514 SEDIMENTARY ENVIRONMENTS/Contourites flows may favour the accumulation of source rocks and robust flows may represent a viable mechanism capable of forming ‘clean’ sands in the deep sea Lowpermeability, fine-grained contourites have been found recently to play a critical role in slope stability by providing potential overpressured glide planes, either when their high-water-content is rapidly loaded or when their rigid biosiliceous microfabric is collapsed by diagenesis Despite its significance, little is still known about this group of sediments There are three reasons for the long-lasting disregard: the inherent elusive nature of these complex deposits, the 50-year dominance of the turbidite paradigm, and the controversy that surrounds these sediments since they were first recognized The elusive and very subtle characteristics of these slowly and continuously accumulated sediments and their occurrence within a spectrum of deep-water deposits (Figure 1) does not allow them to be easily recognized and decoded The monumental efforts to promote the turbidite systems, the simplicity and predictability of turbidity current concepts, and the sense of confidence given by the routine use of widely applicable models has induced the geologic community to ignore alternative, more complex deep-water models The never-ending disputes regarding the occurrence of sandy contourites versus reworked turbidites and the early errors in recognition of fossil examples have prevented the establishment of a widely shared consensus on valid diagnostic criteria for the identification of these deposits The growing level of interest and research in contourites is shown by the recent publication of several special volumes dealing with such systems, covering large parts of present ocean floors and continental margins An increasing number of fossil occurrences in ancient sediments exposed on land have also been documented Nevertheless, wide multidisciplinary approaches and the integrated work of different international specialists (e.g., deep-water sedimentologists, seismic interpreters, physical oceanographers, and palaeoclimatic modellers) are still needed to improve our knowledge of these systems and to help tackle the problems that remain History The systematic study of deep-sea sediments began at the end of the nineteenth century At that time the ocean floor was perceived as a tranquil realm receiving only pelagic clays The possibility that thermohaline bottom currents may influence sedimentation in the deep oceans was suggested by the German oceanographer Wust in 1936 However, the contourite concept was not accepted in marine science until the second half of the 1960s after the American team of Heezen and Hollister had provided geological and oceanographic evidence of this process along the eastern North American continental margin During the next two decades standard facies models for contourite sediments were developed from coring contourite accumulations Other important steps forward were achieved by addressing the link between current strength and grain size and by confronting the problem of distinguishing contourites from other deep-sea facies like hemipelagites and fine-grained turbidites Among the projects that have provided significant contributions to contourite research have been the HEBBLE Figure Schematic model of downslope gravity driven currents and alongslope bottom currents interacting within the continuous spectrum of deep water processes Reprinted from Shanmugam (2000) 50 years of the turbidite paradigm (1950s 1990s): Deep water processes and facies models A critical perspective, Marine and Petroleum Geology 17: 285 342, with permission