The Contourite Sequence: A 21st Century Update

Citation

Stow D, Smillie Z, Jiawei P, Brackenridge R & Bankole S (2018) The Contourite Sequence: A 21st Century Update. The 57th British Sedimentological Research Group (BSRG) Annual General Meeting, Edinburgh, 17.12.2018-20.12.2018. http://www.lyellcentre.ac.uk/news/bsrg.html

Abstract
The past decade, in particular, has seen a greatly renewed interest in contourite systems – modern, ancient and subsurface. IODP Expedition 339 to the Cadiz Contourite Laboratory along the Iberian margin in 2012 recovered over 4.5km of contourites. Current work in the Alboran Sea has further extended understanding of contourites and associated facies. Ancient contourites in outcrop have been well-documented in the Rifean Gateway in Morocco, the Lefkara Drift in Cyprus, and the southern margin of the Ordos Basin, China Miocene-age. Hybrid contourite-turbidite systems are under scrutiny from industry data offshore west and east Africa, and offshore Brazil. The plethora of studies on these sediments have provided a rigorous test for the contourite paradigm and fully endorsed the primary role of alongslope bottom currents in continent margin sedimentation globally. However, considerable debate still surrounds contourite identification and distinction from other deepwater facies. The bi-gradational sequence model is found to be the dominant expression of contourite sedimentation in muddy and mixed muddy-sandy contourite drifts. Statistical analysis of these sequences indicates a mostly irregular sub-Milankovitch cyclicity of between 4,000-12,000 years. This periodicity is most likely due to either an ocean-resonance or sediment-supply control. Evidence from compositional, textural and structural attributes of the sequences implies that a combination of two principal controls has caused individual bi-gradational sequences: (a) variation in bottom-current velocity, and (b) episodic lateral supply from downslope turbidity currents. A third control, that of surface productivity leading to excess pelagic fallout of biogenic material, is found to be less significant. These muddy, silty and muddy-sand contourites are very uniform in their colour, siliciclastic-bioclastic composition, lack of primary sedimentary structures, and an intense bioturbation throughout with a distinctive, small-scale, monotonous ichnofacies and local omission surfaces. The bi-gradational grain-size variation is closely mirrored by other proxies, including mineralogical, geochemical, organic, biogenic, and microfabric. Individual sequences can be correlated over at least 5 km across the drift surface. By contrast, sand-rich contourite facies, which occur within and proximal to oceanic gateways, in contourite channels and plastering contourite terraces, show less distinctive, more-block-like sequences of alternating sand-mud facies. The sandy contourites show clear evidence of parallel-lamination and cross-lamination, with sets from 5cm to 1m in thickness. Composition, geochemistry, and ichnofacies proxies all mirror grain-size variation. Detailed grain-size study reveals a marked trend for deposition of fine muddy to coarser sandy contourites that indicates deposition from uniform suspension at low bottom-current velocity (< 10 cm/s), suspended load with progressive winnowing at increasing velocity (up to about 50 cm/s), and the onset of more dominant tractional bedload transport at higher velocities (>50 cm/s). Distinction from the deposits of other deepwater processes, including turbidity currents, remains difficult. It can rarely be achieved by simple visual examination of facies, but requires a more thorough three-scale approach.

Notes
57th BSRG Annual General Meeting 2018 ; Conference date: 17-12-2018 Through 20-12-2018