Sedimentary Rocks Represent Past Environments
Distinguish among three broad categories of sedimentary environments and provide an example of each. List several sedimentary structures and explain why these features are useful to geologists.
Sedimentary rocks are important to interpreting Earth’s history (Figure 1). By understanding the conditions under which sedimentary rocks form, geologists can often deduce the history of a rock, including information about the origin of its component particles, the method of sediment transport, and the nature of the place where the grains eventually came to rest—that is, the environment of deposition.
An environment of deposition, or sedimentary environment, is a geographic setting where sediment is accumulating. Each site is characterized by a particular combination of geologic processes and environmental conditions.
Some sediments, such as the chemical sediments that precipitate in water bodies, are solely products of their sedimentary environment.
That is, their component minerals originated and were deposited in the same place. Other sediments originate far from the site where they accumulate. These materials are transported great distances from their source by some combination of gravity, water, wind, and ice.
At any given time, the geographic setting and environmental conditions of a sedimentary environment determine the nature of the sediments that accumulate. Geologists carefully study the sediments in present-day depositional environments because the features they find can also be observed in ancient sedimentary rocks.
Geologists apply a thorough knowledge of presentday conditions to reconstruct the ancient environments and geographic relationships of an area at the time a particular set of sedimentary layers were deposited. This process is an excellent example of applying a fundamental principle of modern geology: “The present is the key to the past.”* Such analyses often lead to the creation of maps depicting the past geographic distribution of land and sea, mountains and river valleys, deserts and glaciers, and other environments of deposition.
Types of Sedimentary Environments
Sedimentary environments are commonly placed into one of three broad categories: continental, marine, or transitional (shoreline). Each category includes many specific subenvironments. Figure 2 is an idealized diagram illustrating a number of important sedimentary environments associated with each category. Realize that this is just a sampling of the great diversity of depositional environments. Each of these three categories is an area where sediment accumulates and where organisms live and die. Each produces a characteristic sedimentary rock or assemblage that reflects prevailing conditions.
Continental Environments Continental environments are dominated by the erosion and deposition associated with streams. In some cold regions, moving masses of glacial ice replace running water as the dominant process.
In arid regions (as well as some coastal settings), wind takes on greater importance. Clearly, the nature of the sediments deposited in continental environments is strongly influenced by climate.
Streams are the dominant agent of landscape alteration, eroding more land and transporting and depositing more sediment than any other process. In addition to channel deposits, large quantities of sediment are dropped when floodwaters periodically inundate broad, flat valley floors (called floodplains). Where rapid streams emerge from a mountainous area onto a flatter surface, a distinctive cone-shaped accumulation of sediment known as an alluvial fan forms.
In frigid, high-latitude or high-altitude settings, glaciers pick up and transport huge volumes of sediment.
Materials deposited directly from ice are typically unsorted mixtures of particles that range in size from fine clay to huge boulders. Water from melting glaciers transports and redeposits some of this glacial sediment, creating stratified, sorted accumulations.
The work of wind and its resulting deposits are referred to as eolian, after Aeolus, the Greek god of wind. Unlike glacial deposits, eolian sediments are well sorted. Wind can lift fine dust high into the atmosphere and transport it great distances. Where winds are strong and the surface is not anchored by vegetation, sand is transported closer to the ground, where it accumulates in dunes. Deserts and coasts are common sites for this type of deposition.
In addition to being areas where dunes sometimes develop, desert basins are sites where shallow playa lakes occasionally form following heavy rains or periods of snowmelt in adjacent mountains. They rapidly dry up, sometimes leaving behind evaporites and other characteristic deposits.
Marine Environments Marine depositional environments are divided according to depth. The shallow marine environment reaches to depths of about 200 meters (nearly 700 feet) and extends from the shore to the outer edge of the continental shelf. The deep marine environment lies seaward of the continental shelf in waters deeper than 200 meters.
The shallow marine environment borders all of the world’s continents. Its width varies greatly, from practically nonexistent in some places to broad expanses extending as far as 1500 kilometers (more than 900 miles) in other locations. On average this zone is about 80 kilometers (50 miles) wide. The kind of sediment deposited here depends on several factors, including distance from shore, elevation of the adjacent land area, water depth, water temperature, and climate.
Due to the ongoing erosion of the adjacent continent, the shallow marine environment receives huge quantities of land-derived sediment. Where the influx of such sediment is small and the seas are relatively warm, carbonate-rich muds may be the predominant sediment.
Most of this material consists of the skeletal debris of carbonate-secreting organisms mixed with inorganic precipitates. Coral reefs are also associated with warm, shallow marine environments. In hot regions where the sea occupies a basin with restricted circulation, evaporation triggers the precipitation of soluble materials and the formation of marine evaporite deposits.
Deep marine environments include all the floors of the deep ocean. Far from landmasses, tiny particles from many sources remain adrift for long spans. Gradually these small grains “rain” down on the ocean floor, where they accumulate very slowly. Significant exceptions are thick deposits of relatively coarse sediment that occur at the base of the continental slope. These materials move down from the continental shelf as turbidity currents—dense gravity-driven masses of sediment and water (see Figure 3).
Transitional Environments The shoreline is the transition zone between marine and continental environments. Here we find the familiar deposits of sand or gravel called beaches. Mud-covered tidal flats are alternately covered with shallow sheets of water and then exposed to air as tides rise and fall. Along and near the shore, the work of waves and currents distributes sand, creating spits, bars, and barrier islands. Offshore bars and reefs create lagoons. The quieter waters in these sheltered areas are another site of deposition in the transition zone.
Deltas are among the most significant deposits associated with transitional environments. The complex accumulations of sediment build outward into the sea when rivers experience an abrupt loss of velocity and deposit their load of detrital material.
When we study a series of sedimentary layers, we can see the successive changes in environmental conditions that occurred at a particular place with the passage of time.
Changes in past environments may also be seen when a single layer of sedimentary rock is traced laterally. This is true because at any one time, many different depositional environments can exist over a broad area. For example, when sand is accumulating in a beach environment, finer muds are often being deposited in quieter offshore waters. Still farther out, perhaps in a zone where biological activity is high and land-derived sediments are scarce, the deposits consist largely of the calcite-rich remains of small organisms. In this example, different sediments are accumulating adjacent to one another at the same time.
Different parts of each layer possess a distinctive set of characteristics that reflect the conditions in a particular environment. The term facies is used to describe such sets of sediments. When a sedimentary layer is examined in cross section from one end to the other, each facies grades laterally into another that formed at the same time but that exhibits different characteristics (Figure 4).
The merging of adjacent facies tends to be a gradual transition rather than a sharp boundary, but abrupt changes do sometimes occur.