Low flat deposits of alluvium at the mouths of
streams and rivers that form broad triangular or irregular
shaped areas that extend into bays, oceans, or lakes. They are
typically crossed by many distributaries from the main river
and may extend for a considerable distance underwater.
When a stream enters the relatively still water of a lake or the
ocean, its velocity and its capacity to hold sediment drop suddenly.
Thus, the stream dumps its sediment load here, and the
resulting deposit is known as a delta. The term delta was first
used for these deposits by Herodotus in the 5th century
B.C.E., for the triangular-shaped alluvial deposits at the
mouth of the Nile River. The stream first drops the coarsest
material, then progressively finer material further out, forming
a distinctive sedimentary deposit. In a study of several
small deltas in ancient Lake Bonneville, Grove Karl Gilbert in
1890 recognized that the deposition of finer-grained material
further away from the shoreline also resulted in a distinctive
vertical sequence in delta deposits. The resulting foreset layer
is thus graded from coarse nearshore to fine offshore. The
bottomset layer consists of the finest material, deposited far
out. As this material continues to build outward, the stream
must extend its length and forms new deposits, known as
topset layers, on top of all this. Topset beds may include a
variety of sub-environments, both subaqueous and subaerial,
formed as the delta progrades seaward.
Most of the world’s large rivers such as the Mississippi,
the Nile, and the Ganges, have built enormous deltas at their
mouths, yet all of these are different in detail. Deltas may
have various shapes and sizes or may even be completely
removed, depending on the relative amounts of sediment
deposited by the stream, the erosive power of waves and
tides, the climate, and the tectonic stability of the coastal
region. The distributaries and main channel of the rivers
forming deltas typically move to find the shortest route to the
sea, resulting in the shifting of the active locus of deposition
on deltas. Inactive areas, which may form lobes or just parts
of the delta, typically subside and are reworked by tidal currents
and waves. High-constructive deltas form where the fluvial
transport dominates the energy balance on the delta.
These deltas are typically elongate, such as the modern delta
at the mouth of the Mississippi, which has the shape of a
bird’s foot, or they may be lobate in shape, such as the older
Holocene lobes of the Mississippi that have now largely subsided
below sea level.
High-destructive deltas form where the tidal and wave
energy is high and much of the fluvial sediment gets
reworked before it is finally deposited. In wave-dominated
high-destructive deltas, sediment typically accumulates as
arcuate barriers near the mouth of the river. Examples of
wave-dominated deltas include the Nile and the Rhone
deltas. In tide-dominated high-destructive deltas, tides rework
the sediment into linear bars that radiate from the mouth of
the river, with sands on the outer part of the delta sheltering a
lower-energy area of mud and silt deposition inland from the
segmented bars. Examples of tide-dominated deltas include
the Ganges, and Kikari and Fly River deltas in the Gulf of
Papua, New Guinea. Other rivers drain into the sea in places
where the tidal and wave current is so strong that these systems
completely overwhelm the fluvial deposition, removing
most of the delta. The Orinoco River in South America has
had its sediment deposits transported southward along the
South American coast, with no real delta formed at the
mouth of the river.
Where a coarse sediment load of an alluvial fan dumps
its load in a delta, the deposit is known as a fan-delta. Braiddeltas
are formed when braided streams meet local base level
and deposit their coarse-grained load.
Deltas create unique and diverse environments where
freshwater and saltwater ecosystems meet, and swamps,
beaches, and shallow marine settings are highly varied. Deltas
also form some of the world’s greatest hydrocarbon fields, as
the muds and carbonates make good source rocks and the
sands make excellent trap rocks.
See also CONTINENTAL MARGIN.
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