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2- Geodynamics of Sedimentary Basins
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2.1 Basins due to divergence - Rifts - Subsidence analysis - Passive continental margins
Lakes Tanganyika & Victoria Gulf of Suez & Red Sea USA Atlantic Margin
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2.1.1. Rifts a. stretching Subsidence analysis b. kinematics of rifting c. Rift basin architecture
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a. Stretching
Extension => deformation = stretching
Stretching <=> thinning (volume conservation)
thinitial
linitial
lfinal
thfinal
Stretching factor (β): lfinal/linitial (always >1) Thinning factor: thfinal/thinitial (always <1)
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Stretching (extensional stress)
a) Stretching => thinning b) Deformation = Normal faults in upper crust and
uplift of Lithos/asthenos boundary c) N. Faults => initial subsidence d) LAB isotherm uplift =>increased geotherm
a) Stop of extensional stress => and faults b) Cooling of thinned lithosphere c) Cooling => density increase => subsidence d) Subsidence => sediment deposition
Considered
instantaneous
10’s Millions
of years
Lithospheric Stretching
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Rifting and subsidence processes
Cooling
Stretching/ thinning
Loading
Rifting (lithosphere stretching and thinning) involves the 3 basic subsidences processes,
simultaneously or successively. 0°C
1300°C
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Geological example of a rift : Viking Graben
Moho
Thinned cont. Crust (lithosph. also affected)
Synrift sequence (initial subsidence)
Postrift sequence Rift axis
Postrift basin
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How does it work ? -> the McKenzie model = Subsidence Analysis
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Computing the value of initial subsidence
local
isostacy
instantaneous
Mass
Conservation
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Sediment-fill = load => extra-subsidence
Subsidence without
sediment
sediment
Subsidence with sediment
Extra Subsid. due to
sediment load
local
isostacy
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Computing the subsidence due to Sediment-load
3.15
2.3
1
Sload = 0.60 hs
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subsidence in rift basins
Temporal subdivision
Processes
Driving mechanisms
Syn-rift
(intantaneous, initial)
+ Post-rift (long-term)
Fault activity
+ Thermal recovery
Sediment load + Geodynamics
(“tectonic”)
Total subsidence in rift basins (measured in borehole)
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(“tectonic”)
12 True for any
type of basin
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Exercise: Viking Graben
• Considering a pre-rift crustal thickness of 35km, what is the total subsidence at points A, B, C ? • Same question, considering that Trias + U. Palaeozoic belong to pre-rift. • What are the stretching and thinning factors at each point?
A B C
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Subsidence evolution of sedimentary basins
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Exercices: subsidence evolution of basins
15
Plot the total subsidence curve from different types of data
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Measuring Subsidence in sedimentary basins
Subsidence : Amount of downward vertical movement of the sedimentation surface
= burial of top of the basement = depth of the basement = thickness of sediments and/or water
Data from: • borehole data (stratigraphic) • seismic reflection or refraction • sediment sections measured in the field
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and/or water
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Initial position
must be known
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Synr
ift
Postrift
Prer
ift
time
Sedi
men
t th
ickn
ess fast subsidence
= synrift
postrift subsidence: cooling of the lithosphere
Previously thinned
Subsidence in rift basins
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Postrift subsidence = lithosphere thermal recovery
Theoritical evolution of thermal subsidence for different amounts of stretching, using McKenzie’s model
Following rifting, the stretched mantle lithosphere gets cooler. As it gets cooler, the density increases, and subsides (principle of isostacy) = « thermal subsidence » Subsidence due to sedimentary loading has been removed
geodynamic
subsidence
Exercise: determine initial and thermal tectonic subsidence for a rift basin formed in Late Jurassic (considered instantaneous); Basin is 5km thick, Moho at 25km depth
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Backstripping : geodynamics from total subsidence
Basement burial = total subsidence
Remove effect of sediment loading = backstripping
Computed « geodynamic » subsidence
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Backstripping : 1D modeling softwares
Données pour Backstripping Le puits Chote Name Base Ageb SLb WDb Top Aget SLt Wdt c C o type
