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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
Strat & Sed, Ch. 9 2
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
Strat & Sed, Ch. 9 3
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
Strat & Sed, Ch. 9 4
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Fluvial-dominated deltas
Mississippi delta
Strat & Sed, Ch. 9 7
Fluvial-dominateddeltas
Strat & Sed, Ch. 9 8
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
Strat & Sed, Ch. 9 9
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Tide-dominated deltas
Ganges-Brahmaputra delta
Strat & Sed, Ch. 9 12
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
Strat & Sed, Ch. 9 13
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Wave-dominateddeltas
São Francisco delta,Brazil
Strat & Sed, Ch. 9 16
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
Strat & Sed, Ch. 9 17
Delta facies
Strat & Sed, Ch. 9 18
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
Strat & Sed, Ch. 9 19
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
Strat & Sed, Ch. 9 20
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
Strat & Sed, Ch. 9 21
Coarsening-upwardsequence
Finer-graineddelta plain deposits
Strat & Sed, Ch. 9 22
Delta lobe abandonment
Strat & Sed, Ch. 9 23
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
Strat & Sed, Ch. 9 24
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)
Strat & Sed, Ch. 9 28
Beach morphology
Strat & Sed, Ch. 9 29
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)
Strat & Sed, Ch. 9 30
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
Strat & Sed, Ch. 9 31
Longshore drift & longshore current
Strat & Sed, Ch. 9 32
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
Strat & Sed, Ch. 9 33
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
Strat & Sed, Ch. 9 34
Beach and barrier island facies
Strat & Sed, Ch. 9 35
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
Strat & Sed, Ch. 9 36
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
Strat & Sed, Ch. 9 37
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
Strat & Sed, Ch. 9 38
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
Strat & Sed, Ch. 9 42
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
Strat & Sed, Ch. 9 43
Wave-dominatedestuary
Strat & Sed, Ch. 9 44
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
Strat & Sed, Ch. 9 45
Tide-dominatedestuary
Strat & Sed, Ch. 9 46
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
Strat & Sed, Ch. 9 47
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
Strat & Sed, Ch. 9 48
Transgressiveestuarine facies
Strat & Sed, Ch. 9 49
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
Strat & Sed, Ch. 9 50
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Tidal flat facies
Strat & Sed, Ch. 9 55
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
Strat & Sed, Ch. 9 56
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
Strat & Sed, Ch. 9 57
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
Strat & Sed, Ch. 9 58
Tidal flatsedimentary structures
Strat & Sed, Ch. 9 59
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
Strat & Sed, Ch. 9 60
Tidal flat progradational sequence
Strat & Sed, Ch. 9 61
Verticalfaciesassociations
Coarsening-upwardsequence is producedby prograding beachcomplex