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GEOS 302 Rivers & Fluvial DepositsFluvial Systems

River deposystems and their components1) transport systems2) zones of accumulation

Bow River,Banff Natl. ParkA. Cohen Photo

Importance of Fluvial SystemsEnvironmental issues:

floodingwater transportagriculturesoil erosionnutrient transport

Importance of Fluvial SystemsResource issues-Facies models for hydrocarbon plays and groundwater reservoirs-Sensitivity to changes in climate and tectonics

Channeled Scablands WA Megaflood Bedforms

Fluvial Channels as Hydrocarbon Reservoirs Sands

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Subenvironments in Fluvial Systems

Channels LeveesFloodplainsLakesSwamps

Classification of Fluvial SystemsClassification-based on shape # and stability of channelsSinuosity index (S) = Lthalweg (deepest part of channel) / Lvalley

Meandering (sinuosity >2) -single channel, broad floodplains, channels moderately stable-channel migration

Classification of Fluvial SystemsAnastomosed -multiple, stable channels-prominent levees, extensive wetlands & vegetated islands

l. MacKenzie River Canada), Utrecht U. r. Columbia R., CA H Berendsen Photo

Classification of Fluvial Systems

Braided-multiple, shallow, unstable channels-for a given discharge higher slopes than meandering streams-mid channel bars-floodplains and levees less important

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Why one type of river vs. another?Controlling Variables for Fluvial Systems

Flow Characteristics:(Controlling variables: amplitude, periodicity, amount of change)

Discharge

Slop

e

Independent variables:•Slope (tectonic controls)

•Discharge/variability (climatic control)•Sediment load (amount and particle size)

•Bed roughness

Graph data from Smith 1993

Meandering Stream Facies ModelMost typical of humid regions

zones of more constant discharge, moderate slope conditions)

Channel morphology and channel floor deposits thalweg, x-sectional asymmetry, cut banks,cross-overs, marginal point bars)

Meandering Stream Facies Helical FlowDownstream transport-superelevation of water surface on outside bends (nb typo)-pressure differential causes secondary helical flow

Lateral Accretion

Erosion

Meandering Stream Facies Helical Flow

Particle size fining up point bar with reduced shear stress

Lateral Accretion

Erosion...

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...

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Point Bar- Fining Upwards SequencesFrom channel bottom upwards:1-Basal erosion

2-Large 3-D ripples and large-scale troughs

3-Small troughs and planar x-beds

4-Lower flow regime plane beds

5-Suspension settling

Nichols, 2009

Rt. Shinarump Cgl.-Chinle Fm. Triassic(A. Cohen photo)

Below- Basal erosion and channel depositsLarge-scale troughs (Nichols, 2009)

Meander migration and cutoff

Nichols 2009

Channel Migration-Epsilon Cross BeddingEpsilon-cross bedding & Lateral Accretion SurfacesOften w/superimposed, large straight crested or 3-D ripples)

Nichols, 2009

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Point Bar- Channel, Lateral Accretion and Fining Upwards Sequence

Natural Levees & Crevasse Splays

Crevasse Splay Deposits

Rapid loss of flow competence-Climbing ripplesCrevasse Splays-Tabular sheet sands and muds-thin and fine away from levee

Below-Chinle Climbing Ripples-Crevasse Splay

Nichols, 2009

Oxbow Lakes

Gradual channel abandonment

Photo

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Oxbow Lake DepositsOrganic-rich mudstones, coals

Overbank DepositsMorrison Fm., Colo (l). Fines (silts & paleosols, roots, organic rich)

Channel Sands

Crevasse Splay Sands

Paul Heller photo Gary Nichols photo

Right-Floodplaindeposits (location unknown)

Fossils-moderately common, abraded bones/wood in channels, plants/roots, bones, molluscs, traces in overbank and oxbows

Morrison Fm. Tidwell Mbr Morrison Fm dinosaur bones Chinle Fm. Petrified Woodburrow

Meandering Stream Deposit Fossils

Tabular sands encased in mud rocks (muds predominate)-Epsilon x-beds & fining up sequences characteristic of sands-Abundant and extensive paleosols

Stratigraphic Geometry of Meandering Stream Deposits

Chinle Fm (Petrified Forest) Morrison Fm. CO

P. Heller photo

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Anastomosing Rivers-FormationForm where abundant accomodation space exists for vertical aggradation of channel and bed. Low slope conditions-e.g.: behind dams, during rapid subsidence, rising base level.

Saskatchewan R. N. Smith photo

Anastomosing River ChannelsChannels: Multiple, often (but not always) low sinuosity channels-stable position-Dynamics characterized by avulsion rather than migration-Gravels, trough and planar x-bedded sands, ripple x-laminated sands

Anastomosing River Levees and Overbank Deposits

Levees-vertically accreting alluvial ridges-eventually destabilizeripple lam. sands, massive or mottled silts

Overbank deposits-massive or mottled silts, micritic ls., coalsL. Cosumnes R. (U.C. Davis photo). R. Upper Columbia R. (H. Berendsen photo)

Photo

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Anastomosing River Deposit GeometryIsolated shoestring sand channels from avulsion(100-1000m wide, 10s of m thick, 10s of km long) Encased in mudrocks (overbank, wetland deposits)Fossils-coals/plant fossils in overbank

Channel SandsSt, Sp, Sh

Shoestring SandPaleochannel

LeveeSr, Fsm

Crevasse SplaySr, Sp, Sm

Wetlands, OverbankFm, Fcm

Bioturbated, paleosols

Braided Rivers

Form under conditions of high, mobile sediment supply and high slope-Characterized by multiple, mid channel bars (large scale bedforms). -On scale of channel depth these are large features-Multiple unstable, shallow channels-low sinuosity

Braided River BarsActive during floods of entire channel beltSize- 100s-1000s m long, 10s-100s m wide, few-15m high

Photo

Longitudinal BarsTear drop shaped, elongate downstream pointing downstream,

-typical of channel center in gravelly streams -fine downstream so coarsen upsection as bar migrates-often imbricated)

Toutle R., longitudinal bars. Flow to right.

P. Heller photo

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Transverse BarsLook like large ripples

-typical of sandy systems-often well developed slipface-often assoc. w/ 3-D ripples climbing bar and in channel

Platte R-transverse bars

Braided Streams: Facies AssociationsBar deposits mostly x-strat. or planar strat. sands & gravelsOverbank-minor, weak facies developmentFossils rare/abraded

Braided Stream Deposit Ancient ExampleThe Auriferous Gravels-Eocene,

California

Distinguishing Stream Types in the Stratigraphic Record

Feature Meandering Anastomosing Braided

Fossils Common (verts, inverts, plants, esp. in fines, large bones in channels)

Common (verts, inverts, plants, in fines, large bones in channels)

Uncommon

Paleocurrents Highly variable Intermediate Relatively invariant

Textures/ fabric

Mud dominated, sands in tabular sheets, fining up

Mud dominated, sands in shoe-strings, fine up

Sand-gravel dominated, bars may coarsen up, imbrication

Sand body geometry

Sheet sands. Epsilon x-beds and lateral accretion from channel migration

Lenticular channel forms, shoestring sands

Bars highly variable, stacked multistory bars common

Associated floodplain deposits

V. important (paleosols, mottled & rooted, crevasse splays, coals)

V. important like meandering

Relatively uncommon and unimportant

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Facies Architecture Differences between stream types (from Tornqvist, 1993)