Bottom-current-controlled sand deposits — a review of modern shallow- to deep-water environments

Abstract

Different examples of modern marine-sand accumulations generated or strongly influenced by the action of bottom currents, are here presented. They are drawn from a variety of tectonic and morphological settings and grouped into three water-depth zones: deep-water (>2000 m), mid-water (300–2000 m), and outer-shelf/upper-slope (50–300 m). Deposits in the first two of these depth zones are normal contourites, according to their original definition (Heezen and Hollister, 1971) being those sediments that have been transported and deposited by contour currents in deep-water environments. Those deposited at shallower depths, under the influence of surficial geostrophic currents combined with other hydrodynamic factors (shelf currents induced by wind, tide and waves, gyres, internal waves, etc.), are more properly referred to as outer-shelf/upper-slope bottom-current sands (or shallow-water bottom-current sands). We have elaborated a facies model for each bathymetric zone. Deep-water sandy contourites are relatively rare, thin- and very thin-bedded, highly bioturbated and mainly of bioclastic composition. They are interbedded with muddy contourites and pelagites or, in some areas, with turbidites. In the latter case, thin bottom-current-reworked, sandy tops of turbidites provide a different and distinct facies. Mid-water sandy contourites are more common, ranging up to a metre in thickness, and may form extensive sandy sheets in a variety of slope, bank and channel settings. They are mainly of mixed siliciclastic—bioclastic composition, typically bioturbated, and associated with muddy/silty contourites in coarsening-up/fining-up complete or truncated sequences. Shallow-water bottom-current sands occur in particular outer-shelf/upper-slope settings, where they may develop relatively thick (1–20 m), laterally extensive sheets covered by fields of sandwaves, megaripples and ribbons. Internal structures may be preserved along with much bioturbation. Their composition varies from mainly siliciclastic to bioclastic, and they may be interbedded with both inner-shelf facies and slope hemipelagites. The principal factors that control the deposition of sandy contourites and shallow-water bottom-current sands are the hydrodynamic regime of the basin, the availability of coarse-grained (sandy) sediments and the physiographic context of the area swept by the currents. The greater the depth, the finer and rarer the bottom-current or sandy contourite deposits. Global sea-level and climatic changes and the time involved in the depositional history play an ultimate role in the development of important sand accumulations of this sort by controlling the ocean-circulation pattern and its long-term persistence. From the present analysis, we conclude that mid-depth sandy contourites are the most commonly found in modern environments, and that shallow-water bottom-current sands constitute the most significant potential oil reservoirs to be found in the geological record.