sedimentation on passive margins - géosciences montpellier · 2014. 9. 18. · 1. 1 s. dynamique...
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Sedimentation on passive margins
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rift
Break-up
Young margin
Mature margin
Sedimentation is controled by continental margin geodynamic evolution
Tectonic-sedimentation relationships at passive margin scale
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Stage « rift »
Syntectonic sedim. Outward drainage
=> Starved basin
Lithosphere stretching
Axial lake (restricted) ⇒ M.O. preserved
thermal anomaly
Tectonic-sedimentation relationships on passive margins
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Model: East African rift
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Stage « break-up »
Oceanic accretion => Extensional stress stops => Fault activity stops
Transgression => evaporites
Thermal anomaly ⇒ cooling
cooling => densification => subsidence
Tectonic-sedimentation relationships on passive margins
Young (hot) ocean. lithosph. ⇒ Sub-horizontal ⇒ basement
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Model: Afar & Red Sea
Afar
Marge continentale d’Aden
Rift
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Differential Subsidence
=> tilting
Sed = Accom => aggradation
Gravitational Tectonics
Stage « young margin »
Differential accomm. Proximal->distal facies
Tectonic-sedimentation relationships on passive margins
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Stage « mature margin »
progradation Deep-sea fan
Reduced thermal subsidence
Added sediment load => Subsidence (flexure)
Terrigenous sediment flux
Tectonic-sedimentation relationships on passive margins
Flexural uplift
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- + - +
Acc > Sed
Acc = Sed
Sed > Acc
Sed > Acc
Acc >> Sed
Synrift subsidence
Thermal postrift Subsidence
Water depth Subsidence
0 marin
0 lacustre
Synthetic log of passive margins
Break-up unconf
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Oligocene-Present isopachs
4.5km 4
3
2
1
0km
Rust & Summerfield 1990, Anka 2004
Congo deep-sea-fan
• Angola: thick and focused deposits extensive continental watershed
• Africa SW: distributed and thin deposits
little continental erosion
Integrated study of passive margins « source-to-sink »
Congo
Orange
Watershed - depocentre relationship
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Long-term evolution of W. Africa deep-sea fans
• Congo-Angola: Cenozoic deep-sea-fan
• South-Africa : late Cretaceous margin + deep-sea-fan
Oceanic crust Cont. crust
Syn-rift+SDR
seamount
Cont. Cr.
synrift
Oceanic crust
50km
50km
5km
0km
10km
Séranne &Anka 2005,
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Onset of Cenozoic deep-sea-fan deposition
Base Oligocene
Top Turonian
Miocene latest
0.5
s
1 km
NE SW
Aptian salt SB
Progradation of Tertiary DSF Condensed surface
Tur.
Down lap
On lap
Top Oc. Cr.
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? ?
Condensed interval
Rust & Summerfield 1990, Anka 2004, Leturmy & Lucazeau 2003
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terr
igeo
us
terrigenous
carbonates
Evaporites
mar
ls
terrigenous = syntectonic sedimentation
Con
tinen
tal e
rosi
on
Red
uced
con
tinen
tal
eros
ion
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Record of Climate change on passive margins
Climate change (icehouse) => increased continental erosion => increase terrigenous flux
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Deep sea fans on mature margins Oceanic crust
Continental crust
0-10Ma
100-120Ma
65-100Ma
40-65Ma
10-25Ma
20km
Gravitational tectonics
Rifted Continental Crust Oceanic crust
• Terrigenous sediments over oceanic crust • Huge depocenter => burial + gravitational tectonics • Large sediment flux => large watershed and intense erosion • Sedimentation rate increases in Miocene
Lopez, 1996
Amazon example
DS
F lo
ad
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Flexural loading of Amazon DSF
Watts, 2009
Extra-subsidence (>2km at the center of load)
Flexural uplift (<40m) around the load
Characterizing and computing the extra-subsidence due to the lithosphere flexure in response to the load of the Amazon DSF
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Deep-sea fan : Congo
Angola
Escarpement
(Lopez, 2000)
Congo Turbidite deep-sea-fan (800 km) Margin
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Geometry of Congo DSF
(Anka, 2004)
Isopach map of Oligocene - Present
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Congo watershed
med
io-A
tlant
ic r
idge
Niger fan
Congo fan
Sedimentary load => flexure of the oceanic lithosphere
Modifié d �après Uchupi, 1992
Effect of Congo DSF on the passive margin
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Flexural loading of Congo DSF
Te=20km (Anka, 2004)
Flexural uplift (<70m)
Subs. >2km
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Interactions Margin-DSF
(Seranne Anka, 2005)
⇒ DSF sedimentary record complements that of the margin; ⇒ DSF depocentres are as big as margin�s depocentre, ⇒ Loading increases subsidence in mature margins AND flexural uplift of oshore margin; ⇒ Continental drainage concentrates in few major rivers ⇒ DSF record processes active on continent