Deltaic Systems

Deltaic Systems

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Deltas are highly complex systems that form where a river enters a standing body of water and include a mixture of both fluvial and marine processes. Nearly all the facies associated with meandering and anastomosed river systems and with beach, barrier island, and tidal systems can be found within deltas.

Coastal Depositional Systems

A variety of depositional systems are found along a shoreline. Their distribution, geometry, and extent are determined by proximity to a sediment source, sediment abundance, energy conditions, and fluctuations in base-level.

This lecture focuses on deltaic and estuarine systems, which are produced during regressive and transgressive conditions, respectively.

Isopach PatternIsopach Pattern

Deltas can be recognized in part by a thick, relatively restricted accumulation of terrigenous sediment that interfingers with fluvial deposits in a landward direction and marine systems toward the basin.

Channel BifurcationChannel Bifurcation

As fluvial flow enters a standing body of water, it rapidly loses velocity, decreasing its capacity to transport sediment. The coarsest material is deposited in mid-channel, forming a channel-mouth bar. Flow diverts around the bar, depositing additional sediment in the form of channel margin bars or levees. The process repeats for increasingly numerous, but smaller, channels as the delta advances basinward.

Photo by W. W. Little

Photo by W. W. Little

Splay DevelopmentSplay Development

Splays are particularly common in deltaic settings, forming as smaller deltas within flooded areas adjacent to major channels. Splay growth is the primary process for filling the space between distributary channels in fluvially-dominated deltas.

Homopycnal FlowHomopycnal Flow

If stream flow density is equal to that of the basin, the two mix thoroughly, leading to rapid deposition of both coarse- and fine-grained sediment, producing Gilbert-type deltas. This is typical of delta-building into freshwater lakes.

Gilbert-type Deltas

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Gilbert-type deltas are formed under conditions of homopycnal flow and consist of three major components: bottomset beds of mud deposited by suspension in advance of the delta; foreset beds composed of sand and gravel forming the delta front; and fluvial topset deposits of gravel, sand, and mud.

Hyperpycnal FlowHyperpycnal Flow

If stream flow density is greater than that of the basin, the flow remains in contact with the basin floor, eroding the previous surface and forming turbidite-like deposits. This can occur where cold, sediment-laden water flows into a warm, clear lake or ocean.

Hypopycnal FlowHypopycnal Flow

If stream flow density is less than that of the basin, after dropping the coarse load fraction, fine sediment spreads like a blanket across the sea surface, slowly settling to the sea floor. This is the typical condition for the generation of most marine deltas.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Simplified Models

Deltaic systems are typically subdivided into three major facies associations: subaerial plain (fluvial), delta front (shoreface & tidal), and prodelta (marine).

Basinward Progradation

Photo by W. W. Little

Delta plain (delta top, topset beds): subaerial portion of the delta• Upper Delta Plain (above high tide)

– Fluvial (mostly meandering) channels– Splays– Swamps and marshes

• Lower Delta Plain (between low and high tides)– Tidal deposits– Distributary channels and levees– Interdistributary splays

Delta front (delta margin, foreset beds): coastal portion of the delta• Focus of active deltaic progradation)• Distributary-mouth bars• Beaches and barrier islands• Tidal bars• Bays

Prodelta (delta front, bottomset beds): deep water portion of delta• Below wave/tide base• Quiet water deposition by suspension settling• Turbidity flows• Slumping

Deltaic Sub-systems

Photo by W. W. Little

Delta Classification Delta Classification (morphology)(morphology)

Delta front morphology is determined by a balance between fluvial input and reworking by waves and tides. The relative contribution of these three factors leads to classification as river-, wave-, or tide-dominated.

Constructional

Destructional

River-dominated Delta River-dominated Delta MorphologyMorphology

River-dominated deltas have irregular shorelines that extend significantly away from the general shoreline into the basin. In some cases, distributaries will prograde as finger-like extensions. Conditions that favor river-dominated deltas include high fluvial discharge and sediment load, low wave and tide activity, and a shallow basin.

River-dominated Deltaic Facies River-dominated Deltaic Facies ModelModel

River-dominated deltaic facies coarsen-upward from offshore and prodelta mud through distributary and beach or barrier sand to mixed fluvial and splay deposits.

Wave-dominated Delta Wave-dominated Delta MorphologyMorphology

Wave-dominated deltas have relatively straight shorelines that extend slightly to moderately away from the general shoreline into the basin. In some cases, distributaries are mostly restricted to the major delta plain, and the delta front is dominated by beach ride progradation. Conditions that favor wave-dominated deltas include low fluvial discharge and sediment load, high wave and low tide activity, and a deep basin.

Arno River Delta

Wave-dominated Deltaic Wave-dominated Deltaic Facies ModelFacies Model

Wave-dominated deltaic facies coarsen-upward from offshore and prodelta mud through distributary and beach or barrier sand mixed with lagoonal, washover, and tidal deposits.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Tide-dominated Delta Tide-dominated Delta MorphologyMorphology

Tide-dominated deltas form highly irregular shorelines that extend slightly to moderately away from the general shoreline into the basin. Distributaries tend to be numerous, wide, irregular in shape, and cover most of the delta plain. The delta front is dominated by tidal bars oriented perpendicular to the shoreline. Conditions that favor tide-dominated deltas include low fluvial discharge and sediment load, high tide, low to moderate wave activity, an embayed coast, and a shallow basin.

Photo by W. W. Little

Photo by W. W. Little

Tide-dominated Deltaic Facies Tide-dominated Deltaic Facies ModelModel

Tide-dominated deltaic facies coarsen-upward from offshore and prodelta mud to sandy distributary and tidal bar deposits.

River-dominatedRiver-dominated/tidally-influenced Tide-dominated/wave-influenced

Wave-dominated Wave-dominated/river-influenced Wave-dominated/tide-influenced

Hybrid Delta MorphologyHybrid Delta Morphology

Most deltas show a mixture of fluvial, wave, and tide dominance.

Storm ImpactStorm Impact

Large storms, such as hurricanes can greatly impact delta front depositonal patterns, producing diastems between aggradational packages.

Delta Lobe SwitchingDelta Lobe Switching

As distributaries prograde basinward, channel gradient decreases. Once the gradient reaches zero, the stream can no longer flow. The typical response of a distributary is to avulse and find a shorter, steeper route to the sea, shifting the focus of deposition and abandoning one delta “lobe” to begin another. The abandoned lobe continues to compact and subside and to be reworked by wave and tidal forces. Therefore, both transgression and regression occur simultaneously in adjacent parts of the same delta.

Estuarine SystemsEstuarine systems are deltaic systems that form within drowned river valleys during the latter stages of marine transgression into early stages of regression. They are smaller in scale than deltas and tend to be dominated by a mixture of fluvial and tidal processes.

GEOL 553 - Introduction to Facies Models - Kendall

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little