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Deltas

AP Dr. David MENIERUniversiti Teknologi PETRONAS

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Delta Sedimentary Environments

Why study ancient and present deltas ? Components of a delta Process of formation of a delta Delta classification Fluvial-dominated delta Mixed fluvial to wave-dominated Tide-dominated delta Wave-dominated delta

http://video.nationalgeographic.com/video/places/regions-places/africa-tc/botswana_okavangodelta/http://www.youtube.com/watch?v=adHiQ6Dn8DI

Why study ancient deltas ? Most described ancient deltaic sequences are river-

dominated systems (that are characterized by major coarsening-upward sequences many 10’s m thick, from marine sediments “perhaps limestones” passing up into increasingly non-marine mudrocks - sandstones and then coals).

The deltaic sediments of the Carboniferous strata in W. Europe and E. USA are good examples contain important coal reserves.

The repetition of deltaic cycles in the Carboniferous may be due to: (a) compaction-induced subsidence, (b) tectonic subsidence, or (c) eustatic sea-level changes.

Deltaic sandstones are major oil reservoirs in some basins e.g. (1) the Middle Jurassic Brent Sands of the North Sea.(2) the Tertiary of the Gulf Coast of Texas and Louisiana.

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http://earthobservatory.nasa.gov/blogs/earthmatters/category/uncategorized/page/5/

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Northeast Holden Crater on Mars—10-m resolution DEM of the Eberswalde Delta.Reds are topographic highs, and darker colors are topographic lows. The delta relief is about 60/80 m.To assist in interpretation, this image was generated by hand contouring of color satellite images guided by low-resolution altimetry dataand higher-resolution altimetry of Aharonson and Lewis (CalTech).

Deltas of the SE Asia, modified after Woodroffe 2000

From Ali Wahid, PhD

BORNEO

0 600

kilometres

Sulu Sea

Sandakan Delta

Rajang Delta

Tarakan Delta

Mahakam Delta

Baram Delta

Sabah Delta

Balingian Delta

Barito Delta

Deltas around Borneo island

Deltas of the SE Asia, modified after Woodroffe 2000

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From Ali Wahid, PhD

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Components of a delta modified from Coleman & Wright, (1971)

An active delta consists of one or more river-mouth systems to which prodelta and delta front deposition are directly coupled, a distrubutary network, interdistributary and distrubutary-margin deposits, and a delta shoreline

Subaqueous component

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Deltaic environments

Deltaic environments

Deltas form where a river enters into a standing body of water (ocean, sea, lake) and forms a thick deposit that may or may not form protuberances

The delta plain is the subaerial part of a delta (gradational upstream to a floodplain); the delta front (delta slope and prodelta) is the subaqueous component

Delta plains are commonly characterized by distributaries and flood basins (upper delta plain) or interdistributary bays (lower delta plain), as well as numerous crevasses plays

Upper delta plains contain facies assemblages that are very similar to fluvial settings

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Process of formation of a delta

Deltas develop where river systems debouch into the ocean, inland seas and lakes.

Their form is controlled by a number of factors, chief of which is the relative effectiveness of river discharge compared to the tidal and wave energies of the receiving basin.

- Sediment input- Wave energy-Tidal energy

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Process of formation of a delta

When a stream flows into a large standing body of water, such as a lake, the stream water quickly loses velocity and the heaviest particles drop out, forming a coarse, steeply sloping layer.

Most of the fine suspended load is carried farther out, eventually settling out to form a gently sloping, triangular deposit.

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Delta classification based on the dominant process

Schematic illustration of the construction of a delta body

Bottomset beds Foreset beds topset beds

Deltagrowth

Sea level

According to classical concept, a delta is made up of topset, foreset and bottomset deposits

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Seaward progradation of a depositional environments after Scruton, (1960).

During the grouwth of a delta, different environments migrate toward the sea

Seaward

Marsh

Delta frontSilt & Sand

Prodelta silty clay

Offshore clay

Original depositional surface

Time lines

Depositional surface

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3D View : Carboniferous Delta (U.S.A), in Elliot & Reading., 1986.

Channels

Active prograding delta

Abandoned delta

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Cross section illustrating sedimentary facies of Deltaic and Front delta

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Delta top facies to Marine shelf deposits

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Deposits geometry in function of depth basins

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Sketch graphic log and interpretation of typical sequence produced by delta prograding

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Fluvial-Dominated bird foot delta

Mississippi River (U.S.A)

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Fluvial-Dominated bird foot delta

Levees

Delta(distribury)

channel

Inter-distributary bay

River channel

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Tide-dominated delta

Norman River (Australia)

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Physical characteristics of a tide-dominated delta

Tide-dominated deltas are characterised by relatively high tide energy at the mouth compared with wave energy, and are distinguished from tide-dominated estuaries by high river energy.

Tidal energy is greatest slightly landward of the mouth due to constriction by the funnel shaped mouth.

Wave energy is dissipated on shoals seaward of the mouth, and declines rapidly landwards.

River energy remains moderate to high along the channel, but drops off significantly as the channel widens towards the mouth.

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Wave-dominated delta (cuspate)

Nil River (Egypt)

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Physical characteristics of wave-dominated deltas

Wave-dominated deltas are characterised by relative high wave energy at the mouth compared to tide energy, and are distinguished from wave-dominated estuaries by high river energy.

Total energy at the mouth is high because of high wave energy at the coast.

Total energy declines immediately landward of the mouth because wave energy is dissipated on the barrier. The dominance of river energy further landward means total energy is relatively high along the channel.

Maximum tidal energy occurs in the constricted inlet mouth.

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Mixed Tide-Dominated delta

Mahakam delta (Indonesia)

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A schematic representation of pre-Holocene and Holocene Mahakam stratigraphy (modified after Roberts and Sydow, 2003) with MISEDOR core (Carbonel and Moyes, 1987) location and main time and depth boundaries of important Pleistocene and Holocene surfaces. In the offshore zone several carbonate bioherm structures are situated, which are remains of the late Pleistocene–early Holocene transgressive phase.

From Storm et al; (2005). Sedimentary geology

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Fluvial dominatedbird-foot delta (elongate)

Mixed Fluvial dominated« classical » delta +/- tidal influence

Wave-dominated delta (cupside) Tide-dominated delta

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Mississippi River

http://www.pnronline.com.au/article.php/108/1963

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Conclusion : Delta morphology

Deltaic environments

Delta morphology reflects the relative importance of fluvial, tidal, and wave processes, as well as gradient and sediment supply

• River-dominated deltas occur in microtidal settings with limited wave energy, where delta-lobe progradation is significant and redistribution of mouth bars is limited

• Wave-dominated deltas are characterized by mouth bars reworked into shore-parallel sand bodies and beaches

• Tide-dominated deltas exhibit tidal mudflats and mouth bars that are reworked into elongate sand bodies perpendicular to the shoreline

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Deltaic environments

Mouth bars form at the upper edge of the delta front, at the mouth of distributaries; they are mostly sandy and tend to coarsen upwards

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

Progradation (basinward building) of deltas leads to coarsening-upward successions, and progradation rates depend on sediment supply and basin bathymetry (water depth)

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Deltaic environments

Deltaic environments

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

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

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Thank you for the attention