Megablocks and the stratigraphic record of continental margins: how large an event do they materialise?

Three-dimensional seismic data from continental slopes commonly show submarine landslide deposits that contain large-scale megablocks, suggesting sudden catastrophic instability events. However, the true temporal and volumetric scales of such events are not well documented in the literature. This study reviews data from three independent projects to provide insights into the importance of large-scale instability events to both stratigraphy and petroleum systems. On the basis of data from the three case studies, some conclusions about the impact of these “megaslides” on petroleum systems are shown by modelling leakage tendencies across basal shear zones. Offshore of Japan, a ~65-m landslide drilled during IODP Expedition 333 shows distinct degrees of erosion at its basal shear zone, whose morphology has responded to the tectonic activity of underlying faults during the Quaternary. In Crete, medium-size blocks of a Miocene submarine landslide show that more than 100 m of continental slope strata were disrupted and moved over shaly marine mudstones of a palaeo continental slope, now fully exposed. Offshore of Brazil, ~400-m blocks were left in situ during the sudden collapse of the Espírito Santo continental slope. The data in this study show that these large-scale events can place important limitations on the correlation of seismic and stratigraphic units across sedimentary basins, as the erosional character of such events can be underestimated, thus implying that potential hiatuses and unconformities are not identified when interpreting older landslide successions. However, the erosional character of these large-scale landslides is recorded mainly in proximal areas, where the sudden release of vertical confining pressure has a significant impact on petroleum systems. In this study, results are presented from fluid-flow modelling across landslide strata in the collapse of large-scale (mega) slides