1. GE0-3112 Sedimentary processes and products
Lecture 5. Alluvial fans and fan deltas
Geoff Corner
Department of Geology
University of Tromsø
2006
Literature:
- Leeder Ch. 18
Alluvial fans and fan deltas.

2. Contents 3.1 Introduction - Why study fluid dynamics
2.2 Material properties
2.3 Fluid flow
2.4 Turbulent flow
Further reading

3. Importance of terrestrial fan deposits
Basin –margin fault patterns controlling alluvial fan deposition
Fans common in different tectonic settings:
extensional terranes.
forelands (compressional).
pull-apart basins.
Postglacial fans common in mountain regions.
Thick ancient fan deposits, e.g:
Devonian, Hornelen Basin, Norway.
Jurassic Greenland.
Economic resources:
placer gold in E. Precambian alluvial fan systems of Witwatersrand Supergroup, S. Africa.
petroleum in some fan deltas.
Devonian alluvial fan sandstones, Hornelen Basin, W. Norway

4. Nomenclature Colluvial-alluvial-deltaic system Colluvial Alluvial
dominated by mass-movement processes.
E.g. talus cones, avalanche boulder tongues, debris-flow fans.
Alluvial
dominated by ephemeral and/or permanent streams.
NB. alluvial fans may comprise both mass-movement (debris-flow) and streamflow deposits.
Fluvial
same as alluvial.
Glaciofluvial
Substantial part of the streamflow discharge derives from glaciers.
Colluvial
Alluvial fan
Alluvial or fluvial
Deltaic
Colluvial-alluvial-deltaic system

5. Spectrum of fan deposits
Fan deposits have fan shape.
Coalesced fans are aprons or bajadas.
Deposition occurs:
at foot of slope (gradient change).
through loss of flow momentum or:
through loss of flow volume due to infiltration/evaporation.
NB. A fan deposited in standing water is a delta
Spectrum of 'dry' to 'wet' systems.
Spectrum of unconfined (fan) to confined (valley) deposits.
Fans deposited in standing water are fan deltas.

6. Depositional processes
Snow and rock avalanche
Debris flow
Stream flow (channelized flow)
Sheetflow
Relative importance depends on:
relief
climate and vegetation
sediment texture

7. Fan types
Colluvial fans
Alluvial fans
’Fan deltas’

8. Depositional processes
Snow and rock avalanche
Snow-avalancge and rockfall talus, Lyngen, N. Norway.

9. Talus cones Rock fall processes. Linear profile. Distal coarsening.
Talus cones with bouldery rock-avalanche debris, Varanger, N. Norway.

10. Avalanche talus cones Snow and rock avalanche. Concave profile.
Talus cones and snow-avalanche boulder tongues at Tytebærdalen, Lyngen.

11. Colluvial cones Rock fall, snow-avalanche and debris-flows.
Concave profile.
Distal fining.
Debris-flow channels and lobes formed during torrential rain in August 1999, on talus and colluvial fans at Nordkjosbotn, Balsfjord, N. Norway.

12. Colluvial (alluvial) fan
Debris-flow processes dominate.
Concave profile.
Distal fining.
Colluvial/ealluvial at Disko Bugt, Greenland.

13. Alluvial fan
Ephemeral (flashflood) stream-flow and sheetflow processes.
Gentle, concave profile.
Distal fining.
Alluvial fans, Death Valley, California.

14. Confined and unconfined fans
Unconfined glaciofluvial fan, Lyngen.
Confined glaciofluvial fan (sandur), Steindalen, Lyngen.

15. Spectrum of alluvial fans
Galloway & Hobday 1996

16. Alluvial fan defintion
fan-shaped accumulation of sediment traversed by stream-flow or debris-flow channels.
focused source (point source) of sediment supply, usually an incised canyon, gully or channel from a mountain front or escarpment
radial sediment dispersal pattern in an unconfined position on a basin slope or floor.

17. Controls on fan size Drainage area Climate and process
Bedrock geology/surficial sediments

18. Fan size and gradients Small, steep fans (30 – 5o)
e.g. fans in cold mountainous regions.
Small, moderately steep fans (20 – 2o)
e.g. fans in semi-arid mountains.
Large, moderately steep fans (megafans) (15 – 0,5o)
e.g. Kosi and other fans, Nepalese Himalaya.
Large, gentle fans (<0,5o)
e.g. Okavango fan, southern Africa.

19. Fan area and slope vs. catchment size
Fan gradient

20. Fan development
Flows emerging on fan are free to diverge (expand) and infiltrate.
Fan shape results from frequent radial shifts in feeder channel about the nodal point.
Channel shifts (avulsions) result from blockage and breakout.

21. Nodal points

23. Depositional processes
Snow and rock avalanche
Debris flow
Stream flow (channelized flow)
Sheetflow
Relative importance depends on:
relief
climate and vegetation
sediment texture

24. Debris-flow-dominated fans

25. Occurrence and characteristics
Occur in:
Arctic mountains (e.g. Norway, Svalbard)
Arid/semi-arid mountains (e.g. SW USA, Dead Sea)
Size and morphology:
Relatively small
Relatively steep (5 - 20o)
Concave profile, segments reflect process change
Sediments
coarse (gravels, cobbles), poorly sorted, matrix- to clast supported

26. Debris-flow deposits
Sheet-flow deposits

27. Proximal part of a debris-flow fan

28. Debris-flow fan – idealised long-section

29. Debris-flow fan facies

30. Debris flow deposit from August 2005 event

37. Stream-flow-dominated alluvial fans

39. Stream-flow-dominated fans
Ancient examples
Mesozoic-Cenozoic footwall half-grabens, China
Eocene fan systems, USA
Cambrian, Van Horn Sandstone, Texas
Devonian, Hornelen Basin, Norway
Facies characteristics
Relatively large lateral extent ( often >4 km)
Moderate gradient
Resemble fluvial facies, but with following distinguishing (alluvial fan) characteristics:
uplap onto tectonic highlands
isopach maps show basin margin thickening
radial variation in clast size and dispersal pattern

40. Depositional processes
Stream-flow (channelised) and sheetflow

41. Facies in small gravelly fan

42. Stream-flow-dominated fans
Glacial outwash fan

43. Stream-flow megafans
Humid fan

44. Megafans - Himalaya

45. Large fluvial fans – N Apennines

46. Cambrian Van Horn Sandstone fan

47. Terminal fans
Alluvial fans that loose all discharge through evaporation or infiltration.
Examples:
Fans in semi-arid basins having internal drainage.
Okavago Fan (Okavango ’delta’), Botswana.

49. Further reading