Mainland Shelf 



207 



rock outcrops. Still a third kind of relict 

 sediment is a dark-gray-to-black silt under- 

 lying red sand off Seal Beach at a depth of 

 13 meters. Hydrogen sulfide is present along 

 with fibrous organic debris. R. E. Steven- 

 son (personal communication) considers that 

 this is probably a submerged marsh deposit. 



Areas of nondetrital sediment (neither 

 present nor ancient) are largely restricted to 

 low hills that rise above the general level of 

 the shelf and thus in a sense are islands sur- 

 rounded by a flood of ancient and modern 

 detrital sediments. Good examples exist in 

 Santa Monica Bay, in San Pedro Bay, and 

 off" San Diego. Some nondetrital sediments 

 also occur at or near the shelf break where 

 the slope is so steep that fine-grained detrital 

 sediments bypass. These nondetrital sedi- 

 ments have three origins, residual, organic, 

 and authigenic. Commonly, nondetrital 

 sediments have all three components mixed 

 together, but in a few areas more or less pure 

 single-origin sediment exists. 



Sediments believed to be of residual origin 

 are present at many places on the mainland 

 shelf but are invariably present where bed- 

 rock is exposed on the bottom. They can 

 be recognized by their poor sorting, angu- 

 larity, and mineralogic similarity to the un- 

 derlying or nearby rocks. However, recog- 

 nition is not as simple as for residual 

 sediments on land because on the sea floor 

 these materials have been modified to some 

 extent by water movements which have 

 tended to winnow away the finer debris and 

 to introduce new material. As a result there 

 exists a broad gradation between true resid- 

 ual and true relict sediments that introduces 

 an uncertainty of personal judgment in as- 

 signing a given sediment to a residual or a 

 relict origin. For this reason, the areas of 

 either type might be greater or less than 

 shown by Figure 180. 



Organic sediment is much simpler. On 

 the mainland shelf it consists chiefly of 

 broken and corroded shell fragments mostly 

 of pelecypods and gastropods. The grain 

 size is coarse and the sorting poor. Fora- 

 miniferal tests are included but serve only 

 as a minor constituent, in contrast to the fact 

 that they are more abundant than shell de- 



bris on most island shelves and bank tops. 

 Mixtures with modern detrital, relict, resid- 

 ual, and authigenic sediments are usually 

 present, depending on the origin of the 

 nearby sediments, but a large area of nearly 

 pure shell sand occurs at the shelf break just 

 seaward of Los Coronados Islands off" San 

 Diego (Fig. 180). 



Authigenic sediments are of two types, 

 phosphorite and glauconite. The phospho- 

 rite consists mostly of small-to-large nodular 

 masses which were described in the section 

 on lithology and thus can be neglected here. 

 In addition phosphorite comprises occa- 

 sional small pellets, perhaps originally copro- 

 lites and named sporbo (^mall /polished round 

 b\a.ck objects) by Galliher (1931) from their 

 similar occurrence in Miocene shales of the 

 region. Chief of the authigenic sediments 

 is glauconite, again a sediment that is usu- 

 ally mixed with other kinds of sediment, 

 chiefly organic. Glauconitic sands are far 

 more common on banks than on the main- 

 land shelf, and so their description and dis- 

 cussion of origin will be deferred to the 

 section on bank sediments. For some un- 

 known reason neither phosphorite nor glau- 

 conite has been found in appreciable quanti- 

 ties in San Pedro Bay, in contrast to Santa 

 Monica Bay, and off" Santa Barbara and San 

 Diego. The narrowness of the shelf else- 

 where along the southern California main- 

 land has led to mantling by present-day 

 detrital sediments which have diluted or 

 largely prevented the accumulation of much 

 glauconitic sand on these parts of the shelf. 



Examples of continental shelf sediments 

 similar to those of the present mainland 

 shelf are uncommon in the geological record, 

 although their rarity may be merely a reflec- 

 tion of their having been overlooked by 

 stratigraphers during the past. The shelf 

 sediments off" the Texas coast probably have 

 their older counterparts in ofl'-lapping Ceno- 

 zoic strata of the coastal plain, but that re- 

 gion is one of more or less continuous 

 deposition clearly diff^erent from off" southern 

 California. Here the existing shelf sedi- 

 ments form only a thin mantle atop the 

 eroded shelf surface. Because of low supply 

 of sediments and high turbulence, much de- 



