526 SEDIMENTARY ENVIRONMENTS/Contourites Figure 11 Conjectural standard composite (muddy ỵ sandy) contourite facies model Adapted with permission from Stow DAV, Fauge`res JC, Howe J, Pudsey CJ, and Viana A (2002) Bottom currents, contourites and deep sea sediment drifts: Current state of the art, In: Stow DAV, Fauge`res JC, Howe J, Pudsey CJ, and Viana A (eds.) Deep Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics Geological Society, London, Memoir 22: 20, with permission 9B, 10A and 10B) Grain size may vary from (silty) mud to sand Gravel-rich contourites are common in glacigenic environments due to the presence of icerafted debris Bottom current winnowing and erosion in narrow gateways may also produce gravel lags or shale-chip layers Sorting is generally medium to poor, but some sandy contourites may be well sorted and relatively free of mud, showing low or negative skewness values No offshore textural trend may be observed Composition is generally mixed with a combination of biogenic and terrigenous material High sand-sized content in muddy hemipelagic contourites is often formed by bioclasts (Figures 9B, 10B) The terrigenous component largely reflects the nearby local sources, but alongslope mixing and far-off contributions also occur In contrast the composition is essentially pelagic in biogenic contourites developed in open oceans or in upwelling areas Manganese dioxides or iron oxides may occur as micronodules, stains, coating, and metal-enriched crusts within the manganiferous contourites Fabric (when not exceedingly altered by bioturbation) is indicative of alongslope grain orientation, parallel to the bottom current flow Contourites (Figure 11) are normally arranged in few decimetre-thick sequences showing gradual grain size and/or compositional changes Five divisions may be eventually identified: (C1) lower mud, (C2) lower mottled silt, (C3) middle sand, (C4) upper mottled silt, and (C5) upper mud A complete C1–5 sequence is interpreted to represent a gradual long-term current velocity variation and/or sediment supply change Partial base-only or top-only sequences are also common Facies Continuum and Distinguishing Criteria Deep-sea sediments may be interpreted as the product of three main processes: gravity-driven downslope resedimentation, alongslope bottom current activity, and slow pelagic settling through the water column However, such processes are in fact end members belonging to a continuum spectrum Deep-sea sedimentary facies are often hybrids resulting from interactions within such process continuum (Figure 1) rather than a mere stacking of interstratified deposits produced by discrete processes Imaginably, the distal part of very-lowconcentration (0.025 to 2.5 g l 1), low-velocity (10–50 cm s 1), fine-grained (silt and clay) turbidity currents may be easily deflected by Coriolis force from its natural downslope path to an alongslope direction and then entrained in the indigenous contour current Concurrently, the tests of calcareous and siliceous planktonic organisms and the associated organic matter biosynthesized in the surface layers of the ocean may be significantly contributed by slow vertical