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Rayleigh convection, mass transport, and change in porosity in layers of sandstone
GEOMECHANICS ABSTRACTS
solubility increases with temperature, hence silica is dissolved in the lower, heated part of the layer and precipitated in the cooler upper part. Change in porosity over geological time is examined. Vertical spatial variation of porosity should be a characteristic of past Rayleigh convection.
Geology Tectonic processes and structural geology
912005 Permeabilities, fluid pressures, and flow rates in the Barbados Ridge complex Screaton, E J; Wuthrich, D R; Dreiss, S J d Geophys Res V95, NB6, June 1990. P8997-9007
See also: 912053, 912097. 912125. 912146, 912223 912001 Experimental models of extensional forced folds Withjack, M O; Olson, J; Peterson, E Bull Am Soc Petrol Geoi V74, N7, July 1990. P1038-1054 Single and multilayer clay models have been used to study development of forced folds above normal faults. The deformation patterns resulting from extensional forced folding depend on the dip of the normal fault and the presence of single layer detachments. Deformation patterns from these simple model tests are similar to those observed in the Gulf of Suez and offshore Norway. Knowledge gained will assist structural interpretation of field and seismic data.
912006 Effect of carbonate cementation of underthrust sediments on deformation styles during underplating Sample, J C J Geophys Res V95. NB6. June 1990. P9111-9121
912002 Occurrence of pop-ups in the Quebec City area. Note Wallach, J; Chagnon, J Y Can J Earth Sci V27. NS. May 1990. P698-701 Pop-ups, elongate surficial anticlinal structures, which occur predominantly in layered sedimentary rocks, have been observed in the floors of two quarries. They are stress relief phenomena developed in response to high horizontal surface and near surface compressive stresses. Orientation of most occurrences is consistent with the stress system affecting eastern North America, but other orientations are found, presumably reflecting local stress conditions. 912003 Finite amplitude necking model of rifting in brittle lithosphere Lin. J; Parmenter, E M J Geophys Res 1/95. NB4. April 1990, P4909-4923 Lithospheric extension Iocalised into a narrow zone results in rifting. A mechanical model of rifting as finite amplitude necking of a brittle-elastic layer over a ductile substrate is developed. Evolution of rift width as extension proceeds is followed. Vertical forces due to the mass deficit of the rift will flex the elastic layer outside the yield zone, resulting in flanking uplift. The simple model presented demonstrates the inherent relations between graben formation, crustal thinning, and rift shoulder uplift. 912004 Rayleigh convection, mass transport, and change in porosity in layers of sandstone Palm. E J Geophys Res V95, NB6. June 1990. P8675-8679 Diagenetic processes in sedimentary basins are of great interest in basin analysis. The case where fluid flow is driven by thermal convection is studied for a single layer system. Quartz ~
Recent measurements indicate pore pressures near the toe of the Barbados accretionary prism may be close to lithostatic and the decolletment is a zone with relatively high rates of fluid flow and gas transport. Numerical analysis confirms values of intrinsic pcrmeabilities 3-5 orders of magnitude above those of adjacent prism sediments are consistent with these indications. Fluid flow direction and rate and fluid pressures associated with tectonic processes leading to the observed state are discussed.
Fluid expulsion during deformation of accretionary complexes can lead to a variety of diagenetic reactions, including carbonate cementation. Sandstones of an ancient accretionary complex in Alaska contain significant amounts (2-16%) of carbonate cement formed before underplating occurred. Vein calcite and early calcite replacing framework grains or filling pores, followed by ankerite, mainly as pore filling cement are present. Mean porosity filled is equivalent to that expected at depths 4-Skm in normal compacted sands underthrust at modern convergent margins. Lithification increased strength and cohesion, preventing particulate flow, and contributed to underplating of the formation as a relatively coherent body. 912007 Studies relating to faulting in abyssal plain sediments Stone K J L; Wood. D M Proc Conference on Disposal of Radioactive Waste in Seabed Sediments, Oxford, 20-21 September 1988 P273-289. Publ London: Graham and Trotman, 1989 ( Adt'ances in Underwater Technology, Ocean Science and Offshore Engineering. VI8) Seismic reflection profiles have indicated the presence of faults in areas of the Atlantic abyssal plain. Centrifuge modelling was used to assess the importance of differential settlement over an uneven basement to the formation of these fractures. Ig laboratory tests and mathematical modelling were also undertaken. Shear planes developed in all centrifuge tests, indicating differential settlement to be a likely cause of seabed faults. At Ig, tensile fractures were preferred. Numerical modelling did not indicate likelihood of shear failure, but this is considered due to inadequacy of the soil model used.
