Robert F. Dill

Physiography and Sedimentary Processes of La Jolla Submarine Fan and Fan-Valley, California

The depositional environments of La Jolla canyon, fan-valley, and fan are well known from closely spaced sounding lines, deep-diving vehicle observations, numerous undisturbed box cores, and continuous reflection profiles. The narrow rock-walled canyon changes seaward at 300 fm (549 m) to a wider valley cut into the compacted clayey sediments of a fan, and bordered by discontinuous leveelike embankments. The fan-valley merges gradually into the relatively flat floor of San Diego trough. Numerous dives into the fan-valley have shown precipitous walls along the outside of the bends of the winding channel. Slumping is taking place actively from these walls and large slump blocks of clay are common on the floor. Small scour depressions around isolated erratics suggest the ero ive effect of relatively weak currents in some places but, for the most part, the muddy floor seems to have been little disturbed in recent years. Diagonal tension cracks cut the floor locally. Box cores show that most of the sediment deposited on the valley floor in the past few thousand years consists of poorly sorted clayey silt, underlain by discontinuous layers of well-sorted fine-grained sand with a few coarse sand grains, gravel, and mud balls. Sand layers occur in 94 percent of the valley axis cores, of which 26 percent are graded; 59 percent have parallel laminations; and 41 percent have current-ripple cross-laminations. Sand layers are less common in the cores from levees and from the small discontinuous terraces along the sides of the fan-valley. Cores from the open fan have less and finer grained sand. In all these environments the sand shows no consistent or systematic grain-size variation with increasing water depths. Some of the coarsest sediments, including g avel and mud balls, are found in sand farthest from shore and at the greatest depths. The character of the sand and the finding of shallow-water Foraminifera indicate the probability that sand has been carried from the coastal area along the valley axes and spilled over the levees onto the open fan. However, there is little evidence of recent high-velocity, high-density turbidity currents, because, in general, the covering mud layer is distinctly separated from the underlying sand deposits, and therefore does not suggest deposition at the terminus of a turbidity current. Also, the discontinuous character of the sands and series of laminae with heavy mineral concentrations indicate introduction by a traction type of pulsating current, such as has been seen during vehicle dives, and also has been measured in the few available current-meter records. The locally precipitou fan-valley walls and outcrops of gravel, and the sand layers on the levees and open fan, may be the product of stronger currents that moved down the valley during earlier more pluvial periods, when greater quantities of sediment entered the canyon heads. Possible confirmation of this idea comes from the available C-14 dates in plant layers, which suggest that deposition in the past few thousand years may have been considerably slower than that indicated for the Pleistocene. The finer sediments may be largely the result of slow downslope movement of slightly higher density muddy waters coming from the coastal areas. Continuous reflection profiles have shown that the inner La Jolla fan has only a thin cover of unconsolidated sediments overlying the folded and faulted Miocene-Pliocene rocks. The outer fan and adjacent San Diego trough contain a thick section (more than 1,000 m) of Quaternary sediments with probable buried older channels and possible thick lenses of sand sediments.

Diapiric Intrusions in Foreset Slope Sediments Off Magdalena Delta, Colombia

The Magdalena River of Colombia is prograding its delta rapidly onto the slope that leads north into Colombian basin of the Caribbean. Results of this encroachment are intermittent slope failures that break the jetties at the river mouth and sever the cables on the sea floor outside. Another possible result of the delta encroachment was indicated from the work of a Scripps Institution expedition which led to the discovery of steep-sided domes rising as much as 200 m above the sea floor in water depths of 900-1,200 m. Subbottom profiling showed that these domes, about 1 km wide, have intrusive relations to the foreset beds and can be traced down to at least 700 m below the bottom. The profiles indicate that the domes generally are flanked with ring depressions, which are u derlain by downbent strata, in contrast to the usual upbending of strata flanking salt domes. One explanation for these diapirs is that they are giant mudlumps somewhat akin to those off the passes of the Mississippi. Alternatively, they may be mud volcanoes such as are found on the adjacent lands.

Currents in submarine canyon heads off north St. Croix, U.S. Virgin Islands

Abstract Currents in the heads of two canyons off the north shore of St. Croix, U.S. Virgin Islands are characterized by slow velocity with high-frequency alternations of direction and occasional periods of relatively strong downcanyon flows; the latter occurred during falling tides. The downcanyon flows appear to be density driven by pulses of saline water coming out of either the estuary or harbor located shoreward of the canyon heads.

A diver-held vane-shear apparatus

Abstract An inexpensive instrument, consisting of a small vane attached to the shaft of a calibrated torque screwdriver, has been developed for measuring the shear strength of sea-floor sediments. It can be used either in situ or on sediment samples removed to the ship or laboratory.

Role of Multiple-Headed Submarine Canyons, River Mouth Migration, and Episodic Activity in Generation of Basin-Filling Turbidity Currents: ABSTRACT

The initiation mechanism and source of turbidite sediments to fill both recent and ancient offshore basins can be explained by examining nearshore submarine canyons like those off the Magdelline River, Columbia, and Rio Balsas, Mexico. To generate turbidity currents, large amounts of sediments of mixed grain size must first be stored as a stable deposit in the upper reaches of a canyon, then some mechanisms must create instability and set the entire deposit into motion. Canyons which head at deltas respond to the pulsating sources of sediment and the migration of the river mouth from one area to another. There is a flip-flop in the processes active in the canyon heads from one of erosion when the head is proximal and a large amount of coarse sediment enters directly into he canyon, to one of deposition of fine-grained sediment when the canyon head is distal and processes quiescent. During the distal stage, fine-grained cohesive sediments build up forming V-shaped profiles. The walls literally grow together. The migration of the river mouth back to the vicinity of a formerly quiescent distal canyon head will introduce coarse-grained sediments and reinitiate submarine erosion of the poorly consolidated canyon fill. Erosion forms steep unstable slopes and progressive slumping, creating the mechanism for generating a turbidity current with a large volume of poorly sorted driving sediment. End_of_Article - Last_Page 918------------

Pleistocene Sea-Level Fluctuations off Southern California and Their Relation to Continental Slope Sedimentation: ABSTRACT

Fluctuations of sea level as much as 700 ft during the late Pleistocene have modified the continental margin of southern California and exercised a dominant control on the offshore sediment-distribution patterns. Narrow terraces and low sea cliffs were cut into bedrock during lowered sea stands. They are associated with anomalous occurrences of sand-algal nodules, concentrates of shallow-water shells, phosphorite, and rounded cobbles of mixed rock type. The rock outcrops exposed in the low sea cliffs provide a window that permits sampling and the mapping of bedrock End_Page 457------------------------------ structures from submersibles in an area normally covered with prograding slope deposits. The concentration of relict shallow-water fossils in deep terrace zones and the rough topography can be confused with nonexistent structural features if not recognized during sampling programs. End_of_Article - Last_Page 458------------