Contourites are defined as sediments deposited or substantially reworked by the persistent action of bottom currents (Stow et al., 2002; Rebesco, 2014). The term was originally introduced by Wust (1955) and Heezen et al. (1966) to define sediments deposited in the deep sea by contour-parallel thermohaline currents. With the time, the term widened to embrace a larger spectrum of sediments that are affected by different types of currents. Moreover, contourites may occur interbedded with other sediment types and interaction of processes.

Despite their significance as an important process in the deep marine setting, contourites are still relatively poorly known, especially because the nature of these complex deposits is not clear, and because of the dominance of the “turbidite paradigm” (Shanmugam et al., 1993; Rebesco et al, 2007).

Initially the erosive and depositional features associated to contourites where only attributed to the deep thermo-haline circulation, it is now clear that they are formed under the complex interplay of oceanographic processes, seabed morphology, sediment supply and climate change (Van Rooij, 2013).

The Earth’s rotation tends to steer bottom currents to flow parallel to bathymetry (along the continental margins). Any topographic features such as canyons, seamounts, ridges, banks and mounds, can disrupt the flow. Once the current velocity becomes sufficiently high, the sediment erodes, and as the velocity decreases, the sediment is deposited again (Stow et al., 2002).

Contourite drifts are sedimentary bodies in the open ocean produced by the accumulation of sediment under the control of bottom currents (Stow et al., 2002; Rebesco et al., 2007; 2014). Bottom currents are influenced by a number of factors on the large scale, where LLave et al, (2015) and Rebesco et al (2016) mention many of them, like the global thermohaline circulation, the geostrophic currents, glacioeustatic changes, the tidal system, water mass interphases interacting with the seafloor, local topography, multiple sources of sediment supply.

Frequently, the palaeontological content of contourite deposits is rather similar to that of the surrounding pelagic and/or hemipelagic deposits. It usually comprises planktonic and benthic foraminifera, ostracods, and nannoplankton (Rebesco et al, 2014).

The sedimentary structures described for contourite deposits comprise ripples and cross-laminations (mainly in fine sandy deposits); lenticular and flaser beddings (fine sands, silt and clays); horizontal and sub-horizontal laminations; parallel laminations; large scale cross-stratification; erosive scarps, and grading. (Shanmugam, 1993, 2003).

Contourites occur most of the time closely interbedded with other deep-water facies. The differentiation between contourites and turbidites has been a controversial issue in sedimentology research since the 1970s (Hollister & Heezen, 1972; Bouma & Hollister, 1973).

There is no general agreement on which structures can be used to distinguish contourites from other deep-sea deposits such as fine-grained turbidites. Moreover, contourite processes can trigger certain gravity processes and formation of submarine lobes, and of course, rework previous turbiditic deposits (Shanmugam et al., 1993; Shanmugam, 2012). Therefore, contourite and turbiditic processes can be linked both vertically and laterally, and the distinction of their products is a challenge in any research. (Rebesco et al, 2014).

Deep-marine bottom currents should not be confused with turbidity currents because turbidity currents are sediment gravity flows, whereas bottom currents are not. More importantly, turbidity currents can travel only down-slope, whereas contour-following bottom currents can travel along slope (Shanmugam, 2003). Bottom-current deposits generally exhibit sharp upper contacts, whereas turbidites show gradational upper contacts (Shanmugam, 2008).

Punta Carnero Formation (Middle Eocene), located in Margarita Island, northeastern Venezuela (Figure 1), is an excellent example of a mixed turbidite/contourite system. The formation is composed mainly of fine to coarse grained, Reef-derived bioclastic carbonates, sometimes containing up to 10-30% siliciclastic material, and fine-grained carbonates composed mostly of Globigerinas and Globorotalias, all interlayered with pelagic and hemipelagic deep marine shales. Commonly, within the Punta Carnero section, bioturbation is abundant, and examples of Granularia, Paleodyction and Zoophycus can be identified (Casas & Moreno, 1986a, 1986b). The age of the formation was stablished as Bartonian, because the presence of Orbulinoides beckmanni in hemipelagic mudstones (Casas & Moreno, 1986a).

Figure 1. Margarita Island (Venezuela) and location of Punta Carnero Formation outcrops.

In Punta Carnero Formation there are two types of bioclastic limestones, one composed of shallow origin (shell fragments, Reef-algae fragments, large benthic forams (Nummulites, Lepidocyclina, Eorupertia, etc), quartz, volcanic fragments) transported by turbidity currents to the bottom of the basin and the second type composed of globigerina ooze with evidences of probable transportation by bottom currents like ripples and cross-lamination, gradation, and parallel lamination (Figures 2 and 3), Casas & Moreno (1986a).

Figure 2. Example of a calcarenite composed of globigerina shells, exhibiting ripples, cross-lamination and parallel
Figure 3. Thin section showing a contourite globigerina ooze with subtle cross lamination.

The turbidity-currents system in Punta Carnero Formation, deposited biocalcirudites, biocalcarenites, and biocalcisiltites. They alternate with pelagic and hemipelagic shales containing planktonic foraminifera and sometimes radiolaria. Commonly, there are also intercalations of reworked and sorted, biocalcarenites, biocalcilutites and biocalcisiltite oozes (Figure 4), most probably redeposited by marine bottom currents (Muñoz, 1973).

Figure 4. Thin section showing the alternance of a turbiditic layer with large benthic forams (below) and a contourite globigerina ooze (above).

The occurrence and recognition of the mixed Turbidite/Contourite System in the exposed rock record or in subsurface successions for oil exploration, has become particularly important in recent years. Thick units of sandy contourites together with bottom-current reworked sandy turbidites are potentially important as hydrocarbon reservoirs where buried in association with source rocks. (Stow et al 2002).


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