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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 Cosx ( 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 =hg 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.