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Marginal-marine depositionalenvironments

• Deltas: high sediment supply causes sediment to progradeinto standing body of water

• Beaches: wave and current energy produces well sortedsand, characteristic beach-face profile

• Barrier Island Systems: beach ridge separated from coastby low-energy lagoon

• Estuaries: drowned river valleys, low sediment supply,typical of transgressive coastlines

• Tidal Flats: low-energy mud flats between low and hightide

Coarsening Upward SequenceCretaceous Mesaverde Fm, UT

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Deltas where rivers entering a larger body of water bring in more

sediment than can be carried away by currents or gravityBasic Requirements:

1. stable, shallow shelf2. abundant supply of sediment

Active:– prograde, shift, subside

Importance:– Volumetrically - large component of basins– Coastline stability;– Ecology; fisheries– Economic; coal, oil & gas

• Swampy (coals)• Rapid accumulation and burial of organic matter

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Off Shore Drilling, Mississippi Delta

Deltas• Shape & Size

– Triangular in plan view, wedge-shaped (x-section).– 100-1000 km2 in area & km’s in thickness (w/ subsidence).

• Major Influencing Factors1. rivers

• Sediment load• Density contrast w/ ocean/lake

2. tides• range

3. wave action• Strength, frequency

4. Sea level• Holocene-rising

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Influencing Factors

Salinas River (Winter 2005)

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Buoyancy

Partially-Mixed Estuary

Highly stratified (or salt wedge) estuary

Delta Depositional Environments• delta plain (topsets)

– meandering floodplain & distributaries, swamps, channel,bar complex - channel sands/silts

• delta front/slope (foresets)– sand/silt grades to silt/clay– 1 to 20°

• prodelta (bottom sets)– Clay/mud

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prodelta deposits

U. Carboniferous, S. Wales

3-types of River DeltasBased on dominated

depositional process:1. Fluvial Dominated2. Tide Dominated3. Wave Dominated

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1. Fluvial DominatedSediment laden riversDepositional pattern/facies

influenced by1. Sediment load2. Grain size3. Velocity4. Flux rate (seasonal flooding?)5. River/Basin Density Contrasts

a) Homopycnal - DR=DB

b) Hyperpycnal - DR>DB

• floods (sediment laden)c) Hypopycnal - DR<DB

1. Fluvial DominatedHomopycnal - DR=DB

– Thorough mixing - rapid deposition of sediment load– Coarse grained (gravel/sand) rivers

• Gilbert type delta– Topset, foreset (up to 20° dip), bottomset succession

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Gilbert type delta

• foresets

1. Fluvial DominatedHypopycnal - DR<DB Buoyancy Dominated

– River / ocean or saline lake (common in marine basins)– Horizontally oriented plane jet– Deposition - suspension

• Gradational sorting• Flocculation of clays

– Large Delta front• Foreset dips 1-2°

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1. Fluvial DominatedBouyancy vs. Friction

Dominated• Dependent on

– Density contrasts• Hypopycnal -Flow

detaches from bed• load

– Velocity• Fast-friction dominated

– Sediment texture (size)– Water depth

• Shallow-friction• Deep-buoyant

Mississippi River• Birdsfoot-type Delta• Buoyancy dominated

Depositional Facies?• Interdistributary Bay/Marsh• Natural levee

- Poorly sorted, irregularly beddedsands/silts

• Delta-front, sand bars- X-beds, cut n fill, ripples

• Prodelta clays

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Mississippi River• 7 distinct sedimentary lobes

•Abandoned channels

Atchafalya River• River Avulsion

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Fluvial dominated (i.e., Miss.River) - Vertical Succession;

• Progradation of delta• Shallowing (Coarsening)

upward sequence1. Shelf - fine muds2. Prodelta3. Distal bar4. Mouth bar/Channel5. Beach6. Overbank7. interdistributary deposits8. Bay Marsh deposits

Delta Sequence, Coarsening Upwards

U. Carboniferous, S Wales

mudstones

sandstones

X-bedded sandstones

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Delta Sequence, Channel Cut

U. Carboniferous, S Wales

2. Tide Dominated• Strong Tidal Currents - Bidirectional currents redistribute

river mouth sediments– Sand filled, funnel shaped distributaries– River mouth bar - reworked into linear ridges

• Key Variables– Tidal Range

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Bidirectional Flow(tidal) Bedding

• Flasar

• Wavy

• Lenticular

Ganges-Brahmaputra

• 3 x area of Mississippi• tidal range: 2 m• Low wave energy• Seasonal discharge (monsoon)

– Sand transport/load high– Braided stream

• Depositional facies– Tidal flats

• Flasar bedding– Natural levees

• Poorly sorted/coarsening upward,x-beds

– Interdistributary Flood basins– Tidal sand bars/channels

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3. Wave DominatedWaves modify distributary mouth deposits

– Longshore current-redistribute sediment along shore– Smooth delta front– Absence of interdistributary bay mud– Barrier bars, spits– Beach ridges– Marshes

Sao Francisco Delta, Brazil

• Tidal range: 2 m• Wave Power: 100x Miss./Ganges• Depositional Facies

• Beach-ridge sands aligned parallel to shoreline-sand derived mainly from shelf

• Dunes• Fluvial channel • Channel marsh/mud

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Wave Dominated Delta

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Nile River Delta

Other Deltas

• Mixed Deltas- Waves/Tides

• Braided Delta- hi-sed load

• Fan Delta• Coastal Mtns• Alluvial fan

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Copper River Delta, Alaska• Extreme tidal range (3 m) + Big Waves

General Sediment Characteristics• Upper Delta Plain Sediments

– Above high tide– Channel/levee migration,

overbank flooding, swamps• Lower Delta Plain Sediments

– Low to high tide– Migrating tidal channels,

levees, bays, marshes, swamps• Subaqueous Delta Plain

– Below low tide– Sands near river mouths, finer

sands/silts distal bar, silts claysprodelta

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Progradation• Active delta systems

1. prograding2. switching channels.– build by lateral accretion.– time-transgressive– Coarsening upwards (CU)

• General delta depositional facies:1. complex of discontinuous sandy lenses set within a

silt/mud wedge.2. channel sands

– crossbeds, ripple cross-laminations, scour-and-fill structures,and may contain discontinuous clay lenses.

