DELTA

Delta Facies

What is a delta?A delta, as defined by the USGS (U.S. Geological Survey), is the fan-shaped area at the mouth, or lower end, of a river, formed by eroded material that has been carried downstream and dropped in quantities that can not be carried off by tides or currents. Boggs defines a delta as any deposit, subaerial or subaqueous, formed by fluvial sediments that build into a standing body of water.

Where are deltas found?Deltas can occur in lakes and inland seas as well as in the ocean, but they are most important in the open ocean (Boggs). Deltas are particularly common in the modern ocean owing to post-Pleistocene sea-level rise coupled with high sediment loads carried by many rivers. They often contain extensive wetlands, whose high biological productivity makes them vital nursery grounds for fisheries. Huge amounts of coal, oil, and natural gas are found in subsurface ancient deltaic deposits. Deltas seldom form on active, subducting continental margins because there is no stable shallow shelf on which sediments can accumulate. Twenty-five (25) of the worlds largest deltas are found on passive margins of continents. They are categorized according to features within the delta plain and by energies acting on the delta (tide, wave, and fluvial). The size of a delta is dependent on the size of the drainage basin, the elevation of the land, and climate.

What does a typical delta look like?

What are the different types of deltas?

Tide-Dominated Deltas

The shape of a delta is influenced by sediment input, wave energy, and tidal energy. Deltas which undergo strong tidal interaction are classified as tide-dominated deltas. As sediment travels out of the delta into the sea, high tides and flood tides confine sediment on the delta

plain and low tides carry sediment seaward. Tide-dominated deltas typically occur in locations of large tidal ranges or high tidal current speeds. In situations such as these, where

the sediment supply is over powered by strong tidal currents, the delta tends to be very small. Another resulting feature of a tide-dominated delta is that it has many linear structures

parallel to the tidal flow and perpendicular to the shore (shown below).

Tide-Dominated Deltas

These tidal sand ridges are characteristic of the subaqueous delta plain. They may reach relief heights of up to 15-20 meters close to the shoreline. Relief of the ridges decrease with increasing distances from the distributaries. Ridges are also greater in relief and size when there is more tidal discharge than fluvial discharge. Sediments that make up the sand ridges are fine to medium sands. Characteristic of the lower delta plain are broad tidal flats that grade unidirectionally into the sand bars and the opposite direction into marshes or evaporative flats (depending on climate).

The delta has a gradational, upward coarsening sequence of muds; interbedded sands, silts, and muds; and then fine to medium sands. Due to the eroding effect of tides, the sands display cross-bedding, the sand ridge field can be truncated through erosion, and the

distributary channel sediments contain more slack-water mud drapes than usual.

Wave-Dominated Deltas

An open ocean basin accepts more water input, which means there is potential for greater wave energy, making wave-dominated deltas more likely. High wave interference causes conflicted or deflected river mouths.

There is less influence from fluvial sources. In wave-dominated delta regions, breaking waves cause immediate mixing of fresh and salt water. Typically, the fresh water flow velocity decelerates rapidly. A bar may form in the immediate vicinity of the distributarymouth, often supplemented by landward migrating swash bars. The wave action reworks the sediment, making it much sandier than other types of deltas, coarsening upward. Sediment is delivered by the river and is transported along the coast. The sediment is then deposited as beaches and bars and the development of distributaries is limited

Wave-Dominated Deltas

Geometry:

1. More lobate and have smooth, arcuate to sharp margins. 2. Longshore drift may rework sediment along coastline. 3. Lobes strung out parallel to shoreline.

Controls on the Geometry:

1. substrate gradient 2. wave energy vs. tidal energy 3. sediment supply vs. accommodation volume: supply-dominated areas

produce allochthonous shelves characterized by more rapid sedimentation and thick muddy sequences in which barrier sand bodies maybe encased while accommodation-dominated areas are associated with allochthonous shelves that have sandy sediments and experience extensive reworking.

4. rates of sea level change

Fluvial-Dominated Deltas

Fluvially-dominated deltas are primarily controlled by the water density difference between the inflowing river water and the standing water on the basin.

Different flow types that determine the distribution of sediment and sedimentary structures formed in the delta are homopycnal flow, hyperpycnal flow, and hypopycnal flow

Homopycnal flow occurs when the density of the river water is equal to the density of the standing water in the basin.. Hyperpycnal flow is produced when the density of the river water entering the basin is greater than the density of the standing water in the ocean basin. This higher density river water will flow below the standing water in the basin because of the difference in density. Hypopycnal flow is associated with a lower river water density entering a higher density standing water in the basin. Under these conditions, the river water will flow out over the standing water, gradually depositing the suspended clay portion of the sediment load on the prodelta.