1. Deltaic Depositional Systems
Modern and
Ancient
Arno River Delta (Med)
(a wave dominated and engineered delta)

2. 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)

3. Deltas Rivers When river empties into coastal body of water
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

4. W. W. Norton

5. Modified from Hamblin and Christiansen, 1988
The Nile- Original D
Modified from Hamblin and Christiansen, 1988

6. Importance Of Deltas
Site of substantial fossil fuel resource accumulation
Coal, Oil, and Natural Gas

7. 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)
Tigris&Euphrates River Delta (Persian Gulf)

8. Main Geological Characteristics Of Deltas
Isopach thick... major stratigraphic component of (Terrigenous Clastic) sedimentary basin fill
Mississippi Delta

9. Main Geological Characteristics Of Deltas
Regressive - Progradational successions
Abandonment – Transgressive Stage

10. 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)

11. 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

13. Delta progradation
cross-section results as clastics are deposited in the sub environments
sediments builds out
PROGRADES INTO BASIN

15. Delta Morphology Function of most dominant process Elongate Lobate
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

16. Delta Types Constructional Deltas Destructional 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)

17. 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

18. Subenvironments Delta plain/ delta top Delta front environments
Delta Channels
Floodplain
Delta front environments
Delta Front
Prodelta
Foredelta
Marginal (distal)
Offshore

19. Delta plain/ top Channels and flood plain Delta Channels
part of the fluvial environment
rivers not quite to sea
Delta Channels
coarsest sediments in channels
carries sediments across delta top/ delta plain

20. 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

21. Delta Front Environments
Include river channel
silts and sands
Proximal
Prodelta
silty and clay
Foredelta

22. 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

23. 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

24. 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
up to 35° angle of repose
Fine grains (silty clay) is <1° slope
Fines increase away from river

25. Galloway Classification
Galloway (1975): 3 factors of constraint
fluvial dominated (sediment input)
wave dominated (wave Energy)
tidal dominated (tidal Energy)
Mississippi (fluvial)
Rhone (wave)
Ganges (tidal)

26. Modified from Hamblin and Christiansen, 1988
River Dominated
Modified from Hamblin and Christiansen, 1988

27. Wave Dominated Delta

28. Ganges- Tide Dominated

29. Main Delta Sedimentary Facies
Generic River-Dominated Delta Model
large rivers
broad shelf
low wave energy
low tidal range

30. 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

31. Main Delta Sedimentary Facies
Generic River-Dominated Delta Model
Lower delta plain
between the tides
Distributary channels
Inter-distributary bay fill
levees

32. Main Delta Sedimentary Facies
Generic River-Dominated Delta Model
Subaqueous Delta (Delta Front)
below lowest low tide
distributary mouth bar - bar finger sands
bays

33. Delta Front Progradation

34. 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

35. Transgressive Mississippi Delta Model

36. Transgressive Mississippi Delta Model

38. Wave dominated deltas Wave dominated Morphology limits progradation
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

39. Generic Wave Dominated Delta Model
High wave energy, open coasts, strong longshore currents
Non-marine, swamp to Eolian dune
Arcuate to strand-parallel sand dominated facies, barrier island sequences
Rhone River Delta (Med)
(a wave dominated delta)

40. 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

41. 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
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

42. Generic Tide Dominated Delta Model
High Tidal Range
Extensive lower delta plain/tidal mudflats
Shore perpendicular, elongate sand dominated facies, tidal channel deposits
Ganges - Brahmaputra River Delta (Indian Ocean)
(a tide dominated delta)

43. Tide dominated deltas Coarse grained deltas
bodies of gravelly detritus that form on margins of lakes and seas
needs braided river or alluvial fan

44. 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

45. 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 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.