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- 2. Environments of Deposition •Continental – Fluvial alluvial fan, braided stream, meandering stream – Desert dunes, playa lakes, salinas – Lacustrine – Glacial – Deltaic delta plain, delta front, prodelta • Marginal- Marine – Beach/ barrier island – Estuary/ lagoon – Tidal Flat – Neritic continental shelf, organic reef • Marine – Pelagic continental slope and rise, deep- ocean floor
- 4. Continental (Terrigenous) • Journeyfrom source to sink – Plate tectonics lead to mountain building • Slope! Exposure! Produces rock – Climate causes weathering – Surface processes move sediments, sculpt landscape • Weathering, erosion, transport, deposition
- 5. Deposition • Requires placeto put sediments – Depression/ basin – Deep Marine is best • Lakes are good for short records – Terrestrial is difficult! • Glacial sediments are misleading – Abundant because they are recent
- 6. Deposition • Gravity worksto move sediments – Water – Wind • Change in slope or change in climate impacts sediment transport and abundance
- 7. Alluvial Fan • Localizedareas of enhanced sedimentation downstream of points where laterally confined flows expand – Narrow valley or gorge opens up, onto piedmont or coastal plain – At basin margin • Often structurally controlled (fault or mountain front) • Contribute greatly to the rock record! – 10’s of meters - >100 km cone • “Fanglomerate”
- 8. Alluvial Fan FaciesModels • 3 types – Debris- flow dominated – Braided fluvial fans – Low- sinuosity/meandering fluvial fan • Generally – lacking fossils – Fan shaped – Texturally immature
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- 10. Alluvial Fan FaciesModels • Morphology – Proximal, mid-fan, distal • Streamflow processes dominate – Stream- channel sediments • Long narrow bodies of coarsest materials • Poorly sorted – Sheetflood deposits • Sediments settle out of suspension • VERY high sediment load • Deposit gravels even! – Sieve deposits • Very coarse grains are deposited (> sand) • Fines (and water) settle through
- 11. 3 Alluvial FanFacies Models • Debris- flow dominated – Poorly sorted • Pebbles, gravels, boulders dominate – No sedimentary structures • Possibly reverse graded bedding in base – Flow is initiated when strength is exceeded • Freezes after short distance – Although some travel up to 24 km (Sharp and Nobles, 1953) – Mud- flow dominated • Similar but sand and muds dominate • Braided Fluvial Fan • Meandering Fluvial Fan
- 12. Associated with FluvialFacies • Braided streams – Straight channel streams • Meandering streams • Share characteristics with stream- dominated fan! – Associations! • Large scale (mountains?) • Small scale (flood plain?)
- 13. Alluvial to FluvialTransition • Move downslope, from coarse to fine – Low sediment supply: scoured between systems – High sediment supply: continuous transition to braided streams, then meanders • Steeper slopes – coarser sediments, ephemeral flow – Mountain edges, ice sheets – Rivers are overloaded with sediments • Tend toward BRAIDED streams • Shallower slopes – finer sedimentss, lower velocity flow • Vegetation slows flow on slope; no pre-Devonian meandering streams
- 14. River Basics • Stream –Part of a watershed • Drainage basin (= catchment area); contributive network • Provides water and sediment to stream • Adjacent basins separated by divide – Many scales; continental divide – Stages • Young: Mountain regions – Erosional features dominate • Mature – Formation of flood plains and point bars – Depositional features dominate (old, also) • Old: Coastal regions – Rivers coalesce; Channels reduced in size due to division
- 17. Channel Patterns • Channelsdefine rivers – Function of adjustment of channel to changes • Gradient, Cross-section – Controlled by • Sediment load • River characteristics • Amount and nature of discharge – Three basic channel patterns • Straight • Braided • Meandering – Types Change! Along length or at flood stage
- 21. Fluvial Facies • Meansby which sediment is transferred across the land surface towards its eventual resting place (Collinson, 1996) • Characteristics of streams – gradient- water flows downhill – increase slope, increase velocity • Three styles of flow – alluvial fan- debris accumulates at base of slope – braided stream- high bedload; multiple channels – meandering stream
- 22. Fluvial Processes • Erosion –Provenance • Transport and deposition – Textural, compositional maturity – Wide range of associated sedimentary structures • Post-depositional alteration/ pedogenesis – Often associated with soils • Alluvium • Paleosols (in floodplains)
- 23. Fluvial Processes- Erosion •Wide range of scales! (spatial, temporal) • Incision – Vertical cutting of substrate that deepens the channel • Single flood event • Progressively increasing discharge – May be accompanied by widening • Drop in base level – Strands river banks as terraces • Migration of channels – Lateral erosion of channels • Scouring of loose grains • Mass movement when banks are oversteepened/ undercut – Function of cohesive strength • Overhang – more cohesive material overlies less cohesive
- 25. Fluvial Processes- Transport/Deposition • Debris flow – Freeze when applied shear < strength • Cohesionless debris flow (liquifaction) – Freezes as pore water escapes • Cohesive debris flow (mudflows) – Viscosity incr. by fine grain matrix (+bouyancy inhibits settling) • Bedload – Movement of non-cohesive grains by rolling or bouncing • Mostly sand and gravel sized grains in rock record • Suspended load – Transport due to fluid turbulence (Can includes sands!) • Some deposited on flood plain; most deposited at end of river • Wind
- 27. Basic River Morphology •Thalweg – Deepest part of channel • Point bars – Built on inside of curve (depositional) • Cut bank – On outside of curve (erosional) • Pools – Deep area opposite to point bar • Riffle – Shallower area (cross-over area) • Levee and Flood Plain – Borders river
- 28. Fluvial Facies • Braidedstreams – Straight channel streams • Meandering streams • Share characteristics with stream- dominated fan! – Associations! • Large scale (mountains?) • Small scale (flood plain?)
