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6 DEPOSITIONAL SYSTEMS
humidity. The chemical parameters of an environment include the composition of the
waters that cover a subaqueous sedimentary environment; they include the geochemistry of the rocks in the catchment area of a terrestrial environment. The biological parameters of an environment comprise both fauna and flora. On land these may have
major effects on sedimentary processes. Overgrazing, defoliation, deforestation, and
overcultivation of soils by animals can cause catastrophic increases in rates of erosion
in one area, accompanied by accelerated rates of deposition elsewhere. Conversely, the
colonization of deserts by a new flora has a moderating effect on sedimentary processes.
In marine environments many of the lowliest forms of life are important, both because
their skeletons can contribute to sediment formation, and because their presence in water can change its equilibrium, causing minerals to precipitate (see Section 9.2.3.2). The
morphology and history of carbonates in general, and of organic reefs in particular, are
inextricably related to the ecology of their biota.
This brief review of the physical, chemical, and biological parameters that define a
sedimentary environment should be sufficient to demonstrate how numerous and complex these variables are.
6.1.2 Environments of Erosion, Equilibrium, and Deposition
The classification of sedimentary environments by their physical, chemical, and biological parameters will be returned to shortly. Now consider environments from a slightly
different viewpoint. Examination of the modern earth shows that there are sedimentary environments of net erosion, environments of equilibrium (or nondeposition), and
environments of net deposition. Sedimentary environments of net erosion are typically
terrestrial and consist largely of the mountainous areas of the world. In such erosional
environments weathering is often intensive and erosion is rapid. Locally sedimentation may take place from glacial, mud flow, and flash flood processes. Due to renewed
erosion, however, such deposits are ephemeral and soil profiles have little time to develop on either bedrock or sediment. Erosional sedimentary environments also occur
on cliffed coastlines and, under the sea, in submarine canyons and on current-scoured
shelves. It is, however, in these shoreline and submarine situations that the products of
deposition dominate the process of erosion. Subaqueous deposits must take up some
90% of the world's sedimentary cover. Probably some 60% of this total volume is composed of submarine and shoreline deposits. It appears that depositional sedimentary
environments are predominantly subaqueous.
To environments of erosion and of deposition must be added a third category: environments of equilibrium or nondeposition. These are surfaces of the earth, both on the
land and under the sea, which for long periods of time are neither sites of erosion nor
of deposition. Because of their stability such environments often experience intense
chemical alteration of the substrate. On land, environments of equilibrium are represented by the great peneplanes of the continental interiors (King, 1962). These, in the
case of central Africa at least, are parts of the earth's surface that have been open to the
sky for millions of years. Prolonged exposure to the elements is responsible for the development of weathering profiles and soil formation in the rocks which immediately
underlie an environment of equilibrium (Thiry and Simon-Coincon, 1999). Laterite and
6 DEPOSITIONAL SYSTEMS
humidity. The chemical parameters of an environment include the composition of the
waters that cover a subaqueous sedimentary environment; they include the geochemistry of the rocks in the catchment area of a terrestrial environment. The biological parameters of an environment comprise both fauna and flora. On land these may have
major effects on sedimentary processes. Overgrazing, defoliation, deforestation, and
overcultivation of soils by animals can cause catastrophic increases in rates of erosion
in one area, accompanied by accelerated rates of deposition elsewhere. Conversely, the
colonization of deserts by a new flora has a moderating effect on sedimentary processes.
In marine environments many of the lowliest forms of life are important, both because
their skeletons can contribute to sediment formation, and because their presence in water can change its equilibrium, causing minerals to precipitate (see Section 9.2.3.2). The
morphology and history of carbonates in general, and of organic reefs in particular, are
inextricably related to the ecology of their biota.
This brief review of the physical, chemical, and biological parameters that define a
sedimentary environment should be sufficient to demonstrate how numerous and complex these variables are.
6.1.2 Environments of Erosion, Equilibrium, and Deposition
The classification of sedimentary environments by their physical, chemical, and biological parameters will be returned to shortly. Now consider environments from a slightly
different viewpoint. Examination of the modern earth shows that there are sedimentary environments of net erosion, environments of equilibrium (or nondeposition), and
environments of net deposition. Sedimentary environments of net erosion are typically
terrestrial and consist largely of the mountainous areas of the world. In such erosional
environments weathering is often intensive and erosion is rapid. Locally sedimentation may take place from glacial, mud flow, and flash flood processes. Due to renewed
erosion, however, such deposits are ephemeral and soil profiles have little time to develop on either bedrock or sediment. Erosional sedimentary environments also occur
on cliffed coastlines and, under the sea, in submarine canyons and on current-scoured
shelves. It is, however, in these shoreline and submarine situations that the products of
deposition dominate the process of erosion. Subaqueous deposits must take up some
90% of the world's sedimentary cover. Probably some 60% of this total volume is composed of submarine and shoreline deposits. It appears that depositional sedimentary
environments are predominantly subaqueous.
To environments of erosion and of deposition must be added a third category: environments of equilibrium or nondeposition. These are surfaces of the earth, both on the
land and under the sea, which for long periods of time are neither sites of erosion nor
of deposition. Because of their stability such environments often experience intense
chemical alteration of the substrate. On land, environments of equilibrium are represented by the great peneplanes of the continental interiors (King, 1962). These, in the
case of central Africa at least, are parts of the earth's surface that have been open to the
sky for millions of years. Prolonged exposure to the elements is responsible for the development of weathering profiles and soil formation in the rocks which immediately
underlie an environment of equilibrium (Thiry and Simon-Coincon, 1999). Laterite and