Transcript Slide 1

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
 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
Colluvial-alluvial-deltaic system
Alluvial or
fluvial
Deltaic
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 rockavalanche 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,
o
steep fans (30 – 5 )
 e.g. fans in cold mountainous regions.
► Small,
o
moderately steep fans (20 – 2 )
 e.g. fans in semi-arid mountains.
► Large,
moderately steep fans (megafans)
o
(15 – 0,5 )
 e.g. Kosi and other fans, Nepalese Himalaya.
► Large,
o
gentle fans (<0,5 )
 e.g. Okavango fan, southern Africa.
Fan area and slope vs. catchment
size
Fan area
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