3.deltaic environment

7/21/2019 3.Deltaic Environment

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Contents

Introduction Sedimentology concepts

Fluvial environments

Deltaic environments

Coastal environments

Offshore marine environments

Sea-level change Sequence stratigraphy concepts

Marine sequence stratigraphy

Nonmarine sequence stratigraphy

Basin and reservoir modeling Reflection


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Deltaic environments are gradational to both fluvial and coastalenvironments

The density relationship between sediment-laden inflowingwater and the receiving, standing water body varies

Hyperpycnal: inflowing water has a higher density than basin

water, leading to inertia-dominated density currents Hypopycnal: inflowing water has a lower density than basin water

(buoyancy), leading to separation of bed load and suspended load

Deltas consist of a subaerial delta plain, and a subaqueousdelta front and prodelta

The delta slope is commonly 1-2 and consists of finer (usuallysilty) facies; the most distal prodelta is dominated by even finersediment


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Delta plain

Delta plains are commonly characterized by distributaries andinterdistributary areas

The upper delta plain is gradational with floodplains, lacksmarine influence and typically has large flood basins, commonlywith freshwater peats and lacustrine deposits

The lower delta plain is marine influenced (e.g., tides, salt-waterintrusion) and contains brackish to saline interdistributary bays(e.g., shallow lagoons, salt marshes, mangroves, tidal flats)

Interdistributary areas commonly change from freshwaterthrough brackish to saline environments in a downdip direction(e.g., transition from swamps to marshes)

Minor (secondary) deltas commonly form when distributaries

enter lakes or lagoons


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Delta plain

Delta plains are commonly characterized by distributaries andinterdistributary areas

The upper delta plain is gradational with floodplains, lacks

marine influence and typically has large flood basins, commonlywith freshwater peats and lacustrine deposits

The lower delta plain is marine influenced (e.g., tides, salt-waterintrusion) and contains brackish to saline interdistributary bays(e.g., shallow lagoons, salt marshes, mangroves, tidal flats)

Interdistributary areas commonly change from freshwaterthrough brackish to saline environments in a downdip direction(e.g., transition from swamps to marshes)

Minor (secondary) deltas commonly form when distributariesenter lakes or lagoons


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Delta plain

Distributaries are to a large extent comparable to fluvialchannels, but are commonly at the low-energy end of the

spectrum (meandering to straight/anastomosing) Delta plain distributaries are usually characterized by narrow

natural levees and numerous crevasse splays

Avulsion (i.e., delta-lobe switching) is frequent due to high

subsidence rates, as well as rapid gradient reduction associatedwith channel progradation


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Delta plain

In humid climates, delta plains may have an important organiccomponent (peat that ultimately forms coal)

Hydrosere: vertical succession of organic deposits due to thetransition from a limnic, through a telmatic, to a terrestrialenvironment

Terrestrialization (= hydrosere): gyttja --> fen peat --> woodpeat --> moss peat (commonly a transition from a

minerotrophic to an ombrotrophic environment) Paludification (= reversed hydrosere) is caused by a rise ofthe (ground)water table

Peats are essentially the downdip cousins of paleosols,representing prolonged periods of limited clastic sediment influx


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Delta plain

In humid climates, delta plains may have an important organiccomponent (peat that ultimately forms coal)

Hydrosere: vertical succession of organic deposits due to the

transition from a limnic, through a telmatic, to a terrestrialenvironment

Terrestrialization (= hydrosere): gyttja --> fen peat --> woodpeat --> moss peat (commonly a transition from a

minerotrophic to an ombrotrophic environment) Paludification (= reversed hydrosere) is caused by a rise ofthe (ground)water table

Peats are essentially the downdip cousins of paleosols,representing prolonged periods of limited clastic sediment influx


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Delta front and prodelta

Mouth bars form at the upper edge of the delta front, at themouth of distributaries (particularly in hypopycnal flows); they

are mostly sandy and tend to coarsen upwards Wave action can play an important role in winnowing andreworking of mouth-bar deposits; this may lead to merging withprograding beach ridges and if wave action is very importantmouth bars are entirely transformed

The prodelta is the distal end outside wave or tide influencewhere muds accumulate, commonly with limited bioturbation


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

River-dominated deltas occur in microtidal settings with limitedwave 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 barsthat are reworked into elongate sand bodies perpendicular to the

shoreline


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The typical progradational delta succession exhibits a

transition from prodelta offshore muds through silty to sandy(mouth bar) deposits (coarsening-upward succession), the lattercommonly with small-scale (climbing) cross stratification andoverlain by: Distributary channel deposits (sometimes tidal channel deposits)

with larger scale sedimentary structures

Subaqueous levees grading upward into interdistributary sediments

Transgression occurs upon delta-lobe switching, leading to:

Intense wave reworking and transformation of mouth bar/beach

ridge sands into barrier islands Drowning of barrier islands leading to offshore sand shoals

Increasing salinity and eventual drowning of (part of) the deltaplain


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The typical progradational delta succession exhibits a

transition from prodelta offshore muds through silty to sandy(mouth bar) deposits (coarsening-upward succession), the lattercommonly with small-scale (climbing) cross stratification andoverlain by: Distributary channel deposits (sometimes tidal channel deposits)

with larger scale sedimentary structures Subaqueous levees grading upward into interdistributary sediments

Transgression occurs upon delta-lobe switching, leading to:

Intense wave reworking and transformation of mouth bar/beach

ridge sands into barrier islands Drowning of barrier islands leading to offshore sand shoals

Increasing salinity and eventual drowning of (part of) the deltaplain


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Shallow-water deltas are thinner but larger in area than their

deep-water counterparts

Deformation processes are very common in deltas due to thehigh sediment rates and associated high pore-fluid pressures

Growth faults result from downdip increasing sedimentationrates; they develop contemporaneously with sedimentation

Mud diapirs may form when thick prodelta deposits are coveredby mouth-bar sands

Slumping can lead to the anomalous occurrence of shallow-waterfacies in prodelta deposits


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Other projects at NAFSIG


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Evolution of Western Ganges-Brahmaputra delta plain


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Sarkar et al., QSR, 2009

3.deltaic environment

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