GE0-3112 Sedimentary processes and products Lecture 5. Alluvial fans and fan deltas Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 18 Alluvial fans and fan deltas.

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

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

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

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.

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

Fan types Colluvial fans Alluvial fans ’Fan deltas’

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

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

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

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.

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

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

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

Spectrum of alluvial fans Galloway & Hobday 1996

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.

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

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.

Fan area and slope vs. catchment size Fan gradient

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.

Nodal points

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

Debris-flow-dominated fans

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

Debris-flow deposits Sheet-flow deposits

Proximal part of a debris-flow fan

Debris-flow fan – idealised long-section

Debris-flow fan facies

Debris flow deposit from August 2005 event

Stream-flow-dominated alluvial fans

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

Depositional processes Stream-flow (channelised) and sheetflow

Facies in small gravelly fan

Stream-flow-dominated fans Glacial outwash fan

Stream-flow megafans Humid fan

Megafans - Himalaya

Large fluvial fans – N Apennines

Cambrian Van Horn Sandstone fan

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.

Further reading