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Deep-Water Turbidites in Not-so-Deep Basins


Deep-water turbidites are deposited in a wide range of water depths considered “deep water”, ranging from hundreds to thousands of meters. There are fundamental similarities as well as differences between such deposits. All turbidite systems regardless of basin-floor water depth are characterized by a tripartite system: a proximal tributive feeder system of canyons and slope valleys, a medial single channel/slope valley feeder, and a distal distributive system of channels and lobes. Principal differences lie in the scale of these systems, and most importantly the length of the medial channel/slope valley feeder channel component. In comparatively shallow deep-water systems (i.e., water depth <400m) medial feeder channels can be less than 1-2km long compared with feeder channels in ultra-deep-water settings that can be hundreds of kilometers long. Likewise, the distal distributive component in comparatively shallow deep-water settings is commonly characterized by minimal runout from the base of the slope. 3D seismic data from settings characterized by deep-water less than 400m can afford a complete view of these deep-water turbidite systems. Small medial feeder channels can be imaged, feeding fans with runouts commonly less than 10km from the base of the slope. Rarely, isolated longer runout systems with long-distance runout feeder channels are observed. In general, the more mud prone the system the farther the runout of the medial feeder channels and concomitantly, the smaller the resulting distributive fan system. When viewed in detail, turbidite systems in not-so-deep basins (i.e., <400m) are characterized by the same geomorphic elements that characterize turbidite systems in deep and ultra-deep basins. That is, it is common to observe leveed channels, crevasse splays, crevasse channels, and frontal splays, albeit proportionately smaller in scale. Presumably, this is related to the shorter slope distance that characterizes not-so-deep basins. Shorter slopes would suggest a shorter distance for flow acceleration and hence the basin floor is reached more quickly where flow deceleration would begin to occur. With flows down short slopes acceleration would quickly give way to deceleration and associated decrease in turbulence. The resulting deposits in not-so-deep basins would tend to be less well sorted, possibly tending towards a greater proportion of hybrid event beds.