Deltaic Depositional Systems
Modern and
Ancient
Arno River Delta (Med)
(a wave dominated and engineered delta)
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Deltaic Depositional Systems
• Locus of voluminous terrigenous clastic sediment
accumulation where fluvial dispersal systems
encounter standing water
• Most common in subsiding basin-settings (passive
continental margins) where major river systems
transport large volumes of sediment.
Modern Gulf of Mexico
And the Mississippi River Delta
(a river dominated delta)
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Deltas
• Rivers
– flow seaward
– change slope and velocity
– carry a sediment load.
• When river empties into coastal body of
water
– velocity slows
– sediments are deposited.
• Herodotus (c. 400 BC) - thought the alluvial
plain at mouth of Nile looked like a D
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W. W. Norton
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The Nile- Original D
Modified from Hamblin and Christiansen,
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Importance Of Deltas
• Site of substantial fossil fuel resource accumulation
– Coal, Oil, and Natural Gas
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Importance Of Deltas
• Diverse and prolific ecosystems
• Common site of large human population centers
Nile River Delta (Med Sea)
(an engineered, wave/river dominated delta)
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Tigris&Euphrates River Delta (Persian Gulf)
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Main Geological Characteristics
Of Deltas
• Isopach thick... major stratigraphic component of
(Terrigenous Clastic) sedimentary basin fill
Mississippi Delta
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Main Geological Characteristics
Of Deltas
• Regressive - Progradational successions
• Abandonment – Transgressive Stage
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Main Geological Characteristics
Of Deltas
• Contemporaneous non-marine - marginal marine - to basinal
depositional systems
• Numerous sub-environments (each of a scale similar to that of
most other depositional systems)
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Delta Overview
• Deltas grow steadily from a point source
• Course bedload most proximal
– channel and mouth bar subenvironments
• Fines more distal from point source
• Overall pattern:
coarse
\\\\ medium fine very fine
• Builds out such that fines are offshore
– leads to COARSENING UPWARDS
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• Delta progradation
– cross-section results as clastics are
deposited in the sub environments
– sediments builds out
– PROGRADES INTO BASIN
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Delta Morphology
• Function of most dominant process
• Elongate
– little reworking of sediments; levies form, builds
out into basin
• Lobate
– better reworking (tidal); more blunt shape
• Cuspate
– water concentrates its energy to oppose wave
action
– each ridge is built as it moves out and progrades
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Delta Types
• Constructional Deltas
– Dominated by the fluvial
system
• strongly
progradational/regressive
– Lobate – Elongate
• Destructional Deltas
– Dominated by marine
processes
• common marine reworking
with transgressive
intervals
– Cuspate (transitional to
interdeltaic systems)
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Main Processes Influencing
Delta Depositional Systems
• Climate
• Relief
• Fluvial Discharge (water volume and
time variation)
• Sediment load and type
• River mouth processes
• Tidal Processes
• Wave energy
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Subenvironments
• Delta plain/ delta top
– Delta Channels
– Floodplain
• Delta front environments
– Delta Front
– Prodelta
– Foredelta
• Marginal (distal)
• Offshore
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Delta plain/ top
• Channels and flood
plain
– part of the fluvial
environment
– rivers not quite to sea
• Delta Channels
– coarsest sediments in
channels
– carries sediments
across delta top/ delta
plain
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• Floodplain/ over-bank
areas
– suspended sediments
settle out during floods
• Vegetated
• possible accumulation of
peat
– crevasse splays
• lead to sand lenses on
surface
– interdisciplinary bays
• sheltered areas on delta
plain near delta front
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Delta Front Environments
• Delta front
– Include river channel
– silts and sands
– Proximal
• Prodelta
– silty and clay
• Foredelta
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Delta front environments
•River channel
•Site of deposition of bedload
•Forms sub-aqueous mouth bar
•coarsens up
•coarse sediments reworked by tides, wave
actions
•water often brackish
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Delta front environments
• Prodelta
– finest grained sediments
• aided by plume of sediment rich water that
extends across delta front
• blankets area with fine grains
• suspended sediments
– some coarser sediments from turbidites
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Delta front environments
•
Delta slope
–
inclined area in front of delta top
•
•
Slope delta front is related to grain size
Coarse grains make steep slope
–
•
•
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up to 35° angle of repose
