Transcript No Slide Title

Geophysical Processes in Sedimentary Basin
Formation
3rd Year
The aim of this course is to discuss some of the geophysical processes that are
operating in the Earth's outer layers. The focus is on the evolution of sedimentary
basins which are the world's largest repository of oil and gas deposits. This course
explores the fundamental thermal and mechanical processes that control the
stratigraphic "architecture" of sedimentary basins and the sedimentological
processes that determine the nature of their fill. Practicals provide opportunities
for developing computer skills in the modelling of lithospheric flexure, thermal
subsidence and uplift, crustal structure and, stratigraphy.
Week 6
Lecture Outline
Lecture 1: Sediment loading and unloading
Lithospheric flexure, Hawaii; elastic thickness and its relationship to plate and load age; sediment loading, the lithosphere as a
filter; Amazon Cone; sediment unloading, English Midlands; sediment loading and unloading at passive margins.
Lecture 2: Backstripping
Backstripping; compaction, water depth, and sea-level corrections; crustal structure and paleobathymetry from the backstrip;
flexural backstripping; examples of backstrip curves.
Practical 1: Backstripping
Week 7
Lecture 3: Thermal contraction and uplift
Mid-oceanic ridges and the cooling plate model; McKenzie’s stretching model; initial and thermal subsidence; syn-rift and postrift sediments; refinements; lateral heat flow and flexure;finite rifting; depth-dependant extension; rift flank uplift.
Lecture 4: Flexure, thermal contraction and uplift, and stratigraphy
Progradation and the clinoform break model; aggradation and the steer’s head model; seismic onlap and offlap; yield strength
envelope models; curvature and yielding; time-dependant flexure.
Practical 2: Thermal contraction
Week 8
Lecture 5: Orogenic loading
Foreland basins; surface and buried loads; elastic and viscoelastic models; unconformities; inheritance; encroachment; yoking.
Lecture 6: Forward modelling
Forward stratigraphic models; thermal and maturation history; relative role of tectonics and sea-level; relation between
onlap/offlap patterns and sea-level changes.
Practical 3: Foreland basin stratigraphy
Resources
Books
Allen, P. A. & Allen, J. R. “Basin Analysis”. Blackwells. Chapters 2-10.
Turcotte, D. L. & Schubert, G. “Geodynamics”. John Wiley & Sons. Chapters 3 and 4.
Watts, A. B. “Isostasy and Flexure of the Lithosphere”. Cambridge University Press.
Chapters 4 -7.
All books are on reserve in the library.
Journal Articles
Most journal articles are on reserve in library
Web
http://www.earth.ox.ac.uk/~tony/watts/teaching/geophys_processes/04H.html
Mathcad files
Plate flexure
Seamounts….Sediments (River Deltas)….Trenches….Late
Glacial Rebound
Lithospheric Flexure
The response of the lithosphere to long-term (i.e. > 1 Ma) loads such as the waxing (and waning) of
ice sheets, sediments, volcanoes and, the loads associated with convergent plate boundaries (e.g.
orogenic loading). One of the best examples are the Hawaiian Islands in the Central Pacific Ocean.
The effects of flexure are seen in
the crustal structure…...
Kauai
Oahu
Molokai
Mauai
Bulge
Hawaii
Volcanic
Load
Bulge
Moat
Yellow/Green = high gravity, Blue/Purple = low gravity
Elastic plate (flexure) model
Assumptions :
Parameters:
1. Linear elasticity
2. Plane stress
3. Cylindrical bending
4. Thin plates (i.e. plate
thickness << radius
curvature)
5. Neutral surface,
fixed at the half depth
D = flexural rigidity
y = flexure
m = density of
substrate
infill = density of infill
E = Young’s modulus
Te = elastic thickness
v = Poissons ratio
d4 y
D 4 + ( m –  infill) y g = 0
dx
E Te3
D=
12 (1 –  2)
Handout
Line loads
Continuous plate :
y=
Pb
e -x(Cosx + Sin x)
2 ( m –  infill) g
1/ = flexural parameter
(  m –  infill) g
=
4D
1/4
Discontinuous (ie broken) plate :
y=
2 Pb 
e –x Cosx
( m –  infill) g
Distributed loads can be modelled as one or more line loads
Estimating Te
Te can be estimated by comparing the
amplitude and wavelength of the observed
gravity anomaly to the predicted anomaly
based on an elastic plate model.
Mathcad file
The minimum in the RMS difference between
observed and calculated gravity anomaly
indicate a ‘best fit’ Te ~ 30 km.
Relationship between oceanic Te and plate and load age
Oceanic Te increases with age of the lithosphere at the time of loading but, decreases with load age.
There is therefore a “competition” between thermal cooling, which strengthens the lithosphere, and
a load-induced stress-relaxation which weakens it.
Sedimentary Basins
Pre-requisites: substantial sediment thicknesses (e.g. > 1 km) which have been preserved for
long periods of geological time.
Laske and Masters (1999)
Sediment Loading
Aim: to calculate the deformation of the crust and mantle that is caused by the displacement of water by
relatively dense sediment grains due, for example, to marine sedimentation.