Kimmer 3.063 145 .150 .300 2.824 140 .250 .300 2.6*103 .71 .70 0 Cal-argi Potlan 2.824 140 .150 .300 2.752 134 .250 .200 2.6 .71 .70 0 Cal-argi Kti 2.752 134 .150 .2 00 2.668 110 .150 .100 2.6 .71 .70 0 calc Uncon 2.668 110 .150 .100 2.668 64 .200 .200 Pchi 2.668 64 .200 1.200 2.546 60 .200 1.200 2.42 .39 .56 0 Sab-arg Pchm 2.546 60 .200 1.200 2.320 54 .200 .900 2.42 .39 .56 0 sab>argi Pchs 2.320 54 .200 .900 2.136 49 .200 .400 2.42 .27 .56 0 sab Eg 2.136 49 .200 .400 1.240 39 .200 .200 2.38 .51 .63 0 Argil Chapo 1.240 39 .200 .200 .988 36 .225 .100 2.38 .51 .63 0 Argil Horco .988 36 .225 .100 .831 30 .225 .50 2.42 .27 .49 0 sable Pri .831 36 .225 .050 .639 30 .200 .025 2.42 .27 .49 0 Sabl m Prs .639 30 .200 .025 .368 25 .150 .025 2.42 .27 .49 0 Sabl g Coatzi .368 25 .150 .025 0 22 .100 .010 2.38 .51 .63 0 argi
Data for backstripping Chote well (W. Gulf of Mexico)
Form
atio
n
age
Dep
th B
ase
Sea
leve
l
Bath
ymet
ry
dens
ity
Poro
sity
at
de
posi
tion
Com
pact
ion
fa
ctor
litho
logy
age
Dep
th B
ase
Sea
leve
l
Bath
ymet
ry
Top of formation Base of formation Formation physical properties
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Backstripping
Burial history for each stratigraphic interval Chote well (W. Gulf of Mexico)
Geodynamic subsidence = backstripped total subsidence computed
total subsidence = basement burial observed
Removal of
Sediment load
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b- Kinematics of rifting
- Stretching in crust above stretching in the mantle, - Postrift subsidence centred on the rift axis - conjugate normal faults
- Offset of stretching in crust and in the mantle - surface uplift above mantle thining - postrift subsidence offset - Low-angle extensional detachment - lower crust/mantle denudation
Symetrical (McKenzie, 1978)
Assymetrical (Wernicke, 1981)
uplift
2.1.1. Rifts a. stretching Subsidence analysis b. kinematics of rifting c. Rift basin architecture
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Viking Graben - seismic section
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- Geometry ? - synrift vs postrift ?
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Viking Graben - depth section M
aste
r1 G
éolo
gie
des
Rése
rvoi
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- Geometry ? - synrift vs postrift ? - approx. thinning ratio ? - Prerift?
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c. Rift basin architecture
Tectonic-sedimentation relationships - synrift sediment geometry - Fault profile
Parallel reflections
Divergent reflections
Horizontal, onlap reflections
Rift basin formation
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c. Rift basin architecture
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Relationships between : fault profile and basin-fill geometry
Thin-skinned vs thick-skinned extensional tectonics - Rift geometry - Measurement of extension and consequences on crustal thinning
Faure, 1989 Benedicto, 1996 cf cours P. Labaume
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Active rift basin = Death Valley
www.marlimillerphoto.com/
normal F.
Basement erosion
Alluvial fan in the basin
normal F.
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Active rift basin - Nevada
Basi
n bo
undi
ng f
ault
Small radius aggrading
alluvial fans
Sabkha or lake
Uplifted & eroded footwall
Hanging-wall
Axia
l dra
inag
e
wide radius prograding alluvial fans
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Sedimentary facies distribution ⇒ syn-tectonic sedimentation • Rapid facies change: proximal -> distal • Breccia along active faults - mudstone at rift axis • Growth structures/ progressive unconformities
Breccia against border fault
Sand & conglomerates channels (fluvial)
Lacustrine limestone
Benedicto , 1996
Benedicto , 1996
Fossil rift basin = Les Matelles
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Architecture of rift basin infill
Downlap + thinning away from fault => Proximal breccia -> distal fine clastics
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Prosser, 1993
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Copyright ©
2001. All R
ights Reserved.
Tectonic–sedimentation relationships in rift basin
Gawthorpe & Leeder, 2000
Rift initiation
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Copyright ©
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ights Reserved.
Tectonic–sedimentation relationships in rift basin
Gawthorpe & Leeder, 2000
Rift development
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Copyright ©
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ights Reserved.
Tectonic–sedimentation relationships in rift basin
Gawthorpe & Leeder, 2000
Rift climax
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Active rift basins : Gulf of Corinth
Acc = Sed
Acc > Sed
Acc < Sed
Acc << Sed
Continent.
marine
Rohais & al 2007
dap
rès
Flot
té &
al 2
003
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Type sequence of a rift basin
Synrift unconformity
Fluvial or littoral
Lake delta
Lake turbidites
Lacustrine blackshales (anoxic => organic matter)
Alluvial fan
fluvial
postrift unconformity
prerift
Rift initiation (sediment against
active faults)
Rift climax Accom >> Sedim
Starved basin
Filling sequence Accom < Sedim
End of rifting
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Model of an active rift basin
Erosion in watershed
Footwall
Hanging-wall
Narrow, thick, high accommodation alluvial fans close to border fault
Wide, thin, low accommodation alluvial fans above roll-over
fluvial/lacustrine axial facies
Listric normal fault
Roll-over
Progressive unconformities = growth structures
Footwall Rapid facies change
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Geological example : Moray Firth
W E
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Architecture of post-rift basin infill
Geometry of post-rift sequence = « steer-head » (Porcupine Basin)