6.2: Classification of Sediments

Sediments are separated into categories to facilitate comparisons with each other and communication among scientists. There are a number of classification schemes. The most widely used schemes are based either on the range of sizes of the particles (grains) that make up the sediment or on the origin of the predominant particles in the sediment. Each type of scheme has its advantages, as we shall see in this chapter.

Classification by Grain Size

The size of the grains in marine sediments varies from boulders tens of centimeters or more in diameter to grains that are so small (less than 0.001 mm in diameter) that they cannot be distinguished except under the most powerful electron microscopes. Various grain size classification schemes have been used, but generally sediments are described as clay, silt, sand, or gravel (Fig. 6-2). Sediments with the finest grain size ranges—silt and clay—are often classified together as mud.

Figure 6-2. Various types of particles, each of which has its own characteristic size range, are commonly found in marine sediments. The particles in foraminiferal and diatomaceous muds are, on average, smaller than the sizes of the foraminifera and diatoms from which they are derived because the hard parts of these organisms are partially dissolved and broken mechanically during transport to, and burial in, the sediments.

Sediments with a wide range of grain sizes can be found in different areas of the oceans or at different depths within the layers of sediment at any given location. However, most marine sediments that have accumulated at the same time and in the same area of the oceans consist of only grains that are within a narrow range of grain sizes. Sediments that consist mostly of grains that are within a narrow range of sizes are said to be well sorted. For example, most sediment on the deep-ocean floor consists of well-sorted mud, and many areas near beaches consist of well-sorted sand. Sediments that consist of a wide range of grain sizes are said to be poorly sorted. For example, some undersea landslide deposits are poorly sorted, consisting of a mixture of mud, sand, and gravel. Poorly-sorted sediments are found in only limited areas in the oceans. Transport mechanisms that cause sediment sorting are discussed in CC4, and sorting of beach sand is discussed in Chapter 11.

Classification by Origin

Sediments can be classified as lithogenous, biogenous, hydrogenous, or cosmogenous to indicate the origin of the dominant type of particle. Lithogenous particles consist of fragments of rocks and minerals weathered and eroded from rocks on land, and fragments of ash and rock from terrestrial and undersea volcanic eruptions. Biogenous particles are the remains of organisms. Because most organic matter is decomposed rapidly, biogenous particles are predominantly the mineralized hard parts (shells or skeletal material) of marine organisms. They include, for example, the hard parts of certain phytoplankton and zooplankton species, mollusk shells, fish bones, and whale and shark teeth. Hydrogenous particles are formed by precipitation of dissolved inorganic chemicals from seawater to form solids. Cosmogenous particles are meteorite fragments that have passed through the atmosphere. 

Each category of particles can be subdivided into several different types. For example, biogenous particles can be classified according to the types of organisms from which they are derived (Fig. 6-2).

The distribution of sediment particles in the oceans is determined by the size range and origin of each type of particle, the susceptibility of each type of particle to decomposition or dissolution in seawater, and the mechanisms transporting the particles. The distribution of sediments is discussed later in this chapter. However, to understand the distribution of sediments, we need first to understand the origins and sources of the particles that make up sediments and the processes that transport and transform the particles in the ocean.