User:Shadyae/sandbox/Wave-Dominated Deltas

New article name goes here new article content ...  Introduction Deltas are the depositional products resulting from the sudden energy decrease where a river drains into a larger basin. This energy decrease causes the river to drop its sediment load as it enters the basin, where the sediment fans out into a triangular-shaped deposit. Wave-dominated deltas are coastal environments where wave energy dominates sediment transportation and deposition. Wave energy is supplied by wind patterns and longshore current. These systems are dominated by sand and grain size fines seaward. The geomorphology and sedimentology of wave-dominated deltas differ from that of tidal or river-dominated deltas. Evolution of these depositional systems are controls of sediment supply, bathymetry (or water depth), and sea level. A wave-dominated delta is divided into two major sub-environments: the delta plain and the delta-front.

Geomorphology Geomorphology pertains to the local variables that control formation of geologic features existing within a particular depositional system. In a delta, the ratio of sedimentation rate to sediment reworking are the major variables controlling the stratigraphy and morphology. Stratigraphy and morphology of a delta are dependent upon how the sedimentation rate of the system measures against the amount of sediment reworking by energy sources. Sediment supply and energy levels vary across the system. Wave-dominated deltas typically have an arcuate geometry, although lobate geometries are also common along non-linear coastlines. The two major sub-environments of a deltaic systems consist of a subaerial component called the delta plain and a subaqueous component called the delta-front. The delta plain, also known as the delta-top, is the subaerial, proximal portion of the delta characterized by meandering distributary channels and bars. Deposition is bedload-dominated and grain size is coarse sand, which gradually fines toward the delta-front. The delta-front is the subaqueous, distal portion of the delta dominated by gravity-driven slumps and characterized by the delta slope and prodelta. The slope consists of sets of clinoforms, which prograde seaward during regressive periods. During transgression, a transgressive lag is deposited unconformably above the clinoforms, creating a wave ravinement surface in which the clinoform topset beds are truncated into .. The prodelta is comprised of the finest grains. The geomorphology of a wave-dominated shoreline influences how wave energy is transmitted to the delta system. A steeper continental shelf or delta slope forms larger and more energetic waves. In these settings, longshore current produces bars and troughs on the upper shoreface. In more gently-dipping slope settings, wave energy is lower, resulting in more mud deposition and promoting growth of wetland vegetation, such as marshes. Modern Wave-Dominated Deltas •	The Nile River Delta •	The Rhone Delta •	The Sao Francisco Delta

Depositional Facies and Depositional Model The facies are characterized by the flow regime and sediment transportation mode in which they were deposited. Because depositional processes vary across the system as a function of sediment supply and energy sources, distinct facies are produced in each delta sub-environment, , ,.  Facies Fluvial Channel: lenticular bedded sandstones record multiple flow directions. Trough cross-stratified sandstones with fine to coarse grained grading are produced by unidirectional currents. Cross-stratified, fine to medium grained sandstones are deposited by bidirectional currents, and can include mud drapes and reactivation surfaces. Distributary mouth bars: deposition is dominated by fine to medium grained sandstone. Current ripples, trough cross-stratification, and plane-parallel laminated beds are reflective of a unidirectional current, consistent with fluvial deposition of mouth bars. Delta-front and distal mouth bar: deposits are dominated by turbidity currents caused by gravity-driven slumps along the delta slope. Turbidite sequences exhibit a characteristic fining-upward sequence. Common sedimentary structures associated with turbidity flows are current ripples, climing-ripple laminations, flute marks, and sole marks. The delta-slope is characterized by silt-sized grains, arranged in sea-ward dipping clinoforms, grading basinward into the mud-dominated prodelta.

Models There is no cut and dry model for wave-dominated deltas: facies associations will differ from system to system depending on influence of tidal or fluvial energy, coastal geomorphology, and relative sea level. The stratigraphy of a regressive wave-dominated system will typically exhibit a coarsening-upward succession, while transgressive systems are dominated by a fining-upward succession, as the finer delta-front deposits migrate landward. In some transgressive systems where sediment reworking exceeds sedimentation rate, estuaries can form.

Controls on Depositional System Evolution Sea Level During marine regression, sedimentation exceeds the rate of basin subsidence. The delta plain becomes more exposed as base level is lowered, resulting in an erosional surface or ravinement in the stratigraphy .. Subsequently, deltaic channels are filled with sediment and induce a seaward progradation, or build out, of the delta-front. Relative increase in fluvial energy associated with marine regression results in formation of an asymmetrical delta. The fluvial channel builds out seaward, creating a mouth-bar that protrudes off a lobate coastline. The mouth-bar is an impedance, deflecting littoral currents to transport more sediment to the updrift side of the delta. As a result, the updrift shoreline is characterized by strandplains and beach ridges, deposited parallel to the shoreline. Alternatively, the downdrift side of the delta is relatively sediment-starved, where features such as spits and barriers are formed, enclosing brackish, wave-sheltered water bodies such as lagoons or embayments. In general, tidal energy tends to have a greater influence on transgressive coastlines opposed to regressive coastlines. Symmetrical deltas form along wave-dominated, transgressive shorelines. When sediments are transported from the fluvial channel, littoral currents rework this sediment, evenly depositing it on either side of the delta-mouth and creating a relatively linear shoreline. Wave-energy combined with a low tidal range provide conditions conducive for formation of barrier islands and lagoons during marine transgressional periods, which continuously migrate shoreward With increasing tidal energy in a transgressive wave-dominated deltaic system, the coast becomes embayed with a wave-dominated estuary. If the tidal energy increases further, the delta becomes tide-dominated and the delta plain is inundated with water faster than it can channelize as the shoreline migrates landward, forming an estuary.

Tectonics and Bathymetry When subsidence exceeds sediment influx, the prodelta platform becomes depressed and deepened, enhancing the reworking of sediment by littoral currents. These settings influence the development of lagoons, and even estuaries if tidal energy is sufficient.