Charles D. Hollister

Shaping of the Continental Rise by Deep Geostrophic Contour Currents

Geostrophic contour-following bottom currents involved in the deep thermohaline circulation of the world ocean appear to be the principal agents which control the shape of the continental rise and other sediment bodies.

Deep-sea current evidence from abyssal sediments

Abstract Relatively strong bottom currents in the deep-sea can be inferred from sedimentary structures observed in bottom photographs and cores. The majority of photographs of high and steep topographic prominences, such as seamounts, escarpments and the crests of major ridges, show ripples, scour and rock outcrops. Although photographic current evidence is uncommon in the deeper waters of the ocean basin floor, striking and significant examples do occur. Current scour and ripples are observed beneath the Antarctic bottom current in the western South Atlantic, below the Gulf Stream in the Florida Straits and on the Blake Plateau, beneath the outflowing Mediterranean water west of Gibraltar and beneath the deep currents in the Drake Passage. The three types of deep-sea sands and silts are turbidite, accretionary, and residual. The latter two types are always associated with currents and in some areas turbidites are reworked by bottom currents. Traction velocities necessary for the transport of deep-sea sediment particles probably range from 4–60 cm/sec, velocities similar to those found through dynamic computations of geostrophic currents and observed by recent deep-sea direct current measurements.

Further evidence of contour currents in the Western North Atlantic

A study of compass-oriented sea-floor photographs, echograms and sediment cores on the Atlantic continental margin of North America has been made in order to evaluate the role of deep-sea contour currents in the shaping of the continental rise. n nThe sediments on the upper continental rise consist of lutites which are being deposited on the sea floor in an environment devoid of strong bottom currents. Below an abrupt change in regional slope, that marks the boundary between the upper and lower continental rise, a swift bottom current is observed which flows to the southwest parallel to the contours. Beneath this current the surface sediments are distinctly coarser grained and long cores show many quartz silt laminations in the sedimentary column. Further downslope on the lower continental rise the currents are variable in direction and weaker. Measurements of northerly directions indicate that at certain locations the Gulf Stream may intermittently scour the sea floor. In the area of the Lower Continental Rise Hill a swift southwesterly current is again observed. Hyperbolic echo traces on echograms, prolonged multiple echo sequences, and wedging of sub-bottom reflecting interfaces can be mapped as distinct zones. These zones of sea floor micromorphology parallel the regional contours of the continental rise, and are produced by erosional and depositional processes of bottom currents. We conclude that the continental rise is a large sediment wedge which owes its shape to deep geostrophic contour currents.