- 29. Braided Channel – Mainchannel consists of several channels – Bars • Major depositional feature • Often submerged at high flow • Development – Grow downstream and on lateral areas – Often eroded upstream – Stabilized by deposition of fine grains on top + vegetation
- 30. Braided Channel • Successivedivision and rejoining of flow around islands • Most common in mountain reaches, on alluvial fans and glacial outwash • Requires – high bedload – Steeper gradient (aids in higher bedload) – Relative ease of bank erosion • Grain size – Gravels most common – Can braid fines if discharge is high and banks are weak
- 33. Straight channels • Channelhas low to no sinuosity over its length (relative to its width) – Thalweg is sinuous • Rare! • Exist only over short distances – Limited to 10x the channel width • Leopold et al, 1964
- 34. Straight channels • Channelhas low to no sinuosity over its length (relative to its width) – Thalweg is sinuous • Rare! • Exist only over short distances – Limited to 10x the channel width • Leopold et al, 1964
- 35. Distinctive and CommonSedimentary Facies Associations • Vertical successions principally identified by lithology, associations and vertical arrangement of sedimentary structures – indicative of particular sedimentary depositional environments
- 36. Meandering Channel • Pointbars dominate • Helical circulation leads to meandering – In curves, two flow directions • Strong downstream – greatest at 3, towards the cut bank • Weaker cross-flow at depth – Towards the point bar • When cut bank slumps, material is carried downstream to next point bar
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- 39. Note point bars alonginner bends. The channel belt is defined as the region between the red lines wihtin which the active channel tends to meander. Photo from R.H. Meade, USGS
- 40. Flow Regime andBedforms • Bedload and suspended load – Suspended load • Much moved in suspention; Important in flood plains – Bedload • Dominant in channels and point bars; coarsest grains lag – Floods increase it • Noncohesive materials on stream bed lead to easy formation of bedforms • Lower flow regime is common – Ripples abundant • Upper flow regime is also common – Highest flow ever recorded » 7-8 m/s (25-29 km/hr - Leopold et al., 1964
- 41. Deposition • Rates – Mostdeposition occurs during flood stage • Coleman, 1969 – 1m thick in 24 hours – Migrating dunes » 5-6m thick in 24 hours – Non- flood periods • Non-depositional – Often erosional
- 42. Deposition • Channel deposits –Channel lag – Channel bar (Braided River) – Point bar (Meandering River) – Channel fill • Flood plain deposits – Flood plain – Marsh – Levees – Crevasse splay
- 43. Channel deposits • Channellag – Coarsest sediment available in stream – Accumulates in lenses • Generally gravels, pebbles • May include – Wood – Rip-up clasts – Fossils – Blocks of sediment – Often preserved as fine grains cover it
- 44. Channel Bars • Mostdeposition occurs during floods • Two types based on grain size – Braid bars (coarse material) • Pebbles, etc • Mountain streams – Braid bars (fine material) • High sediment load (seasonal discharge) • Lower reaches of rivers, near deltas – Brahmaputra River
- 46. Channel bar (=channel islands) • Dominant depositional feature in braided rivers – Controlled by • Lateral and vertical deposition • Cutting and abandonment – Upstream end • Steep • Often has pool in front of it – Downstream end • Gentler slope • Migrates downstream (foreset bedding) – Sides • Steep concave (foreset bedding/ laminae) • Gentle convex (foreset bedding/ laminae)
- 47. General Channel BarFacies • Upper – Fine sand, mud in horizontal layers • Muds – Mudcracks, raindrop imprints – Sometimes scour and erosional markings – Convolute bedding – Bioturbation (plant growth) • Lower – Large- scale cross-bedding • Medium sands – ripples and dunes • Coarsest grains at base (pebbles) – Often rests on muds, silts of previous cycle – scour and fill structures
