ch. 9: learning objectives · strat & sed, ch. 9 3 fluvial-dominated deltas • river processes...

Strat & Sed, Ch. 9 1

Ch. 9: Learning objectives• Marginal marine environments are the

transitional areas between terrestrial and fully marine realms

• Understand the sedimentary facies of deltas, beaches and barrier islands, estuaries, and tidal flats– types of sediments; sedimentary structures;

lateral and vertical facies associations; and all other characteristics

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Deltas

• Delta = a discrete deposit formed by fluvial sediments building into a standing body of water– River deltas (geologically most significant)

• Fluvial-dominated• Tide-dominated• Wave-dominated

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Fluvial-dominated deltas

• River processes are more dominant than either tidal or wave processes

• Usually characterized by high sediment and water discharge

• Elongate to lobate in plan shape• Muddy bulk composition (interdistributary

areas), but with sandy channel facies, distributary mouth bar facies, and bar front facies

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Fluvial-dominated deltas

Mississippi delta

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Fluvial-dominateddeltas

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Tide-dominated deltas

• Tidal currents may overwhelm river flow in areas with high tidal range

• River mouth sediments may be redistributed by tidal action

• Estuarine to irregular in plan shape• Mixed sediment composition

– Muddy estuarine areas– Sand-filled channels– Sand ridges

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Tide-dominated deltas

Ganges-Brahmaputra delta

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Wave-dominated deltas

• In areas of strong wave activity, river mouth sediments may be reworked and redistributed to form wave-built beaches, barrier bars, spits, beach ridges

• Overall shape is smooth, arcuate to cuspate

• Composition is mainly sand

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Wave-dominateddeltas

São Francisco delta,Brazil

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Delta facies

• Upper delta plain (above high tide)– River channel deposits, lacustrine delta-fill deposits,

floodplain deposits• Lower delta plain (between high- and low tide)

– Interdistributary bay deposits, crevasse splay, natural levee, abandoned distributary deposits

• Delta front (from low tide to ~ 10m subsea)– Distributary mouth bar deposits, pro-delta distal bar

deposits• Prodelta (seaward of subaqueous delta)

– Fine-grained marine sediments

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Delta facies

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Constructional vs. destructional phases

• Constructive phase occurs when input of sediment is sufficient for progradation– Prodelta fine sediment is overlain progressively

by delta-front silts and sands, distributary mouth sands, and upper delta plain deposits

• Destructive phase occurs when a delta lobe is abandoned or during major transgression– Erosion and redistribution of sediments rather

than seaward growth

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Deltaprogradation

Delta plain:Non-marine fluvial todistributary channel tofiner marsh-, lake-, orinterdistributary bay

Delta front:Distributary mouthBar to distal bar tosilts Prodelta:

Marine siltsand muds

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Vertical facies associations

• Delta progradation results in an overall coarsening-upward sequence (usually 50-150m thick) overlain by finer delta plain deposits (2-15m thick)

• Progradational cycles may be stacked, depending on history of lobe abandonment

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Coarsening-upwardsequence

Finer-graineddelta plain deposits

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Delta lobe abandonment

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Beaches and barrier islands

• Beach and barrier island complexes are best developed on wave-dominated coastlines with small tidal range– Beach = linear sand body attached to mainland– Strand-plain = broader beach ridge system

consisting of multiple parallel ridges– Barrier island = linear sand body separated

from mainland by a lagoon or marsh

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Beach-barrierisland anatomy

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Beach morphology

• Beach morphology is same for beaches and seaward coast of barrier islands– Backshore = above high tide level (including

beach dunes)– Foreshore = swash zone between high and low

tide– Shoreface = low tide to fair-weather wave base

(~10-15m)

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Beach morphology

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Wave processes• In breaker zone and surf zone, breaking

waves translate into bi-directional currents– High turbulence throws sediment into

suspension– Net landward transport of bedload and

suspended load (often with a longshore component)

– Repeated surf action produced a winnowed sediment (well sorted, positively skewed)

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Current processes• Longshore currents are caused by

breaking waves that approach the shoreline at an angle– Some translational wave energy is deflected to

produce a unidirection currentshoreline

wave crests

current

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Longshore drift & longshore current

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Beach and barrier island facies

• Overall depositional system is relatively narrow and produces an elongate set of deposits that are parallel to coastline

• Sediment is dominantly sand– 10’s to 100’s meters wide– 10’s to 100’s km long– 10-20 m thick

