Earth, Our Environment - Class Notes
Chapter 4, The Cornerstones of Geology: Rocks - 10
Types of Sediment

There are three types of sediment:

Most sediments are produced when rocks or minerals interact with the hydrosphere (and biosphere) near Earth's surface. Sediments generally are moved through the Earth system and eventually settle under the force of gravity. Influenced by grain size, mineralogy, or grain orientation, sediments form layers or strata, which tend to form in horizontal "beds".

Clastic Sediments

Clastic sediments are composed of fragments or grains (or clasts) of other rocks and minerals. We classify clastic sediments based on their grain size:

[Study Figure 4.16 on page 89 of the text]

Information in the Clasts

The history of the clasts that eventually lithify into a sedimentary rock is reflected in grain size, degree of sorting, and mineral composition.

Grain Size reflects the amount of bumping and grinding that has occurred. For example, the largest clasts are generally found close to the source of the sediment, since they are harder to transport. The farther away you go from the source, the more grinding occurs between the clasts, and they become smaller and smoother from the transportation process.

Sorting reflects the distance from the sediment source. Near the source, sorting is poor and clasts of many sizes are found together. The difficulty in transporting large clasts leaves results in increased sorting with distance from the sediment source. The smaller grains are easiest to transport and thus the finest clasts can travel far from the sediment source.

Mineral composition will vary with increased transport. In short the stronger and chemically stable minerals survive the journey more often. Here the mineral properties of hardness, cleavage, and the types of bonds holding the atoms together can affect the survivability of minerals. The predominance of quartz in sand is a consequence of its hardness, lack of cleavage, and covalent bonding (which makes insoluble in water). We call well-sorted, rounded-grain, containing mostly quartz mature, reflecting its long history.

[Study Figure 4.17 on page 91 of the text]

Chemical Sediments

Chemical sediments are not formed from the weathering and erosion of other rocks. They form from the precipitation of minerals out of a solution. Most commonly, the solution is sea water, and the precipitates are called evaporites.

As you might have guessed, evaporites form when a liquid containing dissolved ions evaporates. Nevada, Utah, and California contain many dry lake beds that reflect the dry climate in the western US. As the lake dries, the concentration of the ions increases until the remaining water can no longer hold the ions in solution. They precipitate out of solution and onto the lake floor.

The Bonneville salt flats of Utah are the results of this process and as is the Great Salt Lake near Salt Lake City, UT. Large-scale evaporation is the result of a dry and/or hot climate and large salt deposits indicate such conditions. Beneath the Mediterranean Sea, a two-thousand meter sequence of evaporites records the evaporation of that sea around 6 million years ago.

Biochemical Sediments

At the end of the Cambrian era, marine organisms obtained the ability to form protective shells. When these organisms die, their shells fall to the sea floor forming biochemical sediment. Much of this material comes from microorganisms (organisms of microscopic size). Foraminifera and coccoliths secrete calcium carbonate, diatoms and radiolaria produce silica. The primary biochemical rock is limestone. If the shells are not ground finely, the material may be called bioclastic.

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Prepared by: Charles J. Ammon
February 1997