Terrestrial sedimentary environments III - deltas
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 are rare on active margins, but commonly found on passive continental margins.
The simple schematic at right shows their most important distinguishing features:
- Delta deposits prograde or advance their edges into the ocean.
- Because the coarsest sediments are deposited closest to the river mouth, and the finest ones farthest away, the general stratigraphy of a delta environment shows a coarsening upward sequence. Why is that?
The deltaic environment breaks down into three regions:
- The Delta Plain: The terrestrial environment where river meets ocean. It differs from fluvial floodplains and channels considered previously because it develops on top of previous deltaic deposits. In it, we see:
- Distributary channels
- Flood plains
- Interdistributary marshes and swamps
- Beach complexes
- The Delta Front: The steeply sloping portion of the delta where deltaic deposits slope downward from sea level to the level of the sea floor. Characterized by:
- Thinly bedded turbidites
- Slumps, slides and convolute bedding
- Grain size grades from sand and silt near sea level to fine clay farther out. During floods, however, sand may extend farther down slope.
- Prodelta: fine grained distal mudstones on the edge of the delta furthest from shore. Usually finely laminated
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.
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:
Mississippi delta from NAA Earth Observatory
- Setting: At passive continental margins, adjacent to fluvial and shallow marine deposits.
- Geometry: Roughly triangular in map view and wedge shaped in cross section. Tens to thousands of km2 and tens to thousands of meters thick.
- Typical sequence: Coarsening upward from laminated prodelta clays to distributary channel and bar sands, interfingered with interdistributary muds. E.G. Lourinha, Portugal
- Fossils: Marine organisms in prodelta and delta front deposits. Coal in interdistributary muds.