2INTRODUCTIONThe mouths of rivers may be places where the accumulation of detritus brought down by the flow forms a sediment body that builds out into the sea or a lake.In marine settings the interaction of subaerial processes with wave and tide action results in complex sedimentary environments that vary in form and deposition according to the relative importance of a range of factors.Delta form and facies are influenced by the size and discharge of the rivers, the energy associated with waves, tidal currents and longshore drift, the grain size of the sediment supplied and the depth of the water.They are almost exclusively sites of clastic deposition ranging from fine muds to coarse gravels.Deposits formed in deltaic environments are important in the stratigraphic record as sites for the formation and accumulation of fossil fuels.
3RIVER MOUTHS, DELTAS AND ESTUARIES The mouth of a river / Muara Sungai is the point where it reaches a standing body of water, which may be a lake or the sea. These are places where a delta may form (this chapter), an estuary may occur (next chapter) or where there is neither a delta nor an estuary.A delta can be defined as a ‘discrete shoreline protuberance formed at a point where a river enters the ocean or other body of water’, and as such it is formed where sediment brought down by the river builds out as a body into the lake or sea.A delta fed by a river progradinginto a body of water.An estuary / estuari is a river mouth where there is a mixture of fresh water and seawater with accumulation of sediment within the confines of the estuary, but without any build-out into the sea.
4TYPES OF DELTAThe forms of modern deltas:(a) the Nile delta, the ‘original’ delta,(b) the Mississippi delta, a river-dominated delta,(c) the Rhone delta, a wave-dominated delta,(d) the Ganges delta, a tide-dominated delta.
6DELTA ENVIRONMENTS AND SUCCESSIONS Marine deltas form at the interface of continental andmarine environments. The processesassociated withriver channel andoverbank settingsoccur alongsidewave and tidal actionof the shallow marine realm.Deltas are fed by a river and there isa transition between the area that is considered part of the fluvial/alluvial environment and the region that is considered to be the delta top or delta plain. Delta channels can be as variable in form as a river and may be meandering or braided, single or divided channels.Delta deposition can be divided into two subenvironments, the delta top and the delta front.
7DELTA ENVIRONMENTS AND SUCCESSIONS At the mouth of the channels the flow velocity isabruptly reduced as the water entersthe standing water of thelake or sea. Thedelta frontImmediatelyforward of the channelmouth is the site of depositionof bedload material as a subaqueousMouth bar.The delta slope, is often shown as a steep incline away from the deltatop, but the slope varies from only 1o or 2o in many fine-grained deltas to as much as 30o in some coarsegrained deltas.Delta deposition can be divided into two subenvironments, the delta top and the delta front.
8DELTA ENVIRONMENTS AND SUCCESSIONS Fresh river water with a suspended load may have a lower density than saline seawater and the plume ofsuspended fine particleswill be buoyant,spreading outaway from the rivermouth.As mixing occurs depositionout of suspension occurs, with thefinest, more buoyant particles travellingfurthest away from the delta frontbefore being deposited in theprodelta region.Delta deposition can be divided into two subenvironments, the delta top and the delta front.
9DELTA ENVIRONMENTS AND SUCCESSIONS The delta slope, is often shown as a steep incline away from the deltatop, but the slope varies from only 1o or 2o in many fine-grained deltas to as much as 30o in some coarsegrained deltas.
10VARIATIONS IN DELTA MORPHOLOGY AND FACIES Effects of grain size:Differences in the grain size ofthe sediment supplied affect theform of a delta:(a) a high proportion of suspendedload results in a relatively small mouth bar deposited from bedload and extensive delta-front and prodelta deposits;(b) a higher proportion of bedload results in a delta with a higher proportion of mouth bar gravels and sands.A modern Gilbert-typeCoarsegrained delta.
11VARIATIONS IN DELTA MORPHOLOGY AND FACIES Water depth: shallow- anddeep-water deltas(a) A delta prograding into shallow water will spread out as the sediment is redistributed by shallow-water processesto form extensive mouth-bar and delta-front facies.(b) In deeper water the mouth bar is restricted to an area close to theriver mouth and much of the sediment is deposited by mass-flow processes in deeper water.
