Neil F. Marshall

Currents in submarine canyons

Abstract Earlier work indicated that currents move alternately up and down the floors of submarine canyons with greater average speeds and longer duration downcanyon. Now we find there are roughly synchronous movements up to at least 34 m above the canyon floors. The speeds decrease somewhat with height above the canyon floors and upcanyon flows are more significant. The patterns of up- and downcanyon flow at adjacent stations usually can be matched, indicating that the currents are related to internal waves. Crosscanyon flow occurs, particularly during periods of strong crosscanyon winds and usually has a definite sequence of repetition related to the tidal cycle. The strongest and longest transverse flows were in the broadest floored canyon. During storms with strong onshore winds, there are violent downcanyon flows that have carried current meters with them and eroded the floors, but the speeds are unknown.

Current-meter recordings of low-speed turbidity currents

Turbidity currents of high velocity may occur during catastrophic conditions, but our records suggest that currents of little more than 1 knot (= 0.5 m/s) may be quite common in submarine canyons. Our three current-meter records of such currents were obtained (1) during an onshore storm at La Jolla, California, (2) during relatively high swells off the Rio Balsas Delta in western Mexico, and (3) in Abra Canyon off the Abra Delta in northwest Luzon during flood conditions. A pattern observed in all these occurences included a preceding, relatively large, upcanyon flow, a rapid buildup of the downcanyon turbidity current, a slow decay, and a following interval of no current. These currents may be particularly common where deltas have built across the continental shelf and large masses of sediment are being introduced into canyon heads. These relatively weak currents may transport large quantities of sediment down the canyon.

Currents Along Floors of Submarine Canyons

Recordings of currents along the floors of submarine canyons off California and Baja California have provided insight into the nature of their movements. The records indicate that currents of less than 50 cm/sec alternate between upcanyon and downcanyon directions at periods ranging from about 20 minutes to more than 12 hours. In general, the longer periods, some of them related to tides, are found at the deeper stations (more than 250 m), but shorter periods, probably related to internal waves, predominate at the shallower stations. In all but four of 45 recordings in canyons, the net movement was found to be downcanyon. Velocities usually are higher in downcanyon flows, and the duration of flow usually is longer. These currents have sufficient velocity during the peaks f many flows to transport sand in appreciable quantities down the canyon axes. Velocities are at least as high at the deep as at the shallow stations in the same canyon. Variation in wind and wave conditions appears to have little if any effect on canyon currents except during major storms. Fragmentary evidence indicates that more powerful currents operate at rare intervals along the canyons. A device has been installed that we hope will provide better evidence of high-speed flows.

Internal Waves Advancing along Submarine Canyons

Patterns of alternating up- and downcanyon currents have been traced along the axes of submarine canyons off California. The patterns arrive later at stations nearer the heads of coastal canyons. Where a canyon heads between two islands, the patterns advance down the axis. The propagation speeds of these patterns were estimated as 25 to 88 centimeters per second. Internal waves are the probable explanation.

A method for obtaining and analysing undisturbed oceanic sediment samples

Abstract A German coring device ( Reineck , 1958) has been improved to obtain oriented, undisturbed cores at any depth of water. The samples are rectangular in shape, 8 × 12 inches in plan and a maximum of 18 inches high. Good cores have been obtained from clayey material as well as from gravelly sand. No disturbances due to coring were observed on the collected samples. These large samples make it possible to conduct many varieties of investigations, such as study of living organisms and shear strength measurements, as soon as the sample is on deck of a ship; radiography on slices, peeling and impregnation techniques, granulometry, mineralogy, porosity, fossil content, etc. Construction and use of the box corer and applications of some of these analytical techniques are described.

Storm-generated current in La Jolla Submarine Canyon, California

Abstract Current meters were operating in La Jolla Submarine Canyon at 200 m depth during a period of high seas and onshore winds up to 62 km/h (34 knots). The meters were subsequently extracted from a kelp tangle by use of a deep-diving vehicle 0.5 km downcanyon from their emplacement position. The records show a downcanyon speed up to 50 cm/sec, considerably higher than any of our numerous earlier measurements. This was followed by an abrupt termination of data, evidently due to being engulfed in seaward-moving kelp masses. The record may provide evidence of the initial stages of a turbidity current. The conditions for such a current were provided by the piling up of water at the canyon head by the unusually strong onshore wind.

Submarine Canyon Erosion: Contribution of Marine Rock Burrowers

Rocks of the rim and upper walls of Scripps Submarine Canyon are intensely burrowed by marine invertebrates. Important excavators are bivalves, polychaetes, and sipunculoids whose activities culminate in a network of passageways and eventual disintegration of the rocks. In many localities erosion by animals is more important than erosion by physical and chemical processes.

Dives into outer Coronado Canyon System

Abstract Dives in Lockheeds submarine “Deep Quest” followed the floor of the fan valley and outer gorge of Coronado Canyon, southwest of the California—Baja California border. Both the fan valley and outer gorge have meandering courses. Basin depressions and intermittent steep descents characterize the outer gorge. No indications were found of recent activity of turbidity currents at either location, but a sand layer 30 cm below the surface is interpreted as related to a lower stand of sea level at the close of the last ice age. Abundant organisms were seen on the valley floor and in the overlying water.

Sediment waves (giant ripples) transverse to the west coast of Mexico

Abstract A 45-km belt of large symmetrical sediment waves extends along the west coast of Mexico west of Manzanillo between depths of 320 and 770 m. They are thought to be the result of a strong subsurface current that changes seasonally from southeast to northwest.

The measurement and analysis of water motion in submarine canyons

Techniques have been developed whereby it is relatively easy to acquire data on currents in submarine canyons. Savonius rotor current meters, explosive releases, and a free-fall system are utilized. The equipment, its deployment, and general conditions and problems are discussed, along with a brief description of analytical techniques.