GE0-3112GE0-3112 Sedimentary processes and products Sedimentary processes and products

Lecture 5. Alluvial fans and fan deltasLecture 5. Alluvial fans and fan deltas

Geoff CornerGeoff CornerDepartment of GeologyDepartment of GeologyUniversity of TromsøUniversity of Tromsø20062006

Literature:Literature:- Leeder 1999. Ch. 18- Leeder 1999. Ch. 18 Alluvial fans and fan deltas.Alluvial fans and fan deltas.

ContentsContents

►3.1 Introduction - Why study fluid 3.1 Introduction - Why study fluid dynamicsdynamics

►2.2 Material properties2.2 Material properties►2.3 Fluid flow2.3 Fluid flow►2.4 Turbulent flow2.4 Turbulent flow►Further readingFurther reading

Importance of terrestrial fan Importance of terrestrial fan depositsdeposits

► 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 placer gold in E. Precambian alluvial fan

systems of Witwatersrand Supergroup, systems of Witwatersrand Supergroup, S. Africa. 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

NomenclatureNomenclature► ColluvialColluvial

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.

► AlluvialAlluvial dominated by ephemeral and/or permanent streams.dominated by ephemeral and/or permanent streams. NB. alluvial NB. alluvial fansfans may comprise both mass-movement (debris-flow) may comprise both mass-movement (debris-flow)

and streamflow deposits.and streamflow deposits.► FluvialFluvial

same as alluvial.same as alluvial.► GlaciofluvialGlaciofluvial

Substantial part of the streamflow discharge derives from glaciers.Substantial part of the streamflow discharge derives from glaciers.

ColluvialAlluvial

fan DeltaicAlluvial or fluvial

Colluvial-alluvial-deltaic system

Spectrum of fan depositsSpectrum of fan deposits► Fan deposits have Fan deposits have fanfan shape. shape.► Coalesced fans are aprons or Coalesced fans are aprons or

bajadas.bajadas.► Deposition occurs:Deposition occurs:

at foot of slope (gradient at foot of slope (gradient change).change).

through loss of flow momentum through loss of flow momentum or:or:

through loss of flow volume due through loss of flow volume due to infiltration/evaporation.to infiltration/evaporation.

NB. A fan deposited in standing NB. A fan deposited in standing water is a deltawater is a delta

► Spectrum of 'dry' to 'wet' Spectrum of 'dry' to 'wet' systems.systems.

► Spectrum of unconfined (fan) to Spectrum of unconfined (fan) to confined (valley) deposits.confined (valley) deposits.

► Fans deposited in standing Fans deposited in standing water are fan deltas. water are fan deltas.

Depositional processesDepositional processes

►Snow and rock avalancheSnow and rock avalanche►Debris flowDebris flow►Stream flow (channelized flow)Stream flow (channelized flow)►SheetflowSheetflow

Relative importance depends on:Relative importance depends on:- reliefrelief- climate and vegetationclimate and vegetation- sediment texturesediment texture

Fan typesFan types

►Colluvial fansColluvial fans►Alluvial fansAlluvial fans► ’’Fan deltas’Fan deltas’

Depositional processesDepositional processes

► Snow and rock Snow and rock avalancheavalanche

Snow-avalancge and rockfall talus, Lyngen, N. Norway.

Talus conesTalus 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.

Avalanche talus conesAvalanche talus cones► Snow and rock avalanche.Snow and rock avalanche.► Concave profile.Concave profile.

Talus cones and snow-avalanche boulder tongues at Tytebærdalen, Lyngen.

Colluvial conesColluvial 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.

Colluvial (alluvial) fanColluvial (alluvial) fan► Debris-flow processes dominate.Debris-flow processes dominate.► Concave profile.Concave profile.► Distal fining.Distal fining.

Colluvial/ealluvial at Disko Bugt, Greenland.

Alluvial fanAlluvial 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.

Confined and unconfined Confined and unconfined fansfans

Unconfined glaciofluvial fan, Lyngen.

Confined glaciofluvial fan (sandur), Steindalen, Lyngen.

Spectrum of alluvial fansSpectrum of alluvial fans

Galloway & Hobday 1996

Alluvial fan defintionAlluvial fan defintion

► fan-shaped accumulation of sediment fan-shaped accumulation of sediment traversed by stream-flow or debris-flow traversed by stream-flow or debris-flow channels.channels.

► focused source (point source) of sediment focused source (point source) of sediment supply, usually an incised canyon, gully or supply, usually an incised canyon, gully or channel from a mountain front or escarpmentchannel from a mountain front or escarpment

► radial sediment dispersal pattern in an radial sediment dispersal pattern in an unconfined unconfined position on a basin slope or floor.position on a basin slope or floor.

