geos334deltaicsys_...................ppt
- 1. 1 12_G435.pps Deltaic Depositional Systems Modern and Ancient Arno River Delta (Med) (a wave dominated and engineered delta)
- 2. 2 12_G435.pps 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. 3 12_G435.pps 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
- 5. 5 12_G435.pps Modified from Hamblin and Christiansen, The Nile- Original
- 6. 6 12_G435.pps Importance Of Deltas • Site of substantial fossil fuel resource accumulation – Coal, Oil, and Natural Gas
- 7. 7 12_G435.pps 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. 8 12_G435.pps Main Geological Characteristics Of Deltas • Isopach thick... major stratigraphic component of (Terrigenous Clastic) sedimentary basin fill Mississippi Delta
- 9. 9 12_G435.pps Main Geological Characteristics Of Deltas • Regressive - Progradational successions • Abandonment – Transgressive Stage
- 10. 10 12_G435.pps 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. 11 12_G435.pps 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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- 13. 13 12_G435.pps • Delta progradation – cross-section results as clastics are deposited in the sub environments – sediments builds out – PROGRADES INTO BASIN
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- 15. 15 12_G435.pps • 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 Delta Morphology
- 16. 16 12_G435.pps 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)
- 17. 17 12_G435.pps 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. 18 12_G435.pps Subenvironments • Delta plain/ delta top – Delta Channels – Floodplain • Delta front environments – Delta Front – Prodelta – Foredelta • Marginal (distal) • Offshore
- 19. 19 12_G435.pps 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
- 20. 20 12_G435.pps • 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. 21 12_G435.pps Delta Front Environments • Delta front – Include river channel – silts and sands – Proximal • Prodelta – silty and clay • Foredelta
- 22. 22 12_G435.pps 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. 23 12_G435.pps • 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 Delta front environments
- 24. 24 12_G435.pps • 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 Delta front environments
- 25. 25 12_G435.pps • Galloway (1975): 3 factors of constraint – fluvial dominated (sediment input) – wave dominated (wave Energy) – tidal dominated (tidal Energy) • Mississippi (fluvial) • Rhone (wave) • Ganges (tidal) Galloway Classification
- 26. 26 12_G435.pps Modified from Hamblin and Christiansen, 1988 River Dominated
- 29. 29 12_G435.pps Main Delta Sedimentary Facies • Generic River-Dominated Delta Model – large rivers – broad shelf – low wave energy – low tidal range
- 30. 30 12_G435.pps 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. 31 12_G435.pps Main Delta Sedimentary Facies • Generic River-Dominated Delta Model – Lower delta plain • between the tides – Distributary channels – Inter-distributary bay fill – levees
- 32. 32 12_G435.pps Main Delta Sedimentary Facies • Generic River-Dominated Delta Model – Subaqueous Delta (Delta Front) • below lowest low tide – distributary mouth bar - bar finger sands – bays
- 34. 34 12_G435.pps 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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- 38. 38 12_G435.pps • 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 Wave dominated deltas
- 39. 39 12_G435.pps 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. 40 12_G435.pps • 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 Wave dominated deltas
- 41. 41 12_G435.pps • 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 Tide dominated deltas
- 42. 42 12_G435.pps 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. 43 12_G435.pps • Coarse grained deltas – bodies of gravelly detritus that form on margins of lakes and seas – needs braided river or alluvial fan Tide dominated deltas
- 44. 44 12_G435.pps 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. 45 12_G435.pps 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.