1991 Pergamon Press pie. Reproduction not permitted 63A
solubility increases with temperature, hence silica is dissolved in the lower, heated part of the layer and precipitated in the cooler upper part. Change in porosity over geological time is examined. Vertical spatial variation of porosity should be a characteristic of past Rayleigh convection.
Geology Tectonic processes and structural geology
912005 Permeabilities, fluid pressures, and flow rates in the Barbados Ridge complex Screaton, E J; Wuthrich, D R; Dreiss, S J d Geophys Res V95, NB6, June 1990. P8997-9007
See also: 912053, 912097. 912125. 912146, 912223 912001 Experimental models of extensional forced folds Withjack, M O; Olson, J; Peterson, E Bull Am Soc Petrol Geoi V74, N7, July 1990. P1038-1054 Single and multilayer clay models have been used to study development of forced folds above normal faults. The deformation patterns resulting from extensional forced folding depend on the dip of the normal fault and the presence of single layer detachments. Deformation patterns from these simple model tests are similar to those observed in the Gulf of Suez and offshore Norway. Knowledge gained will assist structural interpretation of field and seismic data.
912006 Effect of carbonate cementation of underthrust sediments on deformation styles during underplating Sample, J C J Geophys Res V95. NB6. June 1990. P9111-9121
912002 Occurrence of pop-ups in the Quebec City area. Note Wallach, J; Chagnon, J Y Can J Earth Sci V27. NS. May 1990. P698-701 Pop-ups, elongate surficial anticlinal structures, which occur predominantly in layered sedimentary rocks, have been observed in the floors of two quarries. They are stress relief phenomena developed in response to high horizontal surface and near surface compressive stresses. Orientation of most occurrences is consistent with the stress system affecting eastern North America, but other orientations are found, presumably reflecting local stress conditions. 912003 Finite amplitude necking model of rifting in brittle lithosphere Lin. J; Parmenter, E M J Geophys Res 1/95. NB4. April 1990, P4909-4923 Lithospheric extension Iocalised into a narrow zone results in rifting. A mechanical model of rifting as finite amplitude necking of a brittle-elastic layer over a ductile substrate is developed. Evolution of rift width as extension proceeds is followed. Vertical forces due to the mass deficit of the rift will flex the elastic layer outside the yield zone, resulting in flanking uplift. The simple model presented demonstrates the inherent relations between graben formation, crustal thinning, and rift shoulder uplift. 912004 Rayleigh convection, mass transport, and change in porosity in layers of sandstone Palm. E J Geophys Res V95, NB6. June 1990. P8675-8679 Diagenetic processes in sedimentary basins are of great interest in basin analysis. The case where fluid flow is driven by thermal convection is studied for a single layer system. Quartz ~
Recent measurements indicate pore pressures near the toe of the Barbados accretionary prism may be close to lithostatic and the decolletment is a zone with relatively high rates of fluid flow and gas transport. Numerical analysis confirms values of intrinsic pcrmeabilities 3-5 orders of magnitude above those of adjacent prism sediments are consistent with these indications. Fluid flow direction and rate and fluid pressures associated with tectonic processes leading to the observed state are discussed.
Fluid expulsion during deformation of accretionary complexes can lead to a variety of diagenetic reactions, including carbonate cementation. Sandstones of an ancient accretionary complex in Alaska contain significant amounts (2-16%) of carbonate cement formed before underplating occurred. Vein calcite and early calcite replacing framework grains or filling pores, followed by ankerite, mainly as pore filling cement are present. Mean porosity filled is equivalent to that expected at depths 4-Skm in normal compacted sands underthrust at modern convergent margins. Lithification increased strength and cohesion, preventing particulate flow, and contributed to underplating of the formation as a relatively coherent body. 912007 Studies relating to faulting in abyssal plain sediments Stone K J L; Wood. D M Proc Conference on Disposal of Radioactive Waste in Seabed Sediments, Oxford, 20-21 September 1988 P273-289. Publ London: Graham and Trotman, 1989 ( Adt'ances in Underwater Technology, Ocean Science and Offshore Engineering. VI8) Seismic reflection profiles have indicated the presence of faults in areas of the Atlantic abyssal plain. Centrifuge modelling was used to assess the importance of differential settlement over an uneven basement to the formation of these fractures. Ig laboratory tests and mathematical modelling were also undertaken. Shear planes developed in all centrifuge tests, indicating differential settlement to be a likely cause of seabed faults. At Ig, tensile fractures were preferred. Numerical modelling did not indicate likelihood of shear failure, but this is considered due to inadequacy of the soil model used.
1991 Pergamon Press pie. Reproduction not permitted 63A