Deltas


Deltaic environments are gradational to both fluvial and coastal environments. Examples of deltas on a variety of scales can be seen here and a comprehensive website resource on modern deltas can be accessed here.

Deltas form where a river enters a standing body of water (ocean, sea, lake) and forms a thick deposit that may or may not form protuberances. The sudden decrease in energy causes the river to drop its sediment load. Deltaic deposits therefore become finer grained the farther out into the lake or ocean (distal edge). Across the delta, they are coarsest in the distributary channels and finest away from the channels. Many deltaic deposits resemble lake or shallow marine deposits at their distal margins and fluvial deposits at their proximal margins. Deltas consist of a subaerial delta plain or delta-top (gradational upstream to a floodplain, and a subaqueous delta front (delta-slope and prodelta (Fig.1).

Delta plains are commonly characterized by distributaries and flood basins (upper delta plain) or interdistributary bays (lower delta plain), as well as numerous crevasse splays. Upper delta plains contain facies assemblages that are very similar to fluvial settings.

The delta slope is commonly 1-2° and consists of finer (usually silty) facies; the most distal prodelta is dominated by even finer sediment.

Fig.1

 


The density relationship between sediment-laden inflowing water and the receiving, standing water body varies and this difference in density influences the form of the delta and the sediment distribution.

Hyperpycnal : inflowing water has a higher density than basin water, leading to inertia-dominated density currents.
Hypopycnal : inflowing water has a lower density than basin water (buoyancy), leading to separation of bed load and suspended load

 


Deltaic Environments

Delta Plain

Delta plains are commonly characterized by distributaries and interdistributary areas. The upper delta plain is gradational with floodplains, lacks marine influence and typically has large flood basins, commonly with freshwater peats and lacustrine deposits. The lower delta plain is marine influenced (e.g., tides, salt-water intrusion) and contains brackish to saline interdistributary bays (e.g., shallow lagoons, salt marshes, mangroves, tidal flats).

 

Upper Delta Plain

 

Lower Delta Plain Examples (4 pictures above)

Interdistributary areas commonly change from freshwater through brackish to saline environments in a downdip direction (e.g., transition from swamps to marshes).

Minor (secondary) deltas commonly form when distributaries enter lakes or lagoons.

 

Lower Delta Plain

Distributaries are to a large extent comparable to fluvial channels, but are commonly at the low-energy end of the spectrum (meandering to straight/anastomosing). Delta plain distributaries are usually characterized by narrow natural levees and numerous crevasse splays. Trees are often restricted to the higher elevations of natural levees (see above).

 

Lower Delta Plain

Avulsion (i.e., delta-lobe switching) is frequent due to high subsidence rates, as well as rapid gradient reduction associated with channel progradation.

 

In humid climates, Delta Plains may have an important organic component (peat that ultimately forms coal). Hydrosere: vertical succession of organic deposits due to the transition from a limnic, through a telmatic, to a terrestrial environment. Paludification (= reversed hydrosere) is caused by a rise of the (ground)water table. Peats are essentially the downdip cousins of paleosols, representing prolonged periods of limited clastic sediment influx.

 


Delta Front and Prodelta

Flooded, abandoned, delta-lobe (above left)

Modern, Mississippi Lobe (above right)

Mouth bars form at the upper edge of the delta front, at the mouth of distributaries (particularly in hypopycnal flows); they are mostly sandy and tend to coarsen upwards.

Wave action can play an important role in winnowing and reworking of mouth-bar deposits; this may lead to merging with prograding beach ridges and if wave action is very important mouth bars are entirely transformed.

The prodelta is the distal end outside wave or tide influence where muds accumulate, commonly with limited bioturbation.

 


Detaic Architecture

Delta cycles are the result of repetitive switching of delta lobes, comparable to avulsion in fluvial environments; this leads to characteristic vertical successions with progradational facies and transgressive facies.

A more detailed explanation of deltaic architectures and the role of sequence stratigraphy can be found here.

Prograding Deltas

Progradation (basinward building) of deltas leads to coarsening-upward successions, and progradation rates depend on sediment supply and basin bathymetry (water depth). The typical progradational delta succession exhibits a transition from prodelta offshore muds through silty to sandy (mouth bar) deposits (coarsening-upward succession), the latter commonly with small-scale (climbing) cross stratification and overlain by:

 
 
Shallow-water deltas are thinner but larger in area than their deep-water counterparts.
 

 


Transgressive Delta Systems

Transgression occurs upon delta-lobe switching, leading to:

 

   

 


Delta Morphology

Delta morphology reflects the relative importance of fluvial, tidal, and wave processes, as well as gradient and sediment supply. The process are generally described as having three end members, although most deltas are influenced by a combination of 2 or more of these processes.

 

River-dominated Deltas Examples of River-dominated Deltas
 

 

Wave-dominated Deltas Examples of Wave-dominated Deltas
 

 

 

Tide-dominated Deltas Examples of Tide-dominated Deltas
 

 

Deltas can be classified according to the dominant process (river, wave, tidal) and also by their average grain size. A number of classical deltas are shown below using this classification system.

 

Coarse-grained deltas are composed of gravelly facies and form where alluvial fans or relatively steep braided rivers enter a water body.


Deformation Processes

Deformation processes are very common in deltas due to the high sediment rates and associated high pore-fluid pressures

Outcrop view of growth-faults
Complex slump-faulting, leading to lateral and vertical heterogeneity in facies (including reservoir continuity).