Sedimentologi Kamal Roslan Mohamed DELTA

INTRODUCTION The 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.

RIVER 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 prograding into 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.

TYPES OF DELTA The 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.

Controls on delta environments and facies.

DELTA ENVIRONMENTS AND SUCCESSIONS Marine deltas form at the interface of continental and marine environments. The processes associated with river channel and overbank settings occur alongside wave and tidal action of the shallow marine realm. Deltas are fed by a river and there is a 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.

DELTA ENVIRONMENTS AND SUCCESSIONS At the mouth of the channels the flow velocity is abruptly reduced as the water enters the standing water of the lake or sea. The delta front Immediately forward of the channel mouth is the site of deposition of bedload material as a subaqueous Mouth bar. The delta slope, is often shown as a steep incline away from the delta top, 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.

DELTA ENVIRONMENTS AND SUCCESSIONS Fresh river water with a suspended load may have a lower density than saline seawater and the plume of suspended fine particles will be buoyant, spreading out away from the river mouth. As mixing occurs deposition out of suspension occurs, with the finest, more buoyant particles travelling furthest away from the delta front before being deposited in the prodelta region. Delta deposition can be divided into two subenvironments, the delta top and the delta front.

DELTA ENVIRONMENTS AND SUCCESSIONS The delta slope, is often shown as a steep incline away from the delta top, but the slope varies from only 1o or 2o in many fine-grained deltas to as much as 30o in some coarsegrained deltas.

VARIATIONS IN DELTA MORPHOLOGY AND FACIES Effects of grain size: Differences in the grain size of the sediment supplied affect the form of a delta: (a) a high proportion of suspended load 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-type Coarsegrained delta.

VARIATIONS IN DELTA MORPHOLOGY AND FACIES Water depth: shallow- and deep-water deltas (a) A delta prograding into shallow water will spread out as the sediment is redistributed by shallow-water processes to form extensive mouth-bar and delta-front facies. (b) In deeper water the mouth bar is restricted to an area close to the river mouth and much of the sediment is deposited by mass-flow processes in deeper water.

VARIATIONS IN DELTA MORPHOLOGY AND FACIES Water depth: shallow- and deep-water deltas (a) A delta prograding into shallow water will spread out as the sediment is redistributed by shallow-water processes to form extensive mouth-bar and delta-front facies. (b) In deeper water the mouth bar is restricted to an area close to the river 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.

Process controls: river-dominated deltas A delta is regarded as river-dominated where the effects of tides and waves are minor. This requires a microtidal regime and 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-dominated delta 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.

Process controls: river-dominated deltas When a delta channel avulses a new lobe starts to build out at the new location of the channel mouth. The abandoned lobe subsides by dewatering until completely submerged. Through time the channel will eventually switch back to a position overlapping the former delta lobe. This results in a series of delta-lobe successions, each coarsening-up.

Process controls: river-dominated deltas A schematic graphic sedimentary log of riverdominated delta deposits.

Process controls: wave-dominated deltas Waves driven by strong winds have the capacity to rework and redistribute any sediment deposited in shallow water, especially under storm conditions. The river mouth and mouth-bar areas of a delta are susceptible to the action of waves, resulting in a modification of the patterns seen in river-dominated Deltas. Progradation of the channel outwards is limited because the subaqueous levees do not form and bedload is acted upon by waves as quickly as it is deposited. A wave-dominated Delta brought to the delta front to form coastal sand bars and extensive mouth-bar deposits.

Process controls: wave-dominated deltas A schematic graphic sedimentary log of Wave dominated delta deposits.

Process controls: tide-dominated deltas Coastlines with high tidal ranges experience onshore and offshore tidal currents that move both bedload and suspended load. A delta building out into a region with strong tides will be modified into a pattern that is different to both river- and wave- dominated deltas. The delta-top channel(s) are subject to tidal influence with reverses of flow and/or periods of stagnation as a flood tide balances the fluvial discharge. A tide-dominated delta in a macrotidal regime will show extensive reworking of the delta front by tidal currents and the delta top will have a region of intertidal deposition.

Process controls: tide-dominated deltas This may be seen in strata as reversals of palaeocurrent indicated by cross- stratification, and the formation of mud drapes. The tidal currents rework sediments at the river mouth into elongate bars that are perpendicular to the shoreline. These are modified mouth bars, which May show bidirectional cross-stratification and mud drapes on the cross-bed foresets due to the reversing nature of the ebb and flood tidal currents A tide-dominated delta in a macrotidal regime will show extensive reworking of the delta front by tidal currents and the delta top will have a region of intertidal deposition.

Process controls: tide-dominated deltas A schematic graphic sedimentary log of Tidedominated delta deposits.

Facies succession Delta cycles: the facies succession preserved depends on the location of the vertical profile relative to the depositional lobe of a delta.

Characteristics 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 oxidised facies associations – typically occur overlying shallow- marine facies and overlain by fluvial facies in an overall progradational pattern.

SEKIAN