Contourites and bottom current reworked sands: Bed facies model and implications

• Published in: Marine geology Vol. 428; p. 106267
• Main Authors: de Castro, S., Hernández-Molina, F.J., Rodríguez-Tovar, F.J., Llave, E., Ng, Z.L., Nishida, N., Mena, A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2020
• Subjects: Continental slope; Contourites; Deep-water; Gulf of Cadiz; Mixed-systems; Reworking; Turbidites; Deep-water; Reworking; Continental slope; Contourites; Mixed-systems; Gulf of Cadiz; Turbidites
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/j.margeo.2020.106267

The differentiation of pure turbidites and contourites from mixed deposits —as the bottom current reworked sands (BCRS) — in sedimentary cores and outcrops from the modern or ancient records is still challenging. An accurate evaluation of facies associations calls for detailed understanding of processes controlling these deep-water systems and how they record interactions of along- and down-slope processes. Connecting features to processes is of particularly relevant in the study of contouritic drifts that include sandy deposits. This research describes and interprets BCRS within a Pleistocene contouritic drift located along the middle continental slope of the Gulf of Cadiz. Seismic, wireline, and sedimentological analyses of sediment samples were applied to core material representing a muddy drift that hosts sandy deposits. A sedimentary sequence, comprising five facies (F1 to F5) is defined here; it records background sedimentation influenced by the initial deposition of a gravity-driven flow followed by bottom current reworking. It is moderately bioturbated and contains distinctive trace fossil assemblages, often dominated by ichnofabrics of the Planolites and Thalassinoides. These assemblages represent intermittent deposition between down-slope and along-slope processes during relatively short time scales. The sequence is defined as a partial bi-gradational contourite sequence including BCRS reworked from underlying turbiditic deposits due to the interaction of down- and along- slope processes within the contouritic drift and its adjacent contouritic channel. Longer-term trends include an increase and subsequent decrease in both sediment supply and bottom currents. The sedimentary facies model presented here represents a new end member for a mixed turbiditic-contouritic system in which turbiditic flows influence a contourite drift. As such, it may support basic sedimentological interpretation and petroleum exploration strategies. [Display omitted] •Sedimentological study of the interaction between down- and along-slope processes;•A contourite drift containing reworked turbidites is described;•Turbidites are subjected to reworking and winnowing by bottom currents;•Contourite drifts are not only formed by contourite deposits;•The vertical trend in the sedimentary sequence presented records long- and short- term variation in turbidite-contourite interactions.