Submarine Canyons 



217 



aggregates, although some larger clear crys- 

 tals are within foraminiferal tests. The 

 cyhndrical shape of many concretions sug- 

 gests that water containing dissolved barium, 

 probably as the chloride, moved in tubular 

 channels through the fine-grained sediment. 

 Some thin flat pieces composed of coarser 

 grains of plagioclase, glauconite, and fora- 

 miniferal tests were probably cemented by 

 movement of the solution through the more 

 permeable beds. It is believed that the 

 baritic solution came from magma at an un- 

 known depth and rose along the fault plane 

 that produced the basin slope. On contact 

 with the sulfate-containing interstitial water 

 of the sediments that cover the fault trace, 

 barite was precipitated in concretionary form. 

 Similar barite concretions have been found 

 elsewhere on the sea floor, notably off" Ceylon 

 and in the East Indies, in both instances on 

 steep slopes of probable fault origin. Barite 

 in the form of larger bladed crystals in veins 

 as thick as 1 meter is well known from many 

 exposures along the south side of Palos 

 Verdes Hills where it is clearly of magmatic 

 origin and is associated with a large fault 

 zone. 



Minor features are present in abundance 

 on slopes, as shown by photographs (for ex- 

 ample. Figure 42). Many of the smafler ir- 

 regularities are produced by burrowing 

 activities of animals; others may be effects 

 of small local slides. Folds and crumplings 

 associated with larger slides may be present 

 but are concealed under later sediments; the 

 uniformity of the sediment and the small 

 diameter of core barrels prevent their detec- 

 tion by coring operations. Still larger slides 

 have left scars that are detectable only by 

 sounding surveys (Fig. 43). Some of these 

 features are doubtlessly capable of preser- 

 vation in the geological record. Features 

 probably of related origin have been des- 

 cribed by Fairbridge (1946), Rich (1950), 

 Kuenen (1953^), and Crowell (1957) from 

 various parts of the world. Others are 

 known in the sedimentary rocks of Ventura 

 Basin (Natland and Kuenen, 1951), where 

 some of them represent ancient slope envir- 

 onments, although most are probably from 

 the former basin floor. Similar features 



formed on the continental slope are prob- 

 ably rarely if ever brought to view in out- 

 crops on land. 



Submarine Canyons 



Sediments of submarine canyons occupy 

 an even more restricted environment than 

 do those of slopes. Only one general tex- 

 tural study has been made, by Cohee (1938) 

 on surface samples obtained by Shepard 

 from many canyons. Also, only a single 

 detailed study of sediments in an individual 

 canyon is available, one by Inman (1950) 

 for the shallow part of Mugu Canyon. All 

 other data are incidental to work on organic 

 matter (Trask, 1931), beaches (Marlette, 

 1954), basin sediments (Ludwick, 1950; 

 Gorsline, 1958), or shelf sediments (Terry, 

 Keesling, and Uchupi, 1956). As a result 

 of this heterogeneity of source data, the 

 canyon sediments are imperfectly known, 

 particularly with respect to parameters other 

 than texture. Even so, some generalizations 

 can be formed (Table 12). At the heads of 

 canyons that reach close to shore sediments 

 are coarse-grained and similar in other re- 

 spects to the adjacent beaches. In depths 

 of 100 or 200 meters in the heads of at least 

 some of these same canyons (Mugu, 

 Redondo, Scripps, and La Jolla), sediments 

 that are unusual in the marine environment 

 have been discovered. These are black, 

 smell of hydrogen sulfide, and contain 

 masses of decomposing plant debris, mostly 

 of kelp and surf grass. Clouds of methane 

 bubbles have been observed rising from this 

 sediment. Canyons whose heads are far 

 from shore, such as Santa Monica, San 

 Pedro, and San Gabriel, are floored by 

 sandy green muds. In both kinds of can- 

 yons the surface sediments obtained farther 

 down the canyon axes are more or less 

 progressively finer-grained and grade into 

 normal basin muds. Sediments on the side 

 slopes are green muds like those on the usu- 

 ally less steep basin slopes. 



Core samples in canyons show two dis- 

 tinct kinds of sediment at depth. On the 

 canyon walls the few available cores reveal 

 little change with depth of burial — again 