Assume an “unloaded” and
“loaded” column of crust and
mantle.
The pressure at the base of a column
of height h and density  is :
Wd = the water depth that is
available for sedimentation
pressure = force/unit area
= mass/unit area × g
=hg
 w Wd g + Tc  c g +y  m g = Wd  s g + y  s g + Tc  c g
 y=
Wd (  s –  w)
(  m –  s)
If s = 2500 kg m-3, w = 1030, m = 3300 then, y ~2.5 × available water depth
Example: The maximum sediment thickness that can accumulate on a continental shelf with a shelf
break at 100 m = 100 + 250 = 350 m.
The Lithosphere as a Filter
It is useful when computing the flexural response to consider the lithosphere as a time-invariant filter
which responds to loads in a way that takes into account both the amplitude and wavelength of loading.
First, consider the solution of the general equation for deflection of an elastic plate when subject to a
periodic load i.e.
4 y
D 4 + ( m –
x
 infill) y g = (  c –  w) g h Cos(kx)
The solution is also periodic and of the form :
Flexural response
function
4
( c –  w) h Cos(kx)
D
k
y=
+1
( m –  infill)
( m –  infill) g
where k = wavenumber (2p/,  = wavelength) and h = load height.
-1
e.g. North Sea
e.g. Newark
The figure shows that the lithosphere is behaving as a filter in the way that it responds to sediment
loads; suppressing the short-wavelength deformation associated with local models of isostasy (e.g.
Airy) and passing the long wavelengths associated with flexure.
Flexural response
function
Example: Amazon Cone
The Amazon Cone, which developed on the northeast Brazilian
continental margin during the Late Miocene, is one of the largest
loads (3.5 × 1017 kg) to have formed on the Earth’s crust.
Amazon river
By comparing the topography of the margin in the region
of the Cone to that to the margin to the north and south, we
can determine the sediment load and, hence, the deformation
that it causes of the crust and mantle.
The deformation can be calculated by multiplying (in the frequency
domain) the Airy response to sediment loading by the flexural response
function.
Y(k) =
Wd(k) (  s –  w)
e(k)
(  m –  s)
4
D
k
e(k) =
+1
(  m –  s) g
In 3-D :
k=
-1
k 2x + k 2y
The calculations (left) show that the Amazon Cone has deformed the
lithosphere by as much as 1 km over distances of up to ~500 km. The
Cone experiences subsidence while flanking areas experience uplift.
Node
coincident
with coastline
Additional
Load
Flexure
Constraints on the value of Te can be made by comparing the calculated
flexure to seismic reflection constraints on the depth to the base MidMiocene reflector.
The total sediment load at the Amazon
Cone is greater than is indicated in the
present day bathymetry because the
cone has been superimposed on a
subsiding passive continental margin.
Tectonic subsidence
When the total sediment load is
considered (i.e. the cone + passive
margin), it is clear from the bending
stresses that have accumulated that the
Amazon Cone has loaded the
lithosphere almost to the limits of
its strength.
(Cochran, 1973)
Sediment Unloading
Aim: to calculate the deformation of the crust and mantle that is caused by the removal of sediment loads
due, for example, to river excavation.
Rock uplift
h  s g + T c  g = (h– y)  air g + Tc  c g +y  m g
 y=h
s
m
If s = 2500 kg m-3, m = 3300 then, y ~0.7 × sediment thickness
This is the Airy response. The actual uplift will be less because of the strength of the lithosphere.
Flexure limits the amount of uplift over the centre of the excavation but, increases it in flanking regions .
The flexural response can be found in the same way as before by multiplying (in the frequency domain)
the Airy response by the flexural response function . Therefore,
Y(k) =
where
H(k)  s e (k)
m
4
Dk +1
e(k) = 
m g
-1
Example: River excavation in the English Midlands
Excavation of soft Triassic and Jurassic sediments from the English Midlands by melt-water charged rivers
during the late Pleistocene (~450 ka) and their re-deposition in the Celtic Deep.
Clay Vale
Escarpment
To Bristol Channel and
Celtic Deep
Incised, uplifted, plateau-like
surface
Comparison of the predictions of an asymmetric
flexural unloading model to observed topography
profiles of north Oxfordshire between the valleys
of the Evenlode and Cherwell rivers.
Predictions of an asymmetric flexural
unloading model. The model shows that
sediment removal is associated with
rim uplifts, the amplitude and wavelength
of which depends on the width and depth
of the excavation and the elastic thickness,
,Te, of the lithosphere
Uplift induced
river incision
Example: Deep-sea erosion at passive continental margins
The removal of sediment from continental slopes due,
for example, to corrosive bottom water currents causes
flexural rebound and the formation of local (wide shelf) or
basin-wide (narrow shelf) unconformities. The rebound
may lead to increased downcutting of canyons across the
slope and stream incision on the shelf.
Post-early Oligocene
Post-rift
Oligocene canyons
Syn-rift
McGinnis et al. 1993
Seismic reflection profile offshore Congo showing Late
Eocene to Oligocene deep-sea erosion and Oligocene
canyon cutting across the outer shelf and slope.