Deep Bottom Currents Research Articles

Geomorphological features in the southern Canary Island Volcanic Province: The importance of volcanic processes and massive slope instabilities associated with seamounts

The margin of the continental slope of the Volcanic Province of Canary Islands is characterised by seamounts, submarine hills and large landslides. The seabed morphology including detailed morphology of the seamounts and hills was analysed using multibeam bathymetry and backscatter data, and very high resolution seismic profiles. Some of the elevation data are reported here for the first time. The shape and distribution of characteristics features such as volcanic cones, ridges, slides scars, gullies and channels indicate evolutionary differences. Special attention was paid to recent geological processes that influenced the seamounts. We defined various morpho-sedimentary units, which are mainly due to massive slope instability that disrupt the pelagic sedimentary cover. We also studied other processes such as the role of deep bottom currents in determining sediment distribution. The sediments are interpreted as the result of a complex mixture of material derived from a) slope failures on seamounts and submarine hills; and b) slides and slumps on the continental slope.

Tectonic and oceanographic control of sedimentary patterns in a small oceanic basin: Dove Basin (Scotia Sea, Antarctica)

AbstractDove Basin, a small oceanic domain located within the southern Scotia Sea, evidences a complex tectonic evolution linked to the development of the Scotia Arc. The basin also straddles the junction between the main Southern Ocean water masses: the Antarctic Circumpolar Current (ACC), the Southeast Pacific Deep Water (SPDW) and the Weddell Sea Deep Water (WSDW). Analysis of multichannel seismic reflection profiles, together with swath bathymetry data, reveals the main structure and sediment distribution of the basin, allowing a reconstruction of the tectonostratigraphic evolution of the basin and assessment of the main bottom water flows that influenced its depositional development. Sediment dispersed in the basin was largely influenced by gravity‐driven transport from adjacent continental margins, later modified by deep bottom currents. Sediments derived from melting icebergs and extensive ice sheets also contributed to a fraction of the basin deposits. We identify four stages in the basin evolution which – based on regional age assumptions – took place during the early Miocene, middle Miocene, late Miocene–early Pliocene and late Pliocene–Quaternary. The onsets of the ACC flow in Dove Basin during the early Miocene, the WSDW flow during the middle Miocene, and the SPDW during the late Miocene were influenced by tectonic events that facilitated the opening of new oceanic gateways in the region. The analysis of Dove Basin reveals that tectonics is a primary factor influencing its sedimentary stacking patterns, the structural development of new oceanic gateways permitting the inception of deep‐water flows that have since controlled the sedimentary processes.

Rates of Burial and Disturbance of Experimentally-Deployed Molluscs: Implications for Preservation Potential

Rates of burial and transport of molluscan remains are essentially unknown for deeper continental shelf and slope environments, especially over periods of years. An understanding of the rates of taphonomic loss are critical to paleoecological analyses and to paleoenvironmental studies in general. The post-depositional history of organic remains is highly dependent on the length of time the material remains at or near the sediment/water interface. In order to measure these rates, 100 gastropod and bivalve shells were scattered over a marked area of sea bottom at 21 sites in seven environments of deposition (EOD9s) in the Gulf of Mexico and at five EOD9s on the Bahamas platform edge. A total of over 2600 shells were deployed. Each site was thoroughly documented with video photography. After one year in the Bahamas and after two years in both the Gulf of Mexico and Bahamas, these sites were re-photographed and video-taped to measure rates of burial and movement of shells. Shell condition (e.g., articulation, encrustation, and color loss) for those shells that remained exposed was also determined. Shells deployed in Gulf of Mexico petroleum seep sites, on the open continental shelf, and on the continental slope experienced high rates of burial (0.5-3.0 cm) within two years. Shells at these sites generally were not transported or disturbed, and disarticulation rates were low. In the Bahamas, shells on the platform shelf were completely buried within one year. On the steep platform edge from 70 to 300 m, shells on hardground ledges remained exposed, whereas shells in carbonate sands were buried by up to 3.5 cm of sediment. Transport was more common on the steep slopes of the platform edge. Net sedimentation rates for the outer continental shelf and slope of 0.01-0.06 cm yr-1 are well below our observed burial rates of 31 cm yr (super -1) . Thus, burial rate may be somewhat independent of sedimentation rate due to local reworking of sediments by storms at shallower depths and mechanisms such as deep bottom currents or bioturbation at deeper sites. Therefore, the potential for fossil preservation in offshore areas with low sedimentation rates may be much greater than previously assumed.

