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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
Interdistributary bay
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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
Fluvial discharge Density contrast
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
Fluvial discharge Density contrast
Gradient
Mouth bar aggrades and progrades
(up to 100 m!), forming y-shaped
channel bifurcation
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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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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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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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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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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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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