GE0-3112 Sedimentary processes and products

1. Lecture 5. Alluvial fans and fan deltas GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. 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 • 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 area 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