- 48. Braided Bar types •Longitudinal – Mid-channel (coarsest sediment) • Deposition occurs as stream loses velocity – Long axis // to flow • Grow downstream fill in with finer sediments • Decrease grain size downstream • Massive bedding; cross- bedding • Linguoid and transverse – Lobate or straight; at angle to flow • Found in sandy streams • Steep face downstream • High flood conditions • Lateral – Along sides of channel attached to bank
- 49. Point Bar deposits •Major process of sedimentation in meandering rivers – Usually that which is preserved – Thickness may equal depth of river • Mississippi River= 20-25 m (Fisk, 1944, 1947) • Lateral migration of point bar (and river!) during flooding – But the channel width is maintained (erosion= deposition) – Generally fining upward • Deposition – channel lag at base and mud drape on top • Preservation – Fines at top eroded
- 52. Point Bar Facies •Silts, clays • Planar bedding – May be upper - flow • likely it is sediments settling out of suspension as flood wanes – Often interbedded with ripples • Cross-bedding – Ripples and climbing - ripples – Dune common in lower part • Beds ~ 1m • Scour, fill common • Channel lag at base
- 53. Point Bar Facies •Larger scale point bars develop scroll bars and swales – Scroll-shaped ridges with swales between – Each scroll = migration during one flood – Swales infilled with muds • Mississippi – Few hundred meters wide, 4-5 m thick – Base is concave-upward
- 54. Point Bar LifeHistory • Rapid, periodic deposition! • Arkansas River, Oklahoma (Steinmetz, 1967) – May 19-22, 1957 to October 3-6, 1959 • 459,000 m3 sed deposited; max = 13 m in 156 hours • Mostly its large-scale cross-bedding • overlain by silts (settling) and sand dunes (1 m!) • Sedimentation in a single flood (p. 235, R/S) • Klarälven River, Sweden (Sundborg, 1956) • Develop a longitudinal bar on the meander • Slowly migrates bank-ward • Accumulates drape • New bar begins, migrates
- 55. • Downtown YubaCity December 1955 (Seepage Related Levee Break in upper center) View Southerly http://www.escalera.com/safelevee/1955flood.htm
- 56. Note the manyscroll bars marking the former positions of channels (point bars) across the flood plain.
- 59. Levee Deposits (Natural) •Wedge- shaped ridges of sediment bordering stream channels – Formed as flood waters top banks • Sediment in suspension settles out as velocity drops – Highest at stream banks • Slopes away towards flood plain – Best development on point bank side (concave) • Grades into point bar – Mississippi River • Max: 1.5 km wide, 5-6 m above flood basin
- 60. Levee Deposit (Natural) •Sediments grade laterally – Decrease in size away from channel • Reflects rapid drop in velocity • Coarsest sediments nearest levee – Rate is reduced also • Maximum height of levee = maximum height of floodwaters
- 61. Crevasse Splay • Breachin natural levee – Occurs in floods – Creates drainage network in flood basin – Generally small • 10’s cm to 10’s meters • May be several hundred meters across • Tongues of sediment – Taper in direction of flood basin – Coarser sediments than that of flood plain • Small- scale cross-bedding • Scour and fill • Fossils (plant and animal)
- 64. Channel- fill (Oxbows!) •Stream channels that have been abandoned – Cut-off abandonment – Avulsions: sudden abandonment – Very high rate of sedimentation • Meandering can cause if – Chute cut-off: stream shortens its course – Neck- cut off: new channel but between 2 meander loops
- 65. Channel- Fill deposits •At first, – Rapid and focused at ends • Sands concentrated here • Slower later on – Fill is similar to flood- plain deposits • Clay and silt dominated • Plants also present
- 66. Flood Basin Deposits •Poorly drained, flat, featureless with little or no relieft adjacent to active or abandoned channels (Rheinhold and Singh, 1975) – SLOW accumulation • 1- 2 cm per flood period – (compacts over time!) • Controlled by channel form and pattern – Stationary streams develop thick flood plains – Migrating streams have poorly developed plains • Braided streams, active meanders • Backswamps – may form if the region is humid – Accumulates muds, peats – Congaree National Monument
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