the river avulses (switches course) to a shorter, steeper path to the sea, abandoning the old delta lobe; the abandoned lobe subsides and is reworked/drowned while a new lobe builds elsewhere — this delta-lobe switching (avulsion) cycle, superimposed on shorter-period autocyclic channel/mouth-bar processes and allocyclic (climatic, sea-level) forcing, is what makes deltaic successions strongly cyclic, each cycle recording the birth, growth, abandonment and drowning of one delta lobe.
Each cycle records a characteristic coarsening- and shallowing-upward package: basal prodelta mudstone (dark, laminated, bioturbated, fine grained, deposited from suspension in quiet, relatively deep water beyond the direct influence of the river mouth) grades upward into delta-front interbedded sand and mud showing wave- and current-generated structures (ripple and hummocky cross-stratification, storm beds) as water shallows, which in turn grades upward into a well-sorted, cross-bedded distributary-mouth-bar sand deposited at and just beyond the river mouth itself. The cycle is capped by delta-plain deposits (distributary channel sands with lateral levee/crevasse-splay fines, interdistributary bay muds, and, in humid climates, peat/coal formed in poorly drained interdistributary swamps) once the lobe has built out to sea level and become subaerial. The cycle top is typically an abrupt flooding surface where the next cycle's prodelta mud oversteps the abandoned, subsiding lobe with little or no gradual transition — the sharp juxtaposition of deep-water mud directly on top of shallow subaerial/coal deposits is itself diagnostic of avulsion-driven cyclicity, as opposed to the gradual transitions expected from purely allocyclic (sea-level) forcing alone.
Cyclic deltaic successions produce a stack of laterally-limited (delta-lobe-scale) sand bodies, each isolated above and below by low-permeability prodelta/interdistributary mud — a strongly heterogeneous and anisotropic engineering setting. Practical consequences include: (1) each mouth-bar/channel sand is a discrete, laterally discontinuous foundation or aquifer unit that must be correlated cycle-by-cycle rather than treated as one continuous stratum — a foundation resting on one cycle's sand may sit directly above a very different (weaker, compressible) mud of the underlying cycle; (2) interdistributary/prodelta muds are commonly under-consolidated and highly compressible, producing differential settlement where a structure spans a channel-sand/interdistributary-mud contact; (3) coal seams within the delta-plain cap are a mining and gas (methane) hazard and a source of long-term consolidation settlement once dewatered; (4) the sharp flooding surfaces are potential preferential groundwater flow paths and, where they truncate sand against sand across an abandoned lobe boundary, a plane of hydraulic connection between otherwise separate aquifer sand bodies.
