CLASSIFICATION OF RECENT MARINE SEDIMENTS
Marine deposits are subdivided into two major groups, termed pelagic and terrigenous. The pelagic deposits are those found in deep water far from shore and may be predominantly either organic or inorganic in origin. Pelagic deposits are light-colored, reddish or brown, fine-grained and, generally, they contain some skeletal remains of plankton organisms. Benthic forms are generally rare. The inorganic deposits are referred to as red clay and the organic deposits as oozes. The terrigenous deposits are found near shore and generally contain at least some coarse material of terrigenous origin. They cover a wide range in depth and a great variation in color, texture, and composition. They are known as sands, silts, muds, or intergrading types, depending upon their texture. Although they may have median diameters as small as certain pelagic deposits, the terms clay and ooze are never applied to terrigenous deposits. Nearshore sediments may contain remains of plankton forms but, in addition, skeletons of benthic organisms may be relatively abundant. In certain regions the deposits may be almost entirely made up of the fragments of certain calcareous benthic forms.
Pelagic deposits are classified in the following way:
Inorganic deposits. Those which contain less than 30 per cent of organic remains are known as red clay.
Organic deposits. Those which contain more than 30 per cent of material of organic origin are known as oozes. This class is further subdivided into:
Calcareous oozes. These contain more than 30 per cent calcium carbonate, which represents the skeletal material of various plankton animals and plants. The calcareous oozes may be further divided into three types, depending upon a characteristic type of organism present in the sediment, namely:
― 973 ―
Globigerina ooze, in which the calcium carbonate is in the tests of pelagic foraminifera.
Pteropod ooze, containing conspicuous shells of pelagic molluscs.
Coccolith ooze, containing large numbers of coccoliths and rhabdoliths that form the protective structures of the minute Coccolithophoridae.
Siliceous oozes. These are pelagic deposits which contain a large percentage of siliceous skeletal material produced by planktonic plants and animals. The siliceous oozes are subdivided into two types on the basis of the predominance of the forms represented, namely,
Diatom ooze, containing large amounts of diatom frustules, hence, produced by plankton plants.
Radiolarian ooze, containing large proportions of radiolarian skeletons formed by these plankton animals.
The classification of terrigenous deposits is not so satisfactory as that of the pelagic sediments. A number of systems have been suggested, but the character of the deposits depends so much upon local conditions that no one system has very wide application. Relative to the pelagic deposits, terrigenous sediments cover a small percentage of the sea floor, and since they show a wide range in properties within a relatively short distance from the coast the occurrence of transitional types is more of a problem. As pointed out in the discussion of mass properties, color, texture, and composition form the basis of the classification. Hence it is desirable to indicate the terms applied to terrigenous deposits rather than set up any definite outline of classification.
The color of terrigenous deposits may range from white to black with the addition of blue, yellow, or red, or mixtures of these. In general, sediments of the shelves and slopes are dark in color, ranging from green or brown to blue. The color depends so much on the local environment that it may vary a great deal in a relatively short distance over the bottom, or with depth in the deposit.
Terrigenous deposits are generally coarser than those found in the deep sea, although certain exceptions do occur. The texture will depend not only upon the effective transporting agency but also upon the character of the source material. The following terms have been suggested by Revelle (personal communication) to designate the texture of terrigenous deposits:
Sand. More than 80 per cent of the material coarser than 62 microns in diameter.
Silty sand. Between 50 per cent and 80 per cent coarser than 62 microns.
Sandy silt. More than 50 per cent coarser than 5 microns and more than 20 per cent coarser than 62 microns.
Silty mud. More than 50 per cent coarser than 5 microns and less than 20 per cent coarser than 62 microns.
Clayey mud. Less than 50 per cent coarser than 5 microns.
By definition (p. 957) the sands may be subdivided into very coarse (2000–1000 microns), coarse (1000–500 microns), medium (500–250 microns), fine (250–125 microns), and very fine (125–62 microns). If all the material is within the limits of 62 and 4 microns the term silt may be used. It will be noted that the term mud was not listed in table 105, since it is restricted to terrigenous sediments containing particles with a wide range in size.
The composition is concerned with the character and, hence, with the source of the constituent materials. On this basis terrigenous sediments may be divided into organic and inorganic types. The terms foraminiferal, coral, diatomaceous, or other specific terms may be applied when a single type of organic remains is prominent in the sediment, and if no one form is conspicuous the general terms calcareous or siliceous may be used with the understanding that they indicate material of organic origin. Predominantly inorganic types of sediments may be expected to be related to the character of the material supplied to any area, and in this case no special term need be applied. However, the presence of mineral grains of certain types or sizes may make it desirable to add a term indicating either the source or mode of transportation. Thus, glacial may be used for sediments containing material deposited by glaciers and icebergs, volcanic when there is a large amount of pumice, volcanic ash, and glass in the deposit. Mica is sometimes a readily recognized constituent and if it is abundant the term micaceous may be introduced. In certain areas authigenic glauconite is sufficiently abundant to warrant use of the term glauconitic.
When following the above system of terminology a trinomial nomenclature may be used to describe terrigenous deposits. For example, a sediment from near a continental coast may be a green diatomaceous silty mud, or one from the shelf of an oceanic island may be a gray calcareous sand.