7.6: Sedimentary Environments and Facies

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7.78.png — Figure 7.78: Some of the many environments where sediments may be deposited

The processes that form sedimentary rock occur in many environments, and different minerals or rocks can characterize each. In many places, however, the rate of sediment deposition is slow. In other places, erosion removes sediment as fast as it is deposited. Consequently, most of the sedimentary rocks and minerals that we see are associated with a few special environments where the rate of deposition is relatively fast, and the volume of sediments deposited was relatively large. The most significant such environments are shallow seas and large basins on continents. Lesser amounts of sedimentary rocks are associated with other environments such as shorelines, rivers, lakes, deserts, and glaciers. Because different environments are different physically, chemically, and biologically, the nature of sedimentary rocks is highly variable. The most significant factors that account for the differences are the energy and biology of the environment, the distance from the source of the sediment, and the way the sediment was transported. Figure 7.78 show some of the many different places where sediments are deposited. Sedimentologists divide all of them into smaller categories that produce distinctive sediments and sedimentary rocks.

Most sedimentary environments vary laterally. Large basins, for example, are not the same everywhere. So different kinds of sediment can be deposited in different parts of a basin simultaneously. We use the term facies to refer to the different rocks that characterize a particular process or environment. So, geologists may talk about continental, transitional, or marine facies. More specifically, they may consider reef facies, continental shelf facies, deep ocean facies, and so on.

Different contemporaneous facies often grade into each other. Consider, for example, sedimentation at or near a shoreline. Coarse sediments are typically deposited on dry land and on beaches, somewhat finer sediments in shallow water, and the finest materials farther offshore. If the sedimentary environment changes, the nature of sedimentation changes. With rising sea level, for example, fine deep-water sediments may be deposited over coarser shoreline sediments. With failing sea level, the opposite may occur. So, sedimentary facies vary both laterally and vertically (geographically and temporally).