12VARIATIONS IN DELTA MORPHOLOGY AND FACIES Water depth: shallow- anddeep-water deltas(a) A delta prograding into shallow water will spread out as the sediment is redistributed by shallow-water processesto form extensive mouth-bar and delta-front facies.(b) In deeper water the mouth bar is restricted to an area close to theriver mouth and much of the sediment is deposited by mass-flow processes in deeper water.A schematic sedimentary log of a sandy delta prograding into shallow water.A schematic sedimentary log of a sandy delta prograding into a deep-water basin.
13Process controls: river-dominated deltas A delta is regarded as river-dominated where the effectsof tides and waves areminor.This requires a microtidal regimeand a setting where wave energy is effectively dissipated before the waves reach the coastline. Under these conditions, the form of the delta is largely controlled by fluvial processes of transport and sedimentation.The unidirectional fluvial current at the mouth of the river continues into the sea or lake as a subaqueous flow.A river-dominateddelta with the distributary channels building out as extensive lobes due to the absence of reworking by wave and tide processes. Low-energy, interdistributary bays are a characteristic of river-dominated deltas.
14Process controls: river-dominated deltas When a delta channelavulses a new lobestarts to build out atthe new location of thechannel mouth.The abandoned lobe subsides bydewatering until completely submerged.Through time the channel will eventually switchback to a position overlapping the former delta lobe.This results in a series of delta-lobe successions, each coarsening-up.
15Process controls: river-dominated deltas A schematic graphic sedimentary log of riverdominated delta deposits.
16Process controls: wave-dominated deltas Waves driven by strong winds havethe capacity to rework andredistribute anysediment deposited inshallow water, especiallyunder storm conditions.The river mouth and mouth-bar areas of adelta are susceptible to the action of waves,resulting in a modification of the patternsseen in river-dominated Deltas.Progradation of the channel outwards is limitedbecause the subaqueous levees do not formand bedload is acted upon by waves as quicklyas it is deposited.A wave-dominatedDelta brought to the deltafront to form coastal sand bars and extensive mouth-bar deposits.
18Process controls: tide-dominated deltas Coastlines with high tidal rangesexperience onshoreand offshore tidalcurrents that moveboth bedload and suspendedload. A delta building out into a regionwith strong tides will be modified into apattern that is different to both river- and wave-dominated deltas.The delta-top channel(s) are subjectto tidal influence with reverses of flowand/or periods of stagnation as a floodtide balances the fluvial discharge.A tide-dominated deltain a macrotidal regime will show extensive reworking of the delta front by tidal currentsand the delta top will have a region of intertidal deposition.
19Process controls: tide-dominated deltas This may be seen in strata asreversals of palaeocurrentindicated by cross-stratification, and theformation of mud drapes.The tidal currents rework sedimentsat the river mouth into elongate bars thatare perpendicular to the shoreline.These are modified mouth bars, whichMay show bidirectional cross-stratificationand mud drapes on the cross-bed foresetsdue to the reversing nature of the ebb andflood tidal currentsA tide-dominated deltain a macrotidal regime will show extensive reworking of the delta front by tidal currentsand the delta top will have a region of intertidal deposition.
21Facies successionDelta cycles: the facies succession preserved depends on the location of the vertical profile relative to the depositional lobe of a delta.
22Characteristics of deltaic deposits . lithologies – conglomerate, sandstone and mudstone. mineralogy – variable, delta-front facies may be compositionally mature. texture – moderately mature in delta-top sands and gravels, mature in wave-reworked delta-front deposits. bed geometry – lens-shaped delta channels, mouthbar lenses variably elongate, prodelta deposits thin bedded. sedimentary structures – cross-bedding and lamination in delta-top and mouth-bar facies. palaeocurrents – topset facies indicate direction of progradation, wave and tidal reworking variable on delta front. fossils – association of terrestrial plants and animals of the delta top with marine fauna of the delta front. colour – not diagnostic, delta-top deposits may be oxidisedfacies associations – typically occur overlying shallow- marine facies and overlain by fluvial facies in an overall progradational pattern.