Controls on fan sizeControls on fan size►Drainage areaDrainage area►Climate and processClimate and process►Bedrock geology/surficial sedimentsBedrock geology/surficial sediments

Fan size and gradientsFan size and gradients

►Small, steep fans (30 – 5Small, steep fans (30 – 5oo)) e.g. fans in cold mountainous regions.e.g. fans in cold mountainous regions.

►Small, moderately steep fans (20 – 2Small, moderately steep fans (20 – 2oo)) e.g. fans in semi-arid mountains. e.g. fans in semi-arid mountains.

►Large, moderately steep fans Large, moderately steep fans (megafans) (15 – 0,5(megafans) (15 – 0,5oo)) e.g. Kosi and other fans, Nepalese Himalaya. e.g. Kosi and other fans, Nepalese Himalaya.

►Large, gentle fans (<0,5Large, gentle fans (<0,5oo)) e.g. Okavango fan, southern Africa.e.g. Okavango fan, southern Africa.

Fan area and slope vs. Fan area and slope vs. catchment sizecatchment size

Fan gradientFan area

Fan developmentFan development

►Flows emerging on fan are free to Flows emerging on fan are free to diverge (expand) and infiltrate.diverge (expand) and infiltrate.

►Fan shape results from frequent radial Fan shape results from frequent radial shifts in feeder channel about the shifts in feeder channel about the nodal point.nodal point.

►Channel shifts (avulsions) result from Channel shifts (avulsions) result from blockage and breakout.blockage and breakout.

Nodal points

Depositional processesDepositional processes

►Snow and rock avalancheSnow and rock avalanche►Debris flowDebris flow►Stream flow (channelized flow)Stream flow (channelized flow)►SheetflowSheetflow

Relative importance depends on:Relative importance depends on:- reliefrelief- climate and vegetationclimate and vegetation- sediment texturesediment texture

Debris-flow-dominated fansDebris-flow-dominated fans

Occurrence and characteristicsOccurrence and characteristics

Occur in: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 smallRelatively small Relatively steep (5 - 20Relatively steep (5 - 20oo)) Concave profile, segments reflect process changeConcave profile, segments reflect process change

► SedimentsSediments coarse (gravels, cobbles), poorly sorted, matrix- to coarse (gravels, cobbles), poorly sorted, matrix- to

clast supportedclast supported

Debris-flow deposits

Sheet-flow deposits

Proximal part of a debris-flow Proximal part of a debris-flow fanfan

Debris-flow fan – idealised long-Debris-flow fan – idealised long-sectionsection

Debris-flow fan faciesDebris-flow fan facies

►Debris flow deposit from August 2005 Debris flow deposit from August 2005 eventevent

Stream-flow-dominated alluvial Stream-flow-dominated alluvial fansfans

Stream-flow-dominated fansStream-flow-dominated fans

► Ancient examplesAncient examples Mesozoic-Cenozoic footwall half-grabens, ChinaMesozoic-Cenozoic footwall half-grabens, China Eocene fan systems, USAEocene fan systems, USA Cambrian, Van Horn Sandstone, TexasCambrian, Van Horn Sandstone, Texas Devonian, Hornelen Basin, Norway Devonian, Hornelen Basin, Norway

► Facies characteristicsFacies characteristics Relatively large lateral extent ( often >4 km)Relatively large lateral extent ( often >4 km) Moderate gradientModerate gradient Resemble fluvial facies, but with following distinguishing Resemble fluvial facies, but with following distinguishing

(alluvial fan) characteristics:(alluvial fan) characteristics:► uplap onto tectonic highlandsuplap onto tectonic highlands► isopach maps show basin margin thickeningisopach maps show basin margin thickening► radial variation in clast size and dispersal patternradial variation in clast size and dispersal pattern

Depositional processesDepositional processes

► Stream-flow Stream-flow (channelised) and (channelised) and sheetflowsheetflow

Facies in small gravelly fanFacies in small gravelly fan

Stream-flow-dominated fansStream-flow-dominated fans

Glacial outwash fan

Stream-flow megafansStream-flow megafans

Humid fan

Megafans - HimalayaMegafans - Himalaya

Large fluvial fans – N Large fluvial fans – N ApenninesApennines

Cambrian Van Horn Sandstone Cambrian Van Horn Sandstone fanfan

Terminal fansTerminal fans

► Alluvial fans that loose all discharge through Alluvial fans that loose all discharge through evaporation or infiltration.evaporation or infiltration.

► Examples:Examples: Fans in semi-arid basins Fans in semi-arid basins

having internal drainage.having internal drainage. Okavago Fan Okavago Fan

(Okavango ’delta’), (Okavango ’delta’), Botswana. Botswana.

Further readingFurther reading