Highlights ► A Contourite Depositional System (CDS) is present on the Demerara Plateau. ► The marginal plateau forms a bathymetry high promoting the formation of contourite. ► The formation of longitudinal waves seems to be associated to this contourite.
he IGUANES cruise took place in May 2013 on the R/V L'Atalante along the Demerara passive transform margin off French Guiana and Surinam. Seismic, multibeam and chirp acquisitions were made. Piston cores were collected for pore geochemistry and sedimentology. A mooring was deployed on the sea-bottom for 10 months (temperature, salinity, turbidity and current measurements). This new dataset highlights the lateral variability of the 350 km-long Guiana-Surinam transform margin due to the presence of a releasing bend between two transform segments. The adjacent Demerara Plateau is affected by a 350 km-long giant slide complex. This complex initiated in Cretaceous times and was regularly reactivated until recent times. Since the Miocene, contourite processes seem to be active due to the onset of the North Atlantic Deep Water (NADW) bottom current. A main NADW water vein flows towards SE, eroding slide headscarps and allowing the deposition of contourite drifts. Numerous depressions looking like comet tails or comet scours record this flow. Some of those were interpreted before the cruise as active pockmarks. Pore geochemistry and core analysis do not show any evidence of present-day gas seepage.
The Demerara Plateau is a marginal plateau which forms a bathymetric relief on the sea floor. Here, contourite deposits have been studied in detail, following the recent discovery of contourite sequences likely related to the bottom currents and linked both to contour current and peculiar sea-floor morphology. A chronostratigraphic framework, based on δ18O relative variations and palaeomagnetic events in sediment cores allows correlating sediment processes to current intensity changes and major climate phases (glacial or interglacial). The studied sediments are enriched in glauconitic grains. In addition, the glauconite mineralogical maturity can easily correlate to low sedimentation rate and slightly energetic bottom currents on the seafloor. Based on these data and using the glauconitic authigenic mineral as proxy for inferring the degree of winnowing at the sediment-water interface, we might put forward the hypothesis that the intensity of NADW is higher during the glacial stages and lower during interglacial periods.
Highlights► A contourite depositional system is present on the Demerara Plateau. ► The winnowing effect induced by bottom current allows formation of authigenic mineral. ► The glauconitic content and maturity of glauconitic grains are indicator of the current intensity.
The Demerara Plateau, off the French Guiana and Surinam margin, corresponds to a prominent subhorizontal marginal plateau. The interpretation of a large set of seismic data acquired along the Demerara Plateau and integration with previous seismic stratigraphy and drill studies enables a better understanding of the sedimentary evolution of the outer Demerara marginal plateau. Since the end of the transform activity (late Albian), three sedimentary evolutionary stages have been identified. 1) The Pre-contourite Stage (late Albian to early Miocene) is strongly controlled by the structure of the margin that influences the distribution pattern of post-transform deposits through differential thermal subsidence between transform and divergent segments. 2) The Transitional Stage (middle Miocene to early Pliocene) is expressed as a current-controlled erosive surface resulting from the onset of bottom-current interactions due to the establishment and strengthening of the modern thermohaline circulation that is coeval with the progressive closure of the Isthmus of Panama. 3) The Contourite Stage (middle Pliocene to Holocene) is characterized by the development of a contourite depositional system that is linked to the activity of the North Atlantic Deep Water circulation and by a complex interaction between alongslope and downslope processes. Our study highlights that marginal plateaus might be particularly prone to develop contourite features since they regionally influence ocean current dynamics in guiding and accelerating bottom contour currents. Marginal plateaus should thus be considered as a key target to study bottom current dynamics and paleocirculation patterns. Highlights ► The post-transform sedimentation of the Demerara marginal plateau (DP) is studied. ► A strong relation between topography, downslope and alongslope processes shapes DP. ► Marginal plateaus are particularly prone to develop contourite deposits.
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