FEMA. GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits

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1 Lecture 11: Alluvial Fan Facies FEMA GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits

for mitigation By Wieczorek, G.F., Larsen, M.C.

2 Debris-flow and flooding hazards associated with the December 1999 storm in coastal Venezuela and strategies for mitigation By Wieczorek, G.F., Larsen, M.C., Eaton, L.S., Morgan, B.A. and Blair, J. L. U.S. Geological Survey

3 Today: Examine the morphology of alluvial fans and controls on morphology Outline major processes operating on alluvial fans Examine facies of alluvial fans Consider auto and allocyclic controls on deposition

4 What is an alluvial fan? a fan-shaped deposit of alluvium, often found at a change in slope, for instance where a stream flows out of a mountain onto flatter terrain. it is a distributive system. and a fan delta? where the fan builds out into a standing body of water (we ll come back to these when we look at deltas)

5 The classic alluvial fans of Death Valley Narrow outlet (valley or gorge) Fan apex Radial spreading alluvial fan

6 simple to complex coalesced alluvial fans Marli Miller

8 Controls on Alluvial Fans Climate Tectonics Drainage Area Processes: rockfall, debris flow, stream flow

9 Stages in alluvial fan evolution..

11 Hunza valley, Himalayas, Danny Yee

14 Fan-head entrenchment and avulsion

15 Fan-head entrenchment

16 Death Valley alluvial fans

17 streamflow and sheetflood

18 transitional to

19 Mass flows a spectrum of flow types CREEP Slow intergranular frictional sliding with quasi-static grain contacts SLIDE Coherent mass with minor internal deformation, translational (here) or rotational SLUMP Coherent mass with considerable internal deformation (discrete, nonpervasive shear)

20 Mass flows a spectrum of flow types FLOW with PLASTIC behaviour Remoulded mass, non-turbulent, but possibly with transient large-scale turbulent churning and entrained turbidity current at upper boundary (liquefied flow / debris flow) FLOW with FLUIDAL behaviour Fully turbulent (turbidity current) ROCK FALL Solitary grains or loose grain assemblages

21 a range of flows from streamflow debris flow..

clast-supported, sandy matrix, a(p) fabric inverse grading in basal shear zone wavy base with erosion in places non-cohesive debris flow erosive base normal grading with large clasts at base traction

22 clast-supported, sandy matrix, a(p) fabric inverse grading in basal shear zone wavy base with erosion in places non-cohesive debris flow erosive base normal grading with large clasts at base traction current structures stream flow deposit matrix-supported, sand/fine gravel matrix horizontal fabric no grading non-cohesive debris flow matrix-supported, mud- or sand-rich, subhorizontal fabric, some vertical clasts isolated megaclasts basal pseudolamination tightly cohesive packed (?) gravel debris with flow erosive base, weak normal grading a(p) & a(t) fabric stream flow deposit

23 normal grading to stratified sand near top (indicative of waning flow conditions) crude, thick, irregular cross-strata in gravel clast-supported texture crude imbrication channelised base, possibly with large flutes turbulent streamflood deposit (hyperconcentrated flow?)

24 ... a bit more on debris flows... Debris flow strength = f (cohesive & frictional strength, apparent viscosity, etc.) AND Debris flow strength ~ maximum clast size THEREFORE Maximum clast size = f (cohesive & frictional strength, apparent viscosity, etc.)

25 Flow strength (D) Maximum Particle Size F i * (resultant effect of other factors) Proportional to flow thickness but not necessarily linear : frictional strength viscous resistance dispersive pressure transient turbulence fluidisation F C * (cohesive strength factor) Flow thickness (Y) Bed Thickness Independent of flow thickness Nemec & Steel (1984)

26 Model testing with cohesive strength no cohesive strength New Red Sandstone fanglomerates

27 MPS Model usage To distinguish between dominantly cohesive and dominantly non-cohesive debris flows To distinguish between debris flow conglomerates and conglomerates of streamflow origin, which usually lack correlation between MPS and BTh The correlation coefficient of the best-fit line may serve as a measure for consistency in the physical behaviour of debris flows The slope of the best-fit line can be used to contrast groups of flows with relatively low and high competence subaerial mass flows subaqueous mass flows BTh

28 Longitudinal changes in facies..

29 Longitudinal changes in facies..

30 Facies models for fans: two types?? Debris flow dominated

31 Facies models for fans: two types?? Streamflow dominated

32 Controls: Tectonic regime: uplift Climate Avulsion

33 Tectonic regime

34 Avulsion: Kosi Fan, India

Summary and things to think on.. Debris and streamflow dominated fans Downstream facies changes Debris flow dynamics and paleohydraulics What are changing processes across the fan?

35 Summary and things to think on.. Debris and streamflow dominated fans Downstream facies changes Debris flow dynamics and paleohydraulics What are changing processes across the fan? Controls auto and allo cyclic How do fans reflect climatic and tectonic evolution of area? Reading: B&D, Chapter 13, p. 448 Boggs, Chapter 8 Harold Reading (Ed): Chapter 3 (Alluvial sediments) Mike Leeder: Chapter 18

Lecture 3: Fundamentals of Fluid Flow: fluid properties and types; Boundary layer structure; unidirectional flows

GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits Lecture 3: Fundamentals of Fluid Flow: fluid properties and types; Boundary layer structure; unidirectional flows Why study