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Delta Environments

Nile Delta Mississippi Delta

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Discrete shoreline protuberances, partly subaerial, built by rivers

into a body of permanent water

Source of the sediment?

Source of energy?

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Delta plain: mostly subaerial

part of delta complex (fluvial

processes)Distributary channels

Interdistributary bay

Delta front: shallow subtidalpart of delta, above wave

base (mix of fluvial and

basinal processes)

Prodelta: deeper subtidal part

of delta, below wave base

(basinal processes)

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Progradation of relatively steep delta front (1-10) produces a

bed geometry called clinoforms

Delta front foresets

Topset

Foreset

Toeset Bottomset

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Atchafalaya River, Louisiana

Distributary channel


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Mouth bar governed byphysics of spreading jet

of river water

Friction Buoyancy

Inertia

Fluvial discharge Density contrast

Gradient

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River water is usually less dense

than ocean water (hypopycnal)

Freshwater

lens above

saltwater

wedge


Gradient

Deposits sediment on relatively narrow bar crest

Internal waves and reducedvelocity at interface

What is the proximal-distal grain size trend?

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Flood stage pushes salt wedge out

More sediment = lower rcontrast

Bottom friction dominates


Gradient

Mouth bar aggrades and progrades

(up to 100 m!), forming y-shaped

channel bifurcation

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basinal processes)



(basinal processes)

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What features would you predict in prodelta sediments?

Grain size?

Sedimentary structures?

Bioturbation?

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Hypopycnal plume

Density < seawater

Relatively continuousDeposition from suspension

(=hemipelagic sedimentation)

Hyperpycnal plume

Density > seawater

Episodic, lasts hours-daysDeposition from suspension,

modified by traction

Hyperpycnal flows are slow, may wax

and wane during river flood stage

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Suspension deposition

Normal grading

Waxing/waning flow

Inverse-normal grading

Hemipelagic sediments

Finely laminated or

bioturbated mud

Deposits called hyperpycnites

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Prodelta slopes are comparatively steep (and have rapid

sedimentation rates)

Slumps may be more common than in wave-dominated coasts

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basinal processes)



(basinal processes)

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Grain size?

Sedimentary structures?

Bioturbation?

Delta Front

What features would you predict in delta front sediments?

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High sedimentation rates during river flood events

Traction deposition with significant

contribution from suspension settling

Climbing ripples

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Mouth bar aggrades close to sea level during progradation

Can be emergent

during lower flowDevelopment of small-scale scours

(reactivation surfaces) more common

near the top of the mouth bar

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basinal processes)



(basinal processes)

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Small-scale (3-10 m thick) cycles

formed due to crevasse-splay and

filling of interdistributary bays

Interdistributary bay mudstone

Increasing overbank sediment from river floods

(rippled sandstone)

Rooted supratidal mudstone

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Which delta was deposited during forced regression FSST and

which during normal regression (LST or HST)? Why?

Prodelta

DeltaFr

ont

DeltaFront

P

Delta

Plain

Transgressive lag

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Offlapping clinoforms capped

by subaerial erosion surface

Where shoreface

gradient > wave erosion

profile (0.5), no wave

scoured surface forms

Gradational facies shift

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Gradationally-based forced regressive prodelta

to delta front (no abrupt facies shift)

No delta plain sediments (river incises during forced regression)

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Within-Trend Forced Regressive Surface: conformable facies

contact developed duringforced regression, only forms in river-

dominated deltas

Replaced by regressive surface of marine erosion in wave-

dominated settings

Approximates basal surface of forced regression, which occurs

cryptically in underlying prodelta facies

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Forced regressive deltas often capped by transgressive lag (because

there are no FSST delta plain deposits and LST often thin)

Transgressive ravinement surface (in this case a wave ravinement

surface) reworks the subaerial unconformity

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Gradationally-based shallowing-up shoreface or deltaic

successions

Topsetbeds preserved (no fluvial incision) due to continued

base level rise and facies aggradation

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Gradationally-based shallowing-up

shoreface or deltaic successions

Normal regressive delta succession

Forced regressive delta succession

Transgressive lag over delta front

Delta plain and fluvial (topset)

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Within-Trend Normal Regressive Surface: Conformable facies

contact between prominent shoreline sands and lower energy

supratidal sediments, all within normal regressive regime,

e.g., beach or delta

front to alluvial

floodplain fines

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Within-Trend Normal Regressive Surface (New Idria)

e.g., beach or delta

front to alluvial

floodplain fines

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Progradational delta facies coarsen upward from

prodelta through delta front to (maybe) delta plain

Prodelta

Delta Front

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Coarsening-

upward

cycles

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Prodelta muds: lack current- or wave-produced

structures, may be banded due to slight grain size

differences (fluctuating river discharge)

Slumps and convolute bedding from steeper prodelta

slope and high-porosity clay mineral fabric

Bioturbation usually low due to high sed. rate

Delta front: increasing parallel and lenticular silt

laminae, eventually cross-laminated sand intercalated

with mud (waves, distal river deposition, suspension)

Bar crest: well-sorted sand with cross-lamination,

climbing ripples, flat lamination, deposited during

river floods

Interdistributary: Fine-grained tidal flat or bay

deposits grading upwards to cross-laminated

crevasse-splay units, and/or

Channel/levee: Erosive-based trough or planar x-bedded sandstone from migration of large bedforms

in distributary channel

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Active delta lobe

Abandoned

subsiding lobe

Cycles may be autocyclic(inherent due to delta processes)

rather than allocyclic(due to external forces like base level)

Lobe switching

Deltaic sequences cyclic at medium scale (50 150 m thick)

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1. River builds long distributary channels on delta (progradation)

2. Avulsion occurs and river takes new, more favorable path to

oceanforms new delta and abandons old lobe

3. Abandoned lobe continues to subside (facies retrogradation)

Deltaic sequences cyclic at medium scale (50-150 m thick)

due to lobe switching

d f fl l fl

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By definition, fluvial influence

is important in deltas

What other factors influence

morphology and deposits?

h b difi d b i i li b h id

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Mouth bars modified by wave action into linear beach ridge

sandbody complexes.

Grijalva delta, Mexico

Beach ridge sandbodies

Tiber delta, Italy

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When significant longshore current present, wave-influenced

delta may be substantially asymmetrical

Prograding river levee acts as

groin to block longshore drift

Wave-influenced strandplain

shoreface forms on upcurrent side

Mouth bar modified into

elongate barrier bars

Become emergent, allowing

for fine-grained back-barrier

deposition

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By definition, fluvial influence

is important in deltas

What other factors influence

morphology and deposits?

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Fly River delta, Papua New Guinea

Tide-dominated deltas contain mouth bars that are moulded into

elongate, shore-perpendicular sand bodies by tidal currents

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Typical coarsening-upward deltaic succession,

except capped by tidal flat deposits

Elongate mouth

bars dissected by

erosive-based

tidal channels

Distributary channels

contain tidal bars with IHS

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Delta types may all have similar looking prodelta facies, but differ

in the geometry of their distributary mouth bar sandbodies

lozenge-shaped isolated sandbodies

Increasingly elongate,

shore-parallel

Increasingly elongate,

shore-perpendicular

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