delta

Delta SystemLecture 6#

Introduction• Why studying deltaic system?• Their sandstones constitute the majority of the producing

reservoirs in siliciclastic basin

Rivision• 1. What type of river systems exist?• 2. What are the characteristics of flood plain can be seen from

the imagery?• 3. Describe lithofacies at Point A

Answer:1. Braided, meandering and anastomosing

2. i. Adjacent to the channelsii. Crevasse splay deposits

3.

Electric log (E-log)

Typical delta look like

Where are deltas found?1. Deltas can occur in lakes and inland seas as well as in the

ocean, but they are most important in the open ocean (Boggs).

2. Deltas are particularly common in the modern ocean owing to post-Pleistocene sea-level rise coupled with high sediment loads carried by many rivers.

3. They often contain extensive wetlands, whose high biological productivity makes them vital nursery grounds for fisheries.

4. 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.

5. Twenty-five of the world’s 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.

MorphologyThree geomorphological elements

1. Delta Plain• Subaerial; Flat swampy plain• Fluvial process generally predominate• Incised by fluvial distributary channels

2. Delta front• Shallow submarine platform• Most sand accumulates (mouth bars)

3. Prodelta• Most distal; Water depth > 10-15 m• Bulk suspension settles out

DistributaryChannels

Prodelta

Distributary Mouth Bar

Aluvial Plain

Delta Plain

Delta Front

Previous Coastline

Floodplain BayLow River

Flow

Levee Crevasse Splay

Flood Level

Types of DeltasMorphology reflects the type and intensity of the hydrodynamic processes1. River-dominated delta• elongated distributary and mouth bar

2. Tide-dominated delta• usually funnel-shaped mouth• mouth bar developed as a series of ridges or isolated

bar

3. Wave-dominated delta• mouth bar reworking parallel to the coastline

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

Fluvial-Dominated Deltas

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.

Fluvial Dominated

Tide-Dominated Deltas- Deltas which undergo strong tidal interaction

- 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.

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1. Heights of up to 15-20 meters close to the shoreline.

2. Ridges are greater in relief and size when there is more tidal discharge than fluvial discharge.

3. The delta has a gradational, upward coarsening sequence of muds; interbedded sands, silts, and muds; and then fine to medium sands

4. Due to the eroding effect of tides, the sands display coss-bedding, the sand ridge field can be truncated through erosion, and the distributary channel sediments contain more slack-water mud drapes than usual.

Characteristic of Tidal Delta

Tide Dominated

Wave-Dominated Deltas

- Open ocean basin accepts more water input, which means there is potential for greater wave energy

- High wave interference causes conflicted or deflected river mouths.

- Less influence from fluvial sources & breaking waves cause immediate mixing of fresh and salt water.

A bar may form in the immediate vicinity of the distributary mouth, 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

lithologies – gravel, sandstone and mudstone

mineralogy – variable, delta-front facies may be compositionally mature

texture – moderately mature in delta-top sands and gravels, mature in wave-reworked delta-front deposits

bed geometry – lens-shaped delta channels, mouthbar lenses variably elongate, prodelta deposits thin bedded

sedimentary structures – cross-bedding and lamination in delta-top and mouth-bar facies

palaeocurrents – topset facies indicate direction of progradation, wave and tidal reworking variable on delta front

fossils – association of terrestrial plants and animals of the delta top with marine fauna of the delta front

colour – not diagnostic, delta-top deposits may be oxidised

facies associations – typically occur overlying shallow- marine facies and overlain by fluvial facies in an overall progradational pattern.

Characteristics of deltaic deposits (conclusion)

Questi ons…