LECTURE 6: Deltaic Systems
I.Herodotus (c. 400 BC) - thought the alluvial plain at mouth of Nile looked like a D
Barrell (1912) - described the ancient (Devonian) Catskill Delta and defined a delta as "a deposit partly subaerial built by a river into or against a body of permanent water. The outer and lower parts are necessarily constructed below water level, but its upper and inner surface must be land maintained . . . by a river . . . therefore consists of a combination of terrestrial and marine, or at least lacustrine strata."
So sediment transport by a RIVER is key to having a delta.
II. River processes: sedimentation at the mouth of a river where flow runs into standing sea (or lake) water. Get rapid decrease in shear stress (along with flow expansion at mouth of channel, forcing deposition = distributary mouth bar.
A. In a model described by Coleman, the velocity reduction at the river mouth is not uniform across the region of flow expansion. Thus, sedimentation rates on the bar are not exactly equal. If one area gets a bit higher sedimentation than an ancient area, you can develop a bar that has topography, essentially the mouth bar consists of local highs and lows that causes the flow to split around the highs. This is much like in a braided stream where local areas of flow expansion lead to bar deposition and flow splitting. In the case of delta river mouths this eventually causes the river to split into two, or more, paths. As the river progrades seaward, and the bar builds up above sea level, we find a river pattern on the delta plain (the river dominated, above sea level part of the delta) that is a distributary pattern.
B. Delta Front: The zone just seaward of the river mouth is where the distributary mouth bar is deposited, and represents the area where the coarsest (usually sandiest) marine sediments are found. This area can be reworked by waves etc., but in general the area just seaward of the shoreline on a delta is called the delta front. The block diagram below shows what types of sediment is found in this part of the delta.
Note on the above figure that the area farther seaward of the delta front is a zone where the fine grained, silt to clay, material carried in by the river is deposited. In this zone sedimentation rates of fine grained material are higher than found farther off on the shelf. These muddy build ups are referred to as the Prodelta zone.
III. River-dominated deltas: the type of delta system we have been describing above is dominated by a river that builds a delta pretty much willy nilly into the sea. River dominated deltas are therefore those where other effects, such as much reworking by waves or by tides is minor. These deltas tend to build delta lobes out into the sea. These lobes might have little more than the distributary channel and its levee exposed above sea level. In which case the delta looks something like a bird's foot (upper right example in first figure above). Other times more of the flood plain between individual distributary channels is exposed above sea level, in which case the delta can have more of a lobed shape.
A. Progradation of the distributary channel and its mouth bar into the open sea produces a coarsening and thickening upward sequence. The abandonment of the channel causes the sequence to be overlain (usually pretty abruptly) by mudstones. Note in the example below there is even a mud plug in one of the abandonded channels.
B. Of course, like most rivers, distributary channels can avulse. As a result the entire delta lobe which is being fed by the river will stop getting sediment and become abandoned and an new lobe will start to form and prograde where the newly avulsed river hits the sea. The abandoned delta lobe will tend to subside over time as clays beneath are compacted from the weight of the delta, if the basin is subsiding tectonically and/or as sea level rises and drowns the delta. The result, in all cases, is that the area that had previously been delta deposition becomes submerged and shelf muds are deposited on top and buries these older deposits. Delta lobe abandonment can happen on the time scales of centuries to many millenia as in the example shown below of the Mississippi delta.
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History of delta lobe abandonment on the Mississippi Delta over the past 5000 years. Click figure to go to printable version of the figure. Clicking that figure will return you here. |
C. The resulting vertical stratigraphic sequence of a river dominated delta is shown below. The upper figure is the Mississippi Delta idealized sequence. Compare that to the lower sequence which comes from a wave-dominated delta, and, as a result, has less mudstones and channel associated deposits.
IV. Wave-dominated deltas: Where rivers dump into the sea in areas of significant wave build up. The action of waves is to constantly rework the delta front. Sediment is carried off down the longshore drift direction. This may cause the delta to have a more cuspate shape, where beveled by wave action. The shoreline down drift of the river mouth may have much better developed and extensive beaches and even sandy spits can form in the down drift direction. Muds carried down the river to the delta can get carried much farther down drift leaving large areas of muddy shoreline in those areas away from the river mouths. The lower stratigraphic section in the figure above shows an idealized vertical stratigraphic sequence through a wave-dominated delta.
V. Tide-dominated deltas: where river mouths hit the sea in areas affected by large tidal ranges, the delta shape can be extensively reshaped by the twice a day flood and ebb tidal currents moving in and out of the river mouth. This usually happens in bays and estuaries where the river mouth is protected from much wave activity. The relentless in and out currents of tides can sculpt the sediment into elongate tidal bars, such as seen in the lower left delta of the first figure above. At the head of the bay there may be a classic looking delta, in this location referred to as a bay-head delta, but farther seaward is a zone of lots of tidal bars, islands and inlets caused by tide reworking. A vertical stratigraphic section through this type of deposit will be dominated by lots of muds and sands that show bidirectional (ebb-directed and flood directed) cross bedding and not much evidence of wave reworking (e.g. beaches) nor or strongly prograding rivers (e.g. river dominated lobate deltas).
VI. Result of rapid sedimentation rates: One key feature of deltas is that they are areas of very rapid sedimentation rates. As a result lots of features tend to form caused by lots of loose sediment and rapid loading. Such features include slumps, soft sediment deformation, diapirs, etc., as seen in figure below.
VII.
