A delta is a fan of braided streams and sediment formed when a river discharges into a larger body of water: a sea or a lake. It will be useful if the reader has already read the article on rivers before reading further.
The dynamics of a delta
As a river discharges into a larger body of water, the current disperses and loses energy, and so the river dumps its sedimentary load: naturally, it will shed the heaviest sediments first, where the energy is highest, with progressively lighter sediments being carried further out into the sea or lake.
This means that the river will be constantly blocking up its own course with sediment, causing it to fan out into a web of distributary streams interspersed by bars of sediment. The process is not unlike the mechanisms which produce braided streams along the course of a river (as discussed in the previous article), and which produce alluvial fans as mountain streams disperse into a desert (as discussed in the article on deserts).
The appearance of a delta in profile will consist of flat topset beds of sediment, then foreset beds which slope down into the lake or sea; then horizontal bottom-set beds deposited on the floor of the sea or lake. Over the course of time, the delta will build out (prograde) into the lake or sea, forming a characteristic sedimentary sandwich of coarser topset beds overlying sloping forset beds overlying finer bottomset beds.
Because the distributary streams of a delta will be constantly dumping sediment in their own path, the pattern of streams and sedimentary bars will not be static, but will shift and change, producing a complex pattern of sedimentary deposition in the topset beds.
Indeed, given long enough, the whole course of a river may shift as the delta silts up. The mouth of Mississippi, for example, is known to have shifted several times over the course of the last few thousand years, and it is only by the unstinting efforts of the U.S. Corps of Engineers that the waters of the Mississippi still flow to the sea via the Mississippi River Delta.
Types of delta
Deltas may be categorized as freshwater or marine, depending on whether they discharge into a lake or a sea. Almost everything we have to say about deltas will apply equally to marine deltas and freshwater deltas. However, there is one notable difference in their dynamics. In a marine delta, the river will not be as salty as the sea into which it is discharging, and so the river water will be less dense than the seawater, and so will flow along the surface of the sea, mixing with the sea water in a horizontal layer, resulting in slower mixing and slower dissipation of the current than in a freshwater delta. The practical upshot of this is that the foreset beds of a freshwater delta will slope down at a much greater angle (up to 25° from the horizontal) whereas in a marine delta the foreset beds will have a slope of only a few degrees from horizontal.
Marine deltas may further be categorized by their dynamics as tide-dominated, wave-dominated, or stream-dominated, according to the main factor affecting their form.
- Stream-dominated deltas, such as the Mississippi River Delta, have long distributary channels extending seawards.
- Tide-dominated deltas have, offshore, long bars of sand parallel to the direction of the tide. Inshore, in the main body of the delta, they have tidal flats: beds of mud deposited by the action of the tide. These exhibit cross-bedding produced by the tidal currents: because tides flow in two directions, they will exhibit herringbone cross-bedding, a distinctive sedimentary pattern where alternate sets of cross-beds slope in opposite directions.
- In wave-dominated deltas, longshore drift (a current parallel to the shore) smears the deposed sediment across the face of the delta, so that instead of the tidal bars found in wave-dominated deltas, we get a set of barrier islands at right-angles to the direction of the distributary streams.
Former deltas: how do we know?
Where a river has only recently shifted from disgorging via a delta, then it is perfectly obvious that the delta used to be a delta: not only does it look just like one in terms of its form and position, but also aerial photographs will reveal the former course of the river and its distributaries.
But what of ancient deltas that have been buried and then lithified? Well, in that case they will look just like lithified deltas. Perhaps only a fragment of such a delta will be exposed to our examination, and so the entire topography of the delta may not be visible, but we can still look at the patterns visible in the sediment.
So, looking through a vertical section of the rock, we should expect to see coarser horizontally-layered topset beds, with complex patterns of sedimentation caused by the shifting of streams and bars, overlying sloping forset beds, overlying finer horizontally--layered bottomset beds. This is a very characteristic pattern of deposition produced by no other process.
Looked at horizontally, we expect to see a complex pattern of interfingering of land and marine sediments reflecting the complex ragged shape of the edge of a delta.
When we find fossils, they will reflect the nature of the beds in which they were deposed: so we expect to see land plants and animals in the topset beds, and the fossilized footprints of marine birds and suchlike fauna. In the bottom-set beds we may, to be sure, find a few remains of land plants and animals carried out to sea by the current: but we would expect the fossils in these beds to be dominated by aquatic fossils and by trace fossils such as the burrows of marine worms; we cannot, of course, find fossil footprints in these beds.
Further indications may be given according to the type of the delta: if, for example, it was tide-dominated, then in the topset beds we will find mudrocks displaying the herringbone cross-bedding characteristic of tidal flats, a phenomenon we shall discuss further in the article on nearshore sediments.