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Deltaic successions form a significant
proportion of the Worlds siliciclastic
petroleum reservoirs
1 km10 km
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Review of delta sedimentology andstratigraphy
Coastal processes on deltas
Delta types and reservoirs
Effects of sea level change on deltas
Deltaic reservoirs- correlation & mapping
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Definition, Delta morphology Scale of deltas, size and relationship to
geological time
Deltaic cycles and key surfaces
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Herodotus 445 BC - Nile Delta
a triangular alluvial tract at the
mouth of a river enclosed ortraversed by its distributariesOxford dictionary
a discrete shorelineprotruberance formed when ariver enters the ocean or other
large body of water (Elliot,1986)
shoreline regression typically
associated with progradation
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Alluvial plainfluvial channel
Delta plain
distributaries Delta front
mouth bars Prodelta
shelf muds >10m
10m70m
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Deltas range in scale, from 100m wide to >100km,but are individual delta are typically 10-60m thick
, and
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If sediment supply is high, then most deltas canprograde to shelf edge (10-100s km) in a few
thousand years
Most modern deltas less than 6000yrs old(progradation since global sea level maxima
Most below biostratigraphic resolution, so
geological record comprises deltaic cycles
representing progradation bounded by flooding
surfaces
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Delta classification Fluvial influences and facies
Wave influences and facies Tide influences and facies
Effects of fluvial sediment supply vsaccommodation
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Mississippi
Volga
Fluvial
birdsfoot
lobateWave Tide
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MISSISSIPPI DELTA
Fluvial dominated, no tides,
some waves
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MAHAKAM DELTA
Mixed fluvial and tide
dominated, no waves
Meandering alluvial
plain (incised)
Head of passes
Active distributaries
Abandoned distributaries
Mouth bars Prodelta
10m
70m
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Niger
Baram
Beach ridges on strandplain
Lobate shoreface, tidal channels
Fluvial
Wave Tide
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NIGER DELTA
Wave dominated with tidal influence
Meander belt in
alluvial plain
Abandoned estuarinedistributaries
longshoredrift
Active distributaries
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Landward limit of coastal processes First major bifurcation (head of passes)
Change in gradient, almost flat surface
Decrease in marine or brackish infaunalbioturbation; but bioturbation does occur in
the alluvial plain Alluvial plain typically meandering,
whereas distributaries usually straight
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Distributary channels Mouth bars
Levees Crevasse splays and crevasse deltas
Prodelta muds
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Typically straight, but sinuous thalweg
Depths up to 40m, but mainly 5-20m Widths from 10s to 100s m, up to Kms
Width/thickness ratios 1:50- 1:100
Fluvial dominated channels are zone of sandbypass; lateral bars ->lateral accretion
Tide dominated more heterolithic
May be isolated and mudplugged; can beinterconnnected with mouth bars
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Miocene Mahakam
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Formed when flow velocity declines at the rivermouth and sediment falls from suspension andaccumulates as a bar (coarsening-upward) and can
be fluvial, tidal or wave dominated Fluvial bouyancy dominated flows produce lobate
mouth bars & subaqueous levees
Fluvial friction dominated flows producetriangular mouth bars
Tidal suspension/friction dominated flows produceelongate mouth bars
W/T ratios typically >1:1000, 5-15m thick and
>4km wide; wave dominated >>5km
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Mouth bar
Fluvial channel
Distributary channels
1 km
Crevasse
Splays
Neales Delta, Lake Eyre
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Levees formed in floods, both subaqueousand subaerial; vegetated; climbing ripples;
gentle primary dips
Crevasse Splays; channels and mini-deltas,
with both fining-up and coarsening motifs
Prodelta mainly muddy, dips seaward andonlaps onto shelf
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Floodbasin
Crevasse
Delta
Main
Channel
Distributary Channel
Floodbasin
CrevasseDelta
Distributary Channel
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Delta
successions
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Normal regression during rising or falling
sea level due to greater rate of sediment
supply (progradation) Normal transgression during rising or
falling sea level due to greater rate ofaccommodation than sediment supply
Forced regression (drop in sea level and
resulting incised valleys and lowstand deltas
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Flooding surfaces: transgressive surface and
maximum flooding surface these define
parasequences (or p..sets)
Erosion surfaces: local erosional scour at the base
of fluvial channels or at the fairweather wave base(all minor)
Unconformities & correlative conformable
surfaces that separate older sediment from
younger sediment (sequence boundaries)
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Basic building blocks of the stratigraphic
record produced by one cycle of infilling
accommodation
Represent conformable successions of
genetically related strata bounded by marine
or lacustrine flooding surfaces May either coarsen upward (in deltas,
crevasse splays etc.) or fine-upward
(fluvial/tidal channels)
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If the rate of sediment supply is greater
than the rate of accommodation (S>A)
then sediments will PROGRADE
(progradational stacking pattern,forestepping, or regression)
Eg. deltas!
PROGRADATIONAL PARASEQUENCES
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PROGRADATIONAL PARASEQUENCES
ALLUVIAL PLAIN
UPPER DELTA PLAIN
LOWER DELTA PLAIN
PRODELTA
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If the rate of sediment supply is less than
the rate of accommodation (S
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RETROGRADATIONAL PARASEQUENCES
ALLUVIAL PLAIN
UPPER DELTA PLAIN
LOWER DELTA PLAIN
PRODELTA
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Identify facies (cores, cuttings, fossils)
Identify stacking patterns (wireline logs)
Identify key surfaces (cores and logs)
Use analogues for width/thickness ratios of
distributary channels and mouth bars
Correlate using flooding events notsand
bodies
Interconnectedness of flow units