Terrestrial sedimentary environments III - deltas


The interface between the stream and marine environments.

Until now we have focused on sedimentation in aggrading environments, where tectonic forces or changes in base level allowed new sediments to be deposited on top of old. A delta, in contrast, is primarily prograding - building sediments outward laterally into a basin.

Deltas form when sediment is deposited at river mouths faster than marine processes can remove it. This happens because, streams empty into a standing body of water, flow velocity decreases and the flow quickly drops its sedimentary load, starting with bed load, but eventually including suspended load.

Deltas are rare on active margins, but commonly found on passive continental margins.

The simple schematic at right shows their most important distinguishing features:

The deltaic environment breaks down into three regions:

Because the delta progrades, these environments are superimposed on top of one another.

Deltas in map view:

As a channel current reaches the ocean and loses confinement, it spreads across the surface of the salt water as a tongue-shaped plume. This plume experiences friction with sea water at its edges, so particles drop from suspension at the sides, continuing the river's subaerial natural levees as subaqueous levees.

Bed load, however, is deposited in the thalweg of the channel mouth where the fresh water plume separates from the bottom, as a channel mouth bar. The presence of the channel mouth bar divides the thalweg into two distributary channels. As the delta progrades, these channels lengthen. The channel mouth bar is perched at the top of the delta front. Failures of the front sediments can result in turbitity currents and slumps down its face. Because of current action and rapidly changing salinity, distributary channels are challenging to organisms.

Interdistributary bays: The spaces between distributary channels begin as pieces of open ocean that aggrade through the deposition of overbank muds and crevasse splays into interdistributary marshes and flood plains. These are typically sites of great biological activity, thus, many of the world's major coal deposits are from deltaic environments.

Distributary abandonment: Differences between distributary channels may prompt channels to avulse, abandoning some distributaries in favor of others (generally shorter ones.) Thus, a complication of delta stratigraphy is their tendency to abandon distributaries as may happen with the Mississippi and Atchafalaya.

In the rock record: The schematic at right contrasts the lithologies of deltaic facies. Remember This shows a cross section along a distributary channel. In a cross section through an interdistributary bay fines muds would be more prevalent. In the real world, interdistributary bay and distributary deposits are superimposed on one another as deltas develop and abandon their distributaries.

Factors controlling delta morphology in map view:

Typically, current is not the only thing influencing delta morphology. Tides and waves also do. the spectrum of delta morphology, therefore, has three end members:

Diagnostic features Deltas: