Presentation on theme: "GE Sedimentary processes and products"— Presentation transcript:
1GE0-3112 Sedimentary processes and products Lecture 5. Alluvial fans and fan deltasGeoff CornerDepartment of GeologyUniversity of Tromsø2006Literature:- Leeder Ch. 18Alluvial fans and fan deltas.
2Contents 3.1 Introduction - Why study fluid dynamics 2.2 Material properties2.3 Fluid flow2.4 Turbulent flowFurther reading
3Importance of terrestrial fan deposits Basin –margin fault patterns controlling alluvial fan depositionFans 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
4Nomenclature Colluvial-alluvial-deltaic system Colluvial Alluvial dominated by mass-movement processes.E.g. talus cones, avalanche boulder tongues, debris-flow fans.Alluvialdominated by ephemeral and/or permanent streams.NB. alluvial fans may comprise both mass-movement (debris-flow) and streamflow deposits.Fluvialsame as alluvial.GlaciofluvialSubstantial part of the streamflow discharge derives from glaciers.ColluvialAlluvial fanAlluvial or fluvialDeltaicColluvial-alluvial-deltaic system
5Spectrum 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 deltaSpectrum of 'dry' to 'wet' systems.Spectrum of unconfined (fan) to confined (valley) deposits.Fans deposited in standing water are fan deltas.
6Depositional processes Snow and rock avalancheDebris flowStream flow (channelized flow)SheetflowRelative importance depends on:reliefclimate and vegetationsediment texture
8Depositional processes Snow and rock avalancheSnow-avalancge and rockfall talus, Lyngen, N. Norway.
9Talus cones Rock fall processes. Linear profile. Distal coarsening. Talus cones with bouldery rock-avalanche debris, Varanger, N. Norway.
10Avalanche talus cones Snow and rock avalanche. Concave profile. Talus cones and snow-avalanche boulder tongues at Tytebærdalen, Lyngen.
11Colluvial 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.
12Colluvial (alluvial) fan Debris-flow processes dominate.Concave profile.Distal fining.Colluvial/ealluvial at Disko Bugt, Greenland.
13Alluvial fanEphemeral (flashflood) stream-flow and sheetflow processes.Gentle, concave profile.Distal fining.Alluvial fans, Death Valley, California.
14Confined and unconfined fans Unconfined glaciofluvial fan, Lyngen.Confined glaciofluvial fan (sandur), Steindalen, Lyngen.
15Spectrum of alluvial fans Galloway & Hobday 1996
16Alluvial 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 escarpmentradial sediment dispersal pattern in an unconfined position on a basin slope or floor.
17Controls on fan size Drainage area Climate and process Bedrock geology/surficial sediments
18Fan 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.
19Fan area and slope vs. catchment size Fan gradient
20Fan developmentFlows 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.
39Stream-flow-dominated fans Ancient examplesMesozoic-Cenozoic footwall half-grabens, ChinaEocene fan systems, USACambrian, Van Horn Sandstone, TexasDevonian, Hornelen Basin, NorwayFacies characteristicsRelatively large lateral extent ( often >4 km)Moderate gradientResemble fluvial facies, but with following distinguishing (alluvial fan) characteristics:uplap onto tectonic highlandsisopach maps show basin margin thickeningradial variation in clast size and dispersal pattern
40Depositional processes Stream-flow (channelised) and sheetflow