Contourite drift types and their distribution in the North and South Atlantic Ocean basins

Patterns of sediment accumulation under the control of deep bottom currents depend on interacting factors: intensity of the regional deep circulation, morphology of the sea-floor, Coriolis effects, abundance of terrigenous and biogenic supplies, subsequent turbidity of deep nepheloid layers, and depth of the CCD. Based on many examples of bottom current deposits, from the Atlantic and from other oceans, we recognise three major types of contourite accumulation: (1) giant elongate drifts, (2) contourite sheets, and (3) channel-related drifts. They generally occur in distinct topographic settings and show different geometries, facies and rates of deposition. In the North Atlantic, strong bottom circulation, an open basin system and high sediment supply have led to the construction of large elongate contourite drifts, which have striking relief (200–1000 m) above the surrounding sea-floor and are composed of muddy contourites, rich in calcareous pelagic material. In the South Atlantic, strong bottom-currents are often trapped in gyre-like circulation patterns around topographically closed basins or are accelerated through deep, narrow, connecting passages. The resulting deposits are mainly extensive contourite sheets, covered by fields of large sediment waves, and channel-related drifts, including “contourite fans” constructed at the downstream exit of deep passages.

Deep-Water Bottom-Current Reworked Sands: Their Recognition and Reservoir Potential, Northern Gulf of Mexico: ABSTRACT

Some Pliocene and Pleistocene reservoir sands in intraslope basins of the northern Gulf of Mexico exhibit features that are interpreted to be indicative of reworking by deep-water (bathyal) bottom currents (contour currents). These fine sands have previously been interpreted as turbidites associated with levee overbanks and lobes of submarine fan complexes; however, sedimentological features characteristic of turbidites are rare in these laminated sands. Common features observed are (1) numerous (up to 75 layers/m) thin (<2 cm) sand layers, (2) sharp upper contacts, (3) inverse grading, (4) current ripples, (5) lenticular bedding, (6) flaser bedding, (7) bidirectional cross-lamination, and (8) sigmoidal bed forms with mud drapes (i.e., mud offshoots). These features, dominated by traction structures, can be explained by reworking of overbank turbidite sands by deep-bottom currents. In the present Gulf of Mexico, the surface Loop Current is considered to be a major cause of deep circulation. The authors propose that similar bottom currents and perhaps minor deep tidal currents existed during Pliocene and Pleistocene times. The entire sediment package may be thick and continuous, but individual sand layers within the package are thin and discontinuous. Porosity values of these sands range from 27 to 40%, and permeability ranges from 100more » to 2,000 md at 200 psi. In seismic profiles or seismic facies maps, it is difficult to distinguish the reworked sands from turbidites. Therefore, geologic models based on core and process sedimentology are the key to a better understanding of these often misinterpreted complex reservoir facies.« less

Deep bottom currents off the coast of North Carolina

The direction of bottom currents and their extent from the seaward margin of the continental shelf (300 m) to the Hatteras Abyssal Plain (5335 m) off North Carolina have been determined on the basis of 150 multishot bottom camera lowerings representing approximately 3000 compass-oriented photographs. Correlative data were obtained from sediment samples, submarine topography studies, and 55 hydrographic samples from the bottom water. There are three major currents on the deep-sea floor : the Gulf Stream with a marked seasonal variation in location, the Western Boundary Undercurrent which may split into sections,and one derived from Antarctic Bottom Water. Zones of no motion are identified between each of the three.