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Beach and barrier island facies

• Beach deposits are:– Fine to medium grained sand– Well sorted– Subhorizontal, parallel laminations dominant,

but also low-angle landward- and seaward-dipping cross-beds

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Beach and barrier island facies

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Laterally adjacent facies

• Eolian facies may be developed landward of beach

• Other facies include:– Sandy washover deposits from back of barrier island– Coarser tidal channel deposits (bi-directional

paleocurrent)– Fine-grained lagoon and marsh deposits– Tidal flat sands, silts, and muds (often flaser-bedded

and lenticular bedded)– Offshore finer sediments

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Ch. 9 Marginal marine environments (part 2)

• Estuaries & Tidals flats– Know types of sediments; sedimentary

structures; lateral and vertical facies associations; and all other characteristics

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Estuaries

• “Estuary” is an inlet of the sea that reaches up a river valley as far as the upper limit of the tidal range– Interaction of both fluvial and marine systems– Progradation of an estuary may change it into

a delta

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Estuary types

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Estuary environmental setting

• Influence by river, tides, and waves• Salinity may vary within an estuary at any

given time, or seasonally• Proximal parts of estuaries are river-

dominated• Distal parts of estuaries may be:

– Wave-dominated– Tide-dominated

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Wave-dominated estuary

• High wave energy at mouth of estuary– Estuary mouth sand bar– Quiet water in central part of estuary (muds)

• Water is partially mixed to well-stratified– Fresh vs. marine density stratification

• High river energy at head of estuary– Bay-head delta sediments

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Wave-dominatedestuary

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Tide-dominated estuary

• High tidal energy at mouth of estuary, all the way to tidal-fluvial transition– Estuary mouth tidal sand bars– Generally higher overall energy than wave-dominated

estuaries• Water is well mixed

– no density stratification• Ripple and dune bedforms common

– Foresets may dip in both directions

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Tide-dominatedestuary

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Estuarine sedimentary facies

• Cross-bedded, bioturbated sands form near estuary mouth and in fluvial-tidal channels

• Laminated, bioturbated muds form in non-channel middle and upper parts of estuary

• Fauna is typically low diversity, but possibly high abundance– typically dominated by mollusks

• Oysters, mussels, gastropods

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Vertical succession of estuarine facies

• Depends on whether tide- or wave-dominated

• Transgressive sequence will produce landward migration of environments– Estuarine facies above fluvial facies

• Regressive sequence will produce seawardmigration of facies– Fluvial facies above estuarine facies

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Transgressiveestuarine facies

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Tidal flats

• Environmental setting– Mesotidal to macrotidal (4 to 15 m tidal range),

low-relief coastlines where wave energy is minimal

– Also, behind barriers (barrier islands, spits, reefs)

– Characterized by twice-daily flood and retreat of marine water

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Tidal flat facies

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Tidal flat facies• Subtidal zone

– Below mean low tide– High tidal current velocities– Characterized by bedload transport and sand deposition

• Intertidal zone– Between mean high and low tide; no significant vegetation– Dissected by tidal channels– Both suspended and bedload deposition (mixed mud and sand)

• Supratidal zone– Above mean high tide– Heavily vegetated and incised by tidal channels– Sedimentation from suspension during storm tides

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Tidal flat sediments

• Sediments are mostly sand and muds• Supratidal zone characterized by muds

with evidence of plant roots• Intertidal zone characterized by mixed mud

and sand, with sand in channels• Subtidal zone characterized by sand, with

channels and bars

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Tidal flat sedimentary structures

• Channels may have dunes and cross-bedding with bimodal paleocurrent directions

• Mixed sandy and muddy sediments may exhibit small-scale ripple cross-stratification, flaser bedding, wavy bedding, lenticular bedding, or finely laminated bedding, bioturbation (Skolithos)

• Supratidal deposits may be thinly laminated, but with plant bioturbation and evidence of subaerial exposure

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Tidal flatsedimentary structures

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Vertical facies associations

• Tidal flat progradation (regression) will produce a fining-upward depositional sequence– Subtidal sands overlain by mixed intertidal

sediments, overlain by supratidal muds• Transgression may produce a coarsening-

upward sequence, or it may destroy intertidal and supratidal deposits through reworking

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Tidal flat progradational sequence

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Verticalfaciesassociations

Coarsening-upwardsequence is producedby prograding beachcomplex