Basins and Troughs 



227 



slopes on the mainland side of the basins are 

 aprons of hummocky character (Fig. 43) 

 which are believed to consist of debris 

 dumped there by mass movements from the 

 upper parts of the slopes (Emery and Terry, 

 1956). Many cores from the aprons contain 

 sections that are highly contorted and have 

 some mud clasts, both of which features are 

 considered indicative of deposition from 

 mass movements. Some cores from the base 

 of the slope north of Santa Catalina Island 

 show features that are similar but far less 

 common than in cores from the base of the 

 mainland slope. Beyond the aprons and in 

 areas marginal to the subsea fans, many 

 cores have silt layers, counterparts of sand 

 layers on the fans and basinward of the fans. 

 A schematic representation of the various 

 core features and their areal distribution 

 (Fig. 194) shows a definite relationship to 

 submarine canyons and marginal slopes. 

 The most reasonable agency for depositing 

 the coarse-grained layers is believed to be 

 turbidity currents that develop from mass 

 movements of nearshore sediment at the 

 heads of submarine canyons or down the 

 upper parts of basin slopes, as will be dis- 

 cussed more fully in a later section. 



Support for turbidity currents is provided 

 by the common upward decrease of grain 

 size of sands within many individual layers, 

 as though in response to slower settling of 

 finer grains carried by turbidity currents 

 after most of their coarse load had been de- 

 posited. Some of the layers grade into silt; 

 others have sharply defined top surfaces of 

 sand. Commonly, charcoal-like sticks, twigs, 

 and leaves are also incorporated into the top 

 parts of the graded layers. Near the mouths 

 of canyons some of the sands are banded or 

 laminated as though deposited at intervals 

 too short for accumulation of green muds 

 which elsewhere intervene between the 

 sands. 



Additional examples of abundant deep- 

 water sands are provided in the San Diego 

 Trough at the mouths of La JoUa and Coro- 

 nado Submarine Canyons (Fig. 195). The 

 Pliocene shales of the Los Angeles Basin also 

 contain many sand layers, particularly in the 

 middle area of originally deep water and to 



BASIN FLOOR 



I I OLIVE CLAY- SILT |=^ | T.RAY SILT | " "| FINE GRAY SAND 



|:y;;'v:>;.^ MEDIUM GRAY SAND |^;'^9'/j SLUMPED SILT 



Figure 194. Schematic representation of typical sedi- 

 mentary features in basin sediments. From Gorsline and 

 Emery (1959, Fig. 5). A. canyon-fan-basin sequence; 

 B, slope-apron-basin sequence. 



the northeast of it (Slosson, 1958; Conrey, 

 1959). Still other deep-water sands are well 

 known in the Pliocene shales of Ventura 

 Basin where they have been described by 

 Natland and Kuenen (1951) and Baldwin 

 (1959). The areal distribution and minor 

 characteristics of the sands in both of these 

 ancient basins correspond very closely with 

 those of the modern basins and doubtlessly 

 were produced by similar processes. 



E. Organic matter. Trask (1932) and 

 Emery and Rittenberg (1952) found that 

 California basin sediments contain large per- 

 centages of organic matter as compared with 

 most other marine sediments, values locally 

 exceeding 10 per cent by dry weight of whole 

 sediment. Determinations of nitrogen by 

 Kjeldahl analysis reported by Emery and 

 Rittenberg, supplemented by a nearly equal 

 number of additional analyses, 350 alto- 

 gether, are given in Figure 196 for both sur- 

 face and subsurface samples. These nitrogen 

 values may be converted to total organic 

 matter by multiplying by the factor 17, as 

 will be discussed more fully in a later section 

 on composition of organic matter. 



