Sizes of submarine slides and their significance

Abstract

A striking discrepancy is shown between the sizes of submarine slide or slump sheets recognised from seismic profiling on present continental margins and the sizes of possibly analogous sheets described from the ancient on-land record. The recent slides are, on average, several orders of magnitude larger in cross-sectional area than their supposed ancient equivalents. Where then are the true ancient analogues of these large recent slide sheets? Are they genuinely absent in the geological record or have they not been recognised? A list is given of characteristics of large continental margin slides which could be recognised in the on-land record. These include their size and the extent to which they are allochthonous, their dominant content of continental margin sediments, their high-level, non-metamorphic deformation and, most critically, their superposition of shallower over deeper water sediments. Using these criteria on typical ancient allochthonous sediment masses shows that most could not have been down-margin slides; only two convincing pre-Pleistocene examples have been found. Therefore the problem remains of whether more examples could exist or whether post-glacial conditions make present margins unique.