Depositional systems and environments
Depositional systems are descriptions of the interrelationships of form and the physical, chemical, or biological processes involved in the development of stratigraphic sequences. Depositional environments are the locations where accumulations of sediment have been deposited by either mechanical or chemical processes.
Depositional systems
Traditional stratigraphic analysis, which emphasized the physically descriptive aspects of strata, has changed; a critical genetic dimension has been added. Where once, for example, a formation may have been described in physical terms as a fine- to medium-grained quartzose sandstone, overlying a thick dark-gray shale sequence and underlying a coal-bearing sequence with discontinuous sands, the same sandstone may now be recognized as a delta-front sandstone, the underlying shale as a prodelta facies, and the overlying coal-bearing formation as the product of deposition on a delta plain. In this interpretation, the three distinct stratigraphic units become part of a genetically related sequence, each a component facies of a prograding delta system. See also Stratigraphy.
In a modern setting, a particular system of deposition is directly observable and known, whether it is a major delta, an alluvial fan, a meandering river, a barrier bar, a carbonate platform, or the like. Through specific observation, description, and delineation, it may be determined that a variety of depositional processes are active. At the terminus of rivers, sands may be deposited as bars in river mouths, creating the delta front; muds may be carried by suspension into the oceanic waters and deposited through flocculation, creating the prodelta; on the delta plain, a distributary channel may be depositing bed-load sands and, during flooding, may be carrying suspended muds to flanking flood basins or breaching the natural levee to form crevasse splays. Each process or combination of processes gives rise to distinct, specific environments of deposition, with each resulting in a deposit which can be characterized by such features as lithologic composition, texture, sedimentary structures, geometry, size, and relationship to other deposits. See also Delta; Floodplain; River.
Distinct physical, or in some cases biologic or chemical, products of deposition can be related directly to definable, operative processes. Such data permit the development of models of modern deposition in which processes and resulting deposits or facies are linked. By recognizing comparable physical, chemical, and biologic attributes of ancient strata, modern depositional analogs can be applied, and the original processes forming the ancient deposit can be inferred. Such an ancient deposit is called a genetic facies. It contains the sedimentary record and constitutes a three-dimensional stratigraphic (ancient) depositional system. See also Facies (geology).
An ancient depositional system is a three-dimensional, genetically defined, physical stratigraphic unit that consists of a contiguous set of process-related sedimentary facies. Several corollaries have evolved from the application of this concept. Depositional systems, such as delta, fluvial, and shelf systems, (1) are the stratigraphic equivalents of major physical geographic units; (2) form the principal building blocks of the sedimentary basin fill; and (3) can be applied where principal boundaries of the systems are preserved and where the geometry of the framework facies can be mapped.
The major realms of deposition may be classed broadly as terrigenous clastic depositional systems and biogenic-chemical depositional systems. Each of these major systems is subdivided according to particular systems of deposition, and within each of the subdivisions is an assemblage of genetic facies, which are the fundamental units of depositional systems.
Terrigenous clastic systems, composed chiefly of sands and shales, embrace eight major systems: (1) fluvial or river systems; (2) delta systems; (3) strike coastal systems; (4) fan or clastic wedge systems; (5) lacustrine systems; (6) continental eolian systems; (7) shelf systems; and (8) slope and abyssal systems.
The biogenic-chemical systems consist of three major systems: (1) carbonate systems; (2) glauconitic and authigenic shelf systems; and (3) evaporite systems.
Depositional environments
Depositional environments may be distinguished from erosional environments, in which erosion of the Earth's surface is taking place. Both depositional and erosional environments are of interest to geomorphologists. However, most attention to depositional environments has come from sedimentologists, particularly in order to understand the origin of sedimentary rocks. See also Erosion.
Sediment is derived mainly from source areas that are actively undergoing uplift and erosion, and is deposited mainly in areas that are undergoing subsidence. Location of the source and basin of deposition is mainly controlled by large-scale geophysical processes acting within the Earth's mantle, so a major factor affecting the nature and distribution of sedimentary environments is the overall structural development, or tectonics, of the area. See also Plate tectonics.
Tectonics determines the major geological structure or setting of an environment of deposition, including the location and nature of the main areas undergoing uplift or subsidence. Areas with high relief, such as mountains and volcanoes, suffer rapid erosion and supply much more sediment to basins of deposition than larger areas of low relief. One investigation, for example, found that 82% of the suspended solids (mud) discharged by the Amazon River were supplied by the 12% of the drainage basin located within the Andes Mountains.
A second important major control is climate. This includes the average temperature, the range of temperature variation, the aridity or humidity (ratio of evaporation to precipitation), and the magnitude and frequency of floods and storms. Climate in turn has an important influence on such physical factors as the salinity and energy of the environment (wind and water speeds and degree of turbulence, for example), as well as on the abundance and types of plants and animals.
In areas of subsidence and sedimentation, topography results from and controls sedimentary environments. Along a coastline of low relief, for example, spits and barrier islands are produced by waves generated in the open sea. Shallow lagoons on the landward side of barrier islands are protected from wave action by the islands themselves. The distinctive features of the lagoon environment are a result of a topography which has been produced by the accumulation of sediment in another sedimentary environment (the barrier island). See also Barrier islands.
Sedimentary environments can be classified into three categories: terrestrial, including alluvial fans, fluvial plains, sandy deserts, lakes, and glacial regions; mixed (shore-related), including deltas, estuaries, barrier island complexes, and glacial marine environments; and marine, including terrigenous shelves or shallow seas, carbonate shelves or platforms, continental slopes, continental rises, basin plains, ocean ridges, and ocean trenches.
Although the importance of tectonics and climate in controlling sedimentary environments is widely recognized, most classifications are based mainly on topography. Almost all distinguish terrestrial (sub-aerial or fresh-water) from marine environments, and also recognize an important group of mixed or shore-related environments.
A number of major processes operate within environments and determine the types of sediment deposited in the environment, including water depth, energy (waves and current), temperature, and salinity. Biological factors also exert a very strong influence. There would be little or no oxygen in the atmosphere if it were not for the photosynthetic activity of plants. Deposition of calcium carbonate and silica in lakes and the oceans takes place largely through the action of plants and animals, and organic matter deposited along with mineral particles is largely responsible for the development of reducing conditions within sediments after deposition. Vegetable material accumulates in swamps to form peat and coal, and fine organic detritus settles with marine muds and is the ultimate source of oil and gas. Both terrestrial and aquatic plants exert a trapping and binding action that tends to immobilize sedimentary particles, as, for example, when coastal sand dunes or tidal flats become stabilized by the growth of salt-tolerant grasses. Terrestrial vegetation plays an important role in rock weathering.
