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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 49 (1965)

Issue: 3. (March)

First Page: 338

Last Page: 338

Title: Bathyscaph Observations in the La Jolla Submarine Fan Valley: ABSTRACT

Author(s): Robert F. Dill

Article Type: Meeting abstract


Observations, made from the bathyscaph Trieste during six dives to depths ranging from 1,800 to 3,000 feet, reveal that submarine erosion is actively modifying a series of step-like terraces forming the internal walls of the La Jolla Submarine Fan Valley. The innermost terrace is cut by a narrow, steep-walled, flat bottomed channel that forms the longitudinal axis of the valley. Beneath a thin mud cover, the channel contains sand and plant fill that is entirely different from that found on or forming the internal terraces. The steep slopes (up to 70 degrees), found where the innermost terraces lead down to the inner channel, have slump scars, striations, and fresh burrows that indicate they are presently being eroded by marine processes. Terrace sediments are bedded and s mi-consolidated and do not appear to have been deposited in the present erosional environment of the canyon. On one of the dives, large rounded rock boulders, up to 3 feet in diameter, were found scattered through the interbedded sand, mud, and plant material found in the inner channel. The nearest possible source for these large erratics is more than one-fourth of a mile from their present location.

The following characteristics are arguments against dense, high-velocity turbidity currents as agents of erosion or transportation in the present day La Jolla Fan Valley: (1) the lack of scour depressions around large man-made objects found in the sands of the inner channel, (2) the sinuous course of the inner channel, (3) a low axial gradient, (4) a lack of inner channel sediment on terraces 10 feet above the channel bottom, and (5) the heterogeneous mixture of fragile sea-grass mats, large boulders, and micaceous sands.

Pulsating bottom currents with velocities up to 0.45 knots have been measured. These currents were observed to have sufficient strength to transport fine micaceous sand and unconsolidated clay-sized particles along the bottom of the inner channel. However, the large boulders associated with sands and organic debris, foreign to the surrounding sea floor, must have been transported by another mechanism; gravity creep and progressive slumping of the entire fill of the erosion channel are suggested agents.

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Copyright 1997 American Association of Petroleum Geologists