3. Interchannel areas• finer-grained muds & clays & organic matter

Prograding Deltas, Coarsening Upward Sequence

KR Mesaverde Fm,UT

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Distal, thin-bedded fine mud

Mesaverde Fm, UT

Mid-Delta, fine grain sands, muds

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thin coal (delta top marsh)

Low-standDeltas

• Gulf of Maine– Sea level rise

m= mud (modern)d=deltagm=glacial marine

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Eberswalde delta

Beaches and Barrier Islands

• Classification by tidal range:1. Microtidal 0-2 m2. Mesotidal 2-4 m3. Macrotidal >4 m

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Coastlines/Beaches

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Offshore to Onshore

• Offshore - below storm wave base– Bioturbated muds, thin sand layers

• Shoreface - storm wave base to sea level– lower - below fair weather wave base (<10-20 m)

• wave effect is weak• Fine sand, muds - Planar lamination

– upper - fair weather base to low tide (surf zone)

Lower shoreface

• Mud interbedded with thin lenticular fine grainsand lenses (Pleistocene (Eemian), Denmark)

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• Upper Shoreface– Erosion in winter, multi-directional flow– Trough x-beds, low angle planar x-beds,– Landward dipping planar reflectors in shoreface/foreshore transition

• Foreshore (includes swash zone)• Coarser sands• Gently seaward dipping (2-10°), laminated planar bedding

Shoreface to foreshore to Bedforms

biot

urba

tion

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Shoreface

• Shannon Sandstone Member trough-cross-beddedsandstone, Edgerton, Wyoming (USGS)

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• Backshore– Berm - sandy terrace, horizontal to landward dipping

plane beds– Dunes, eolian trough, x-strata, meters thick

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• Backshore– Washover fan, landward dipping horizontal strata, thin,

lobate sheets– Lagoon - laminated, fine grain silt and clay, organic

rich

Back Beach Erosional/DepositionalFeatures

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Wave Refraction• What happens to as a wave line approaches a beach?

– shallow water - Speed (m/s) = 3.1 √ d

• bottom interference (L /2)– leading edge feels bottom first- wave line to bend

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LONGSHORE TRANSPORT of SEDIMENT

longshore current - wavesstrike at an angle- net motion parallel to

shorevelocity (0.3 to 1.0

m/sec) controlled by:1. beach slope2. angle of breakers3. size of waves

Long shore drift -“river of sand”

• transport of sand1. suspension2. saltation– sand to pebble size clasts– Low-angle cross bedding, imbricated

pebbles• Equilibrium - fine balance ~ current

velocity & sediment load– disrupt balance ~ erosion or deposition

Monterey Bay, CA

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Long-shore DriftDepositional

Features

• Spits• Bay Barriers• Barrier Islands

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Lateral accretion of spitMost features related to

long shore drift• HOW?

– Current loses velocity-drops sediment

Spit; Racoon Key, SC

• Large scale (2 m) high angle, x-beds– Plio-Pleistocene, London Clay, Essex

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bay barrier

Barrier Islands• Long, narrow accumulations of sand - wave

dominated coasts• Small to moderate tidal range

– Microtidal, Mesotidal (<4 m)– Wave energy concentrated - transports sand

– Barrier Beach– Lagoons– Marsh

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LAGOONScoastal bodies of shallow water, w/ restricted

connections to the sea, bounded by a reef or islands– Low energy - tidal currents– Moderate Salinity variations - brackish, salty– Muds, silts, organic matter, shells & washover sand (floods)

Estuary/TidalFlat

• Mud• Flaser beds• Sand/mud• Sand

– x-beds,bidirectional

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Sunburst Member of the Lower CretaceousKootenai Formation, western Montana

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Flaser Bedding - symmetric Wave-ripples

• Sunburst Member of the Lower Cretaceous Kootenai Formation, westernMontana (Schwartz1 and Vuke, 2007)

Rhythmic mud and bioturbated sandstone tidal laminations.

• Sunburst Member of the Lower Cretaceous Kootenai Formation, westernMontana

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• Sunburst Member of the Lower Cretaceous Kootenai Formation, westernMontana

Biweekly tidal cycles?

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Lagoons and Coastal Waterways

Beach

Dunes

Barrier Islands/Lagoons

Lagoon

Depositional Facies & Sealevel• interbedded & interfingering sandstone, shale, siltstone, &

coals (w/oyster shells)• Vertical sequence - sealevel & sediment supply

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Vertical Succession;Barrier Island, BackBeach Lagoon

Shallowing upward sequence• Mid/Upper-Shoreface

– Plane bed sands– laminations

• Channel– Trough X-bedded sands

• Marsh-tidal flat– Sand, coal

• Subtidal lagoon– Shale, oysters

Cretaceous, St Mary River Formation,Southern Alberta Canada

Channel Cap - trough x-stratification

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Prograding Beach and Barrier IslandSystem

Tidal Flat Facies

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Idealized VerticalSuccession (offshore-onshore)

• Progradation• Shallowing upward

sequence– increase grain size– Decreased burrowing

• Holocene– Sealevel rising– Trangressive sequence