Fine grains (silty clay) is <1° slope
Fines increase away from river
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Galloway Classification
• Galloway (1975): 3 factors of constraint
– fluvial dominated (sediment input)
– wave dominated (wave Energy)
– tidal dominated (tidal Energy)
• Mississippi
• Rhone
• Ganges
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(fluvial)
(wave)
(tidal)
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River Dominated
Modified from Hamblin and Christiansen,
1988
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Wave Dominated Delta
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Ganges- Tide Dominated
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Main Delta Sedimentary Facies
• Generic River-Dominated Delta Model
–
–
–
–
large rivers
broad shelf
low wave energy
low tidal range
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Main Delta Sedimentary Facies
• Generic River-Dominated Delta Model
– Upper Delta Plain
• above highest high
tide
– low gradient/
meandering river
systems
– fresh water lakes
– swamps
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Main Delta Sedimentary Facies
• Generic River-Dominated Delta Model
– Lower delta plain
• between the tides
– Distributary channels
– Inter-distributary
bay fill
– levees
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Main Delta Sedimentary Facies
• Generic River-Dominated Delta Model
– Subaqueous Delta
(Delta Front)
• below lowest low tide
– distributary mouth
bar - bar finger
sands
– bays
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Delta Front Progradation
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Main Delta Sedimentary Facies
• Generic River-Dominated Delta Model
– Prodelta
• Offshore transitional
to open marine
– Normal Marine Shelf
• High biological
productivity
– Abundant slumps and
syndepositional
deformation
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Transgressive Mississippi Delta Model
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Transgressive Mississippi Delta Model
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Wave dominated deltas
• Wave dominated
–
–
–
–
wind- driven waves agitate surface
rework sediments in shallow water
affects mouth bars in basin and mouth of river
modifies river –dominated delta
• Morphology limits progradation
– can’t form sub-aqueous levees
• bedload is immediately reworked
• if waves hit obliquely (and usually do), get lateral
migration of sediments and development of spits
– beach and mouth bars form // to coast
• waves sort grains
• mouth bar is better sorted sediments
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Generic Wave Dominated Delta Model
• High wave energy, open coasts, strong longshore
currents
– Non-marine, swamp to
Eolian dune
– Arcuate to strandparallel sand dominated
facies, barrier island
sequences
Rhone River Delta (Med)
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(a wave dominated delta)
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Wave dominated deltas
• Progradation
– waves don’t transport ALL material from
river mouth
– mouth bars build to form new beaches
– River mouth bars aren’t as continuous and
have more overbank deposits
– probably similar delta slope and front
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Tide dominated deltas
• Tide dominated
– onshore/offshore currents move bedload/
suspended load back and forth
– very different features
– delta plain
– tidal currents are bidirectional
• Herringbone cross-bedding
• Mud lenses as suspended sediments settles out in
slack tide
–
–
–
–
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lots of sediment in surface in form of tidal flats
lobate shape to mouth bars; perpendicular to shore
look for bi-directional flow indicators
can confuse with estuarine systems
» look at over all sequences
» delta is progradational; estuary often
retogradational
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Generic Tide Dominated Delta Model
• High Tidal Range
– Extensive lower delta
plain/tidal mudflats
– Shore perpendicular,
elongate sand dominated
facies, tidal channel
deposits
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Ganges - Brahmaputra River Delta
(Indian Ocean)
(a tide dominated delta)
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Tide dominated deltas
• Coarse grained deltas
– bodies of gravelly detritus that form on
margins of lakes and seas
– needs braided river or alluvial fan
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Environmental Issues in Modern Deltas
• Damming, Dredging, Diverting
– Coastal Land loss (erosion/subsidence)
– Coastal Pollution
• Nutrient loading,
– anoxic events
• Petroleum contamination
– Habitat Destruction
• land loss,
• contamination, and
• development
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Environmental Issues in Modern Deltas
•
Mississippi Delta Coastal Land loss
–
Louisiana's coastal wetlands, a national resource
supporting 30% of the nation's fisheries and
most of the wintering ducks in the Mississippi
Flyway, are at risk from the annual conversion of
an estimated 35-45 mi2 of wetlands to open
water. Louisiana's wetland loss rate is the
highest of any state in the nation. The processes
causing wetland loss in coastal Louisiana are
complex and varied.
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