CLASSIFICATION OF RECENT DEEP-SEA SEDIMENTS 



15 



Suggested Modifications 



The primary separation of deep-sea sediments into 

 terrigenous and pelagic types is a valuable one and should 

 be retained but, as has been stated, the criteria which 

 have been used for the distinction of the two types are 

 often contradictory. A more precise delimitation maybe 

 attained by redefining pelagic deposits on the basis of 

 the actual properties of the sediments, namely; color, 

 texture, and composition. The term pelagic deposits 

 would then be restricted to those deep-sea sediments 

 which are red, brown, yellow, or white in color, and 

 which contain only small amounts of neritic organic re- 

 mains and of detrital or volcanic mineral grains of di- 

 ameters greater than 5 microns. It is difficult to give a 

 numerical value for texture, since relatively few mechan- 

 ical analyses of deep-sea sediments have been published; 

 but a maximum of 20 per cent of detrital or volcanic 

 particles greater than 5 microns in diameter is valid at 

 least for the samples described in this report and appar- 

 ently agrees well with the average values given in the 

 above tabular summary. The doubtful deposits discussed 

 in preceding pages will be placed in the terrigenous 

 group by this definition and should be called Globigerina, 

 diatom, or other- types of mud. 



Besides texture and color, the most important char- 

 acter of deep-sea sediments is the presence or absence 

 in large amounts of skeletal remains of organisms, and 

 this long has been recognized in pelagic deposits by the 

 separation into oozes and red clay. A corresponding di- 

 vision in terrigenous deposits should be made by the use 

 of descriptive adjectives signifying the nature of the 

 principal organic remains when these are present in 

 amounts greater than 30 per cent. An important distinc- 

 tion between the various types of organic remains may 

 be based on whether they are calcareous or siliceous. 



Further subdivisions reflecting the varying conditions 

 of formation of deep-sea sediments may be made on the ba- 

 sis of the nature of the inorganic materials, which should 

 be indicated by the adjectival terms applied to the deposits. 

 In pelagic deposits these are chiefly of four types, namely: 

 (1) Free iron and aluminum hydroxides, and iron manga- 

 nese oxides. These may be lumped together as ferruginous 

 constituents. (2) Clay minerals or argillaceous constitu- 

 ents, that is, the minerals of the kaolinite, beidellite- 

 nontronite, montmorillonite, and possibly other groups. 

 These first two types of components (together with phil- 

 lipsite and other products of diagenesis) may be regard- 

 ed as alteration minerals. When unaltered minerals oc- 

 cur in pelagic deposits, they are usually either of (3) 

 detrital (continental) or (4) volcanic origin. In terrige- 

 nous deposits the prominent altered and diagenetic min- 

 eral constituents are clays and glauconite. The unaltered 

 materials are again either of detrital or volcanic origin. 

 It is suggested, therefore, that the adjectives argillaceous, 

 ferruginous, glauconitic, detrital, or volcanic be used 

 when it is necessary to designate the characteristic type 

 of inorganic material in a sediment. The inorganic con- 

 stituents of most pelagic deposits are usually argilla- 

 ceous, whereas terrigenous sediments usually contain 

 argillaceous and detrital minerals, and these may be 

 considered as the normal types for which no descriptive 

 adjective is necessary. Furthermore, it is only when 

 deposits contain important amounts of both mineral and 

 organic debris that it is necessary to use adjectives for 

 both constituents. 



From the previous discussion of the significance of 



color, it is evident that the terms blue and green mud 

 used for terrigenous sediments should be applied only to 

 deposits which are actually bluish or green in color when 

 collected (regardless of the presence of glauconite); and 

 the descriptive adjectives brown, black, or gray should 

 be used for other terrigenous deposits such as those 

 mentioned above, described by Stetson and Boggild. 



The nouns used to describe terrigenous sediments, 

 namely, sand and mud, indicate something of the texture 

 of these deposits but, as has been stated previously, 

 texture is of genetic significance, principally with 

 regard to the size of the unaltered mineral or rock grains 

 of the sediment. For muds in which the inorganic con- 

 stituents have been mechanically analyzed, or in which 

 there is a low content of remains of organisms, the ad- 

 jectives silty and sandy for sediments of median particle 

 diameters greater than 5 microns, and clayey for sedi- 

 ments of particle diameters less than 5 microns, may be 

 used. 



Summary 



We may now summarize the above discussion and 

 the suggested modifications of the usual classification 

 and nomenclature of marine sediments. 



1. The classification should be based on the physical 

 properties and composition of the sediments themselves, 

 namely: color, which is an index of the oxidation-reduction 

 potential in a sediment and, therefore, of the rate of dep- 

 osition; texture of allogenic mineral or rock particles, 

 which indicates the effectiveness of the agents of trans- 

 portation involved in the production of the sediment; and 

 the kinds of organic and inorganic components, which in- 

 dicate the origin of the materials in the deposit. 



2. Since one of the fundamental distinctions between 

 pelagic and terrigenous sediments is the relatively slow 

 rate of deposition of the former, indicated by color and 

 texture, the term "pelagic deposits" should be restrict- 

 ed to sediments of red, brown, yellow, or white color 

 which have less than a certain amount (about 20 per cent 

 in the Pacific) of allogenic mineral and rock particles 

 larger than 5 microns in diameter and which contain only 

 small amounts of neritic organic remains. 



3. The accepted types of pelagic deposits should be 

 retained, but sulxlivided when necessary on the basis of 

 the inorganic constituents into argillaceous, ferruginous, 

 volcanic, and detrital Globigerina and other oozes, and 

 red clay. 



4. Terrigenous sediments should be distinguished by 

 any one or more of the following characteristics: ablack, 

 bluish, green, or gray color, or the presence of more 

 than a certain amount of allogenic mineral and rock par- 

 ticles larger than 5 microns in diameter, or the presence 

 in appreciable amounts of neritic organic remains. 



5. The types of terrigenous deposits should be des- 

 ignated when necessary by adjectival terms (restricted 

 to their actual descriptive meanings) for color, nature 

 of organic and inorganic materials, and texture. In ter- 

 rigenous sediments which contain more than thirty per 

 cent of organic skeletal remains the adjectives Globiger- 

 ina, pteropod, calcareous (for neritic organisms), dia- 

 tom, or radiolarian should be used to indicate the pre- 

 dominant type of organic material. For terrigenous 

 sediments which contain appreciable amounts of other 

 than argillaceous inorganic material the terms glauconit- 

 ic, volcanic, or detrital should be used when necessary. 



