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.
A debris flow is a gravity-driven, sediment-gravity-flow mass movement in which clasts of all sizes (mud to boulders) are held in transport by the strength and buoyancy of a dense, cohesive, water-saturated matrix (matrix strength/plastic rheology and buoyancy support the load, not fluid turbulence), so the deposit is characteristically poorly sorted, ungraded (or only crudely graded), matrix-supported, and can show large "rafted" clasts floating with no internal stratification. A turbidite is the deposit of a turbidity current — a sediment-gravity flow in which the excess density comes from sediment held in suspension by fluid turbulence itself (a true, if dense, turbulent fluid flow, not a plastic mass); as the current decelerates, grains settle out of turbulent suspension in order of decreasing settling velocity, producing the diagnostic normally-graded Bouma sequence (Ta massive/graded sand, Tb plane-parallel lamination, Tc ripple cross-lamination, Td upper parallel lamination, Te pelagic/hemipelagic mud drape).
The diagnostic field distinction is grading and fabric: turbidites are normally graded, internally structured (Bouma divisions), sharp-based with a scoured/flute-marked sole, and thin (cm–dm scale, basin-plain sheet geometry); debris-flow deposits (debrites) are ungraded or only crudely graded, matrix-supported, structureless, commonly thick and lobate/channelized, and can carry very large outsized clasts far beyond what any turbulent current could suspend. Both are gravity-flow deposits common on continental slopes, submarine fans and in front of deltas, and they commonly interfinger — a debris flow can transform downslope into a turbidity current as it dilutes and becomes turbulent (a debris-flow-to-turbidity-current transformation), so a single event bed can show a debrite base grading up into a Bouma-graded top.
Paleontological/rock-record significance: both processes are important taphonomic (fossil-preservation) agents — a turbidity current or debris flow can rapidly transport and re-deposit shallow-water fossils (reef-derived bioclasts, shallow benthic fauna) into deep basinal settings, producing "exotic" or reworked fossil assemblages in deep-water rock that do not reflect the in-situ depositional environment (a classic pitfall for paleoecological interpretation, directly analogous to the biofacies-vs-migratory-event distinction discussed elsewhere in this bank); rapid burial by a debris flow or turbidite can also produce exceptional preservation (obrution deposits) of otherwise fragile organisms. Turbidite sequences are also central to the rock record as the dominant deep-marine clastic reservoir/facies architecture studied in submarine-fan systems (e.g. classic ancient examples: the Ross Formation, Ireland; Marnoso-Arenacea, Italy; Tanqua Karoo, South Africa), while large-scale debris-flow/debrite complexes (e.g. the giant submarine slide deposits of passive-margin slopes) record catastrophic slope failure and are a key geohazard indicator in the rock record.
