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

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Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 5. Alluvial fans and fan deltas Geoff Corner Department of Geology University of Tromsø 2006 Literature:"— Presentation transcript:

1 GE 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. Alluvial fans and fan deltas.

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

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

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

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

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

7 Fan types Colluvial fans Colluvial fans Alluvial fans Alluvial fans Fan deltas Fan deltas

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

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

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

11 Colluvial cones Rock fall, snow-avalanche and debris-flows. Rock fall, snow-avalanche and debris-flows. Concave profile. Concave profile. Distal fining. 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. Debris-flow processes dominate. Concave profile. Concave profile. Distal fining. Distal fining. Colluvial/ealluvial at Disko Bugt, Greenland.

13 Alluvial fan Ephemeral (flashflood) stream-flow and sheetflow processes. Ephemeral (flashflood) stream-flow and sheetflow processes. Gentle, concave profile. Gentle, concave profile. Distal fining. 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. 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 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. radial sediment dispersal pattern in an unconfined position on a basin slope or floor.

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

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

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

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

21 Nodal points

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23 Depositional processes Snow and rock avalanche Snow and rock avalanche Debris flow Debris flow Stream flow (channelized flow) Stream flow (channelized flow) Sheetflow 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) Arctic mountains (e.g. Norway, Svalbard) Arid/semi-arid mountains (e.g. SW USA, Dead Sea) Arid/semi-arid mountains (e.g. SW USA, Dead Sea) Size and morphology: Size and morphology: Relatively small Relatively small Relatively steep ( o ) Relatively steep ( o ) Concave profile, segments reflect process change Concave profile, segments reflect process change Sediments Sediments coarse (gravels, cobbles), poorly sorted, matrix- to clast supported 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 Debris flow deposit from August 2005 event

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37 Stream-flow-dominated alluvial fans

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

40 Depositional processes Stream-flow (channelised) and sheetflow 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. Alluvial fans that loose all discharge through evaporation or infiltration. Examples: Examples: Fans in semi-arid basins having internal drainage. Fans in semi-arid basins having internal drainage. Okavago Fan (Okavango delta), Botswana. Okavago Fan (Okavango delta), Botswana.

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49 Further reading


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