Characterisation of coherent submarine mass-wasted sandstone packages from outcrop and in the subsurface
Gravity-driven sediment transport is a major mechanism occurring on active and passive margins. Past research on outcrops was limited by the incomplete preservation and limited 2D exposure. Over the last few decades geophysical techniques have made it possible to map modern sea-floors and to obtain subsurface images from seismic surveys. Each of the data has advantages and disadvantages; hence investigating and comparing gravity-driven sediments using complementary approaches was undertaken in this project to gain a more complete picture. Proximally, slide blocks form a planform jigsaw-pattern with an elongated axis parallel to the headscarp. Underperformed slide bodies (10s km) are found in the lower slope. Exposures of small-scale (meters) slide blocks show that the initial break-up stage of sand sheets forms a jigsaw-pattern of rafts, which down-dip transform into deformed and tilted blocks. At the base of slumps the end-member facies is typically a ‘rolled-up’ sandstone body. Preservation of the internal stratigraphy within slides is observed even in only partly consolidated sediments. Slide blocks may displace over long distances without loosing their internal geometry and deformation is only observed close to the shear surface, at the front and at the lateral margins. This makes the identification of slides difficult on seismic data and also at outcrop. Slide blocks are overlooked as petroleum reservoirs since it is commonly inferred that the internal geometries are discontinuities and because the sediment is overconsolidated. This may be a premature assumption as the lack of internal deformation implies continuity and minimal overconsolidation, especially in slides, which have only moved a little.