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

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)

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

W. W. Norton

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

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

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)

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

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

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)

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

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

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

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)

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

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

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

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

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

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

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

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

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)

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

Wave Dominated Delta

Ganges- Tide Dominated

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

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

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

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

Delta Front Progradation

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

Transgressive Mississippi Delta Model

Transgressive Mississippi Delta Model

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

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)

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

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

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)

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

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

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.