C3 Clastic Sediments Lecture 4

1. C3 Clastic SedimentsLecture 4 Muds and Mass movement: Sediment gravity flows

2. Mud Material finer than 63 μm Silt is mainly quartz, calcite, chert, feldspar Fine and very fine silt has increasing clay mineral content Clay (<2µm) is mainly clay mineral (Kaolinite, Illite, Smectite, Chlorite)

3. Elastic: Strain linearly proportional to stress; strain recoverable. Earth’s crust Plastic: Above yield stress, material deforms permanently (by flow), With no additional increase of stress. Ice sheet Pseudoplastic Many muds Viscous: Strain linearly proportional to stress; strain permanent. Flow velocity ~ stress. Water du/dz, s-1. RHEOLOGY

4. Aggregation and flocculation • Fine particles stick together • particles must be brought together, and (b) they must stick. • Collision mechanisms: • Brownian motion,Shear, Differential settling, Biological intervention d < 1 μm dominant Brownian aggregation. d > 1 μm dominant Shear aggregation Pictures of mucous coatings that will bind/not bind in weak/ strong electrolytes Electrostatic forces next to a particle in an electrolyte

5. Settling behaviour of mud suspensions

6. Critical erosion conditions for muds – use of yield strength Erosion rate E (kg/m2/s) is related to (τo – τc): E = M(τo – τc)n

7. Critical deposition conditions for muds Below a certain shear stress τd the concentration in suspension Ct decreases exponentially; Ct = Coexp(-wstp/D) where D is the depth of flow, Co is initial concentration, and p is the probability of deposition. For Co < 0.30 kg/m3, p = (1 – τo/ τd), where τd is the limiting shear stress for deposition, the stress below which all the sediment will eventually deposit. So: rate of deposition Rd = Cbws (1 - τo/ τd) Cb is the value near the bed. If there is no flow this reduces simply to the settling flux Cws. τd is not well known but is probably f(ws) whether aggregates or single grains. One expression for single grains of density ρs is: τd ≈ 0.048 (ρs – ρ)gd Higher concentrations, C = 0.3 to 10 kg/m3 C declines logarithmically with log of time. log C = -K log t + const in which K = 103 (1 - τo/ τd)/D. Some material is deposited at τo > τd; an equilibrium concentration is attained which reduces as τo is reduced to τd. The graph below is of Ceq/Co vs τo for an initial concentration Co of 1.25 kg/m3 This means that some mud can be deposited from relatively fast flows, as long as the concentration is high (> 1 kg/m3). (In this case τd is 0.18 Pa)

8. Modes of failure of sediments on slopes; slides and slips Failure condition: ΔρtghS > τy τy is yield strength, Δρt is (ρt - ρ) where ρt is the bulk density of the saturated soil mass and ρ the is that of the ambient fluid (air, or water for lakes and seas), S is slope (tan a)

9. Transformations of mass flows with increasing entrainment of fluid

10. Mechanisms and facies products

11. Sallenger’s demonstration of dispersive pressure as a support mechanism in grain flows ‘dispersive pressure’ P is given by T/P = tan α; α is the angle of friction, T is the tangential stress P = f(C).ρs.d2(du/dz)2 cos α At a given level in the flow the grains must experience the same P, du/dz and C. So, ρsH.d2H = ρsL d2L, or d H = d L (ρsL/ρsH)½ for heavy (H) and light (L) grains

12. Surges at the base of turbidity currents could lead to formation of inversely graded layers as shown

13. It is supposed that the mud behaves like a Bingham plastic with a yield strength and that above a basal sheared layer, at a height zp above the base, the shear is equal to the yield strength of the mud. Above that the mud is not sheared and its strength can support larger grains. But, experimentally, not boulders.

14. Kitimat Delta failure, note runout slopes <1o

15. Kitimat Delta failure, note outrunner blocks on slopes <1o

16. Flow speed of turbidity currents A similar expression to that for rivers governs flow of turbidity currents: Ub= [(gDr/rt ).(hS/CD(t+b))]½ Ub is the mean flow speed of the body of the flow, ∆ρ is ρt – ρ (ρt is the total current density, ρ is overlying fluid), (because the flow is underwater the effect of gravity is reduced so g' = g∆ρ/ρt termed the ‘reduced gravity’ is often used), h is the thickness of the flow and S = the slope. The drag coefficients includes top and bottom CD(t + b). A common value of CD(b) for a deep flow is 0.002 and a rule of thumb is top drag = half bottom, t = 0.5 b, so CD(t + b) ≈ 0.003. Remember CD = (u*/Ū)2 e.g. if ρt = 1100 kg/m3, ρ = 1050, h = 200 m, S = 0.003, CD = 0.003 U ≈ (0.5x200) ½= 10 m/s The head of the current moves as: Uh = C(g'h)½  with C = 0.75 (experiment, 0.74 by theory), i.e. independent of slope. With the values above Uh≈ 7.5 m/s, i.e. slower than the body.

17. Autosuspension Bagnold argued that turbidity currents containing particles of a certain size could flow indefinitely down a slope because the potential energy lost going downhill generated turbulence intense enough to keep the grains in suspension. Bagnold said that the critical particle setting velocity was ws < ŪS . This neglects the fact that suspension is an inefficient process, less than 10% efficient so ws < ŪSe, where e is suspension efficiency. e.g. For a 3 m/s flow on a 1/100 slope with e = 0.10 the autosuspended grains would be of ws < 3.0 mm/s or coarse silt to very fine sand. Implication is that very fine grains – mud – can stay in suspension a long time and push a turbidity current at speed a very long way

18. Dimensional analysis Dimensional analysis of the problem by Dade and Huppert (1995) here in Cambridge has given:  . length of current at time t Lt = k(g'q)1/3 t2/3 (1) k = 1.47 ± 0.05 final runout length xr = 3(g'qo3)/ws2)1/5 (2) runout time tr = ≥ 6qo/(wsxr) [or ≥ 2(qo2/g'ows3)1/5] (3) deposit thickness η = (Co/Cb) (qo/xr) (4) . In this, the important parameter qo is the initial volume per unit width of the current, hoLo (initial height x initial length). For these flows the Froude No. is Fr' = U/(g'h)½ and is generally around 1. Co is the initial volume concentration and Cb is the concentration in the deposited bed, Cb ≈ 0.5. ho h L x Lo

19. S. Grand Banks failure, St Lawrence Canyon turbidity current

20. Nice Airport failure, Var Canyon turbidity current