468
10 SEDIMENTARY BASINS
changes in sea level, which affect the top of the accommodation space, and tectonism,
principally rate of subsidence, which affects the floor of the accommodation space.
The rate of sediment supply into a basin is dependent on many variables. These include the type and intensity of weathering, erosion, and transportation within the drainage catchment area. Most of these variables are closely related to climate (Leeder et al.,
1998). Glacial, temperate, arid, and tropical climates all generate characteristic weathering profiles and have their own erosional and transportational characteristics, as described in Chapters 2 and 4.
Vegetation is another important controlling parameter, closely related to climate.
Prior to the colonization of the land by plants in the Devonian Period, runoff was uninhibited by plants, their roots, and thick humus-rich soil. Except in arid climates, runoff has been much slower in post-Devonian times, due to the presence of vegetation.
This is demonstrated by the fact that all Pre-Cambrian and pre-Devonian Phanerozoic
alluvium shows the characteristics of deposition from ephemeral braided channel systems. Characteristic features of deposition from meandering channel systems occur only
in post-Silurian alluvium (refer back to Section 6.3.2.2.3).
The chemical composition and physical parameters of the rock in the source area will
also play an important part in controlling both the volume and mineralogy of the sediment supplied to a basin. For instance, a hinterland of igneous and metamorphic rocks
will supply a diverse range of detrital minerals to a basin, together with sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, and carbonate ions, carried in
solution in groundwater. By contrast, a hinterland of pure quartz sand can only contribute a detrital load of the same, or, to take another extreme situation, a limestone terrain
will contribute little detritus to a basin, though the runoff will be rich in calcium, magnesium, and carbonate ions.
Figure 10.1 attemps to display the various factors just outlined that control the generation of sediment in a catchment area, and its dispersal within the accommodation space
of a basin.
10.1.2 Basic Concepts and Terminology
The geographic distribution and organization of sediments is now described. This topic
can be loosely referred to as basin analysis (Miall, 1984; Klein, 1987; Leeder, 1999). Sedimentary rocks cover a large part of the earth's surface, including some 75% of the land
areas. Yet sedimentary rocks make up only 5 % of the lithosphere (data from Pettijohn,
1957, p. 7). From this it follows that sedimentary rocks cover the earth only as a thin and
superficial veneer. This cover is not evenly distributed. Thick sedimentary deposits were
laid down in localized areas, loosely termed "sedimentary basins." Large areas of the
continents lack a thick sedimentary cover; Pre-Cambrian igneous and metamorphic
rocks crop out at or near the surface. These stable continental cores are termed "cratons." Sedimentary basins are of many types, ranging from small alluvial intermontane
valleys to vast mountain ranges of contorted sediment, kilometers thick. Before proceeding to describe these various basins, some terms and concepts are first defined.
Basins are of three types: topographic, structural, and sedimentary. Topographic basins are low-lying areas of the earth's surface naturally surrounded by higher areas. Topographic basins are both subaerial and subaqueous. Subaerial topographic basins
range from bolsons (intermontane plains) to transcontinental alluvial valleys such as the
10 SEDIMENTARY BASINS
changes in sea level, which affect the top of the accommodation space, and tectonism,
principally rate of subsidence, which affects the floor of the accommodation space.
The rate of sediment supply into a basin is dependent on many variables. These include the type and intensity of weathering, erosion, and transportation within the drainage catchment area. Most of these variables are closely related to climate (Leeder et al.,
1998). Glacial, temperate, arid, and tropical climates all generate characteristic weathering profiles and have their own erosional and transportational characteristics, as described in Chapters 2 and 4.
Vegetation is another important controlling parameter, closely related to climate.
Prior to the colonization of the land by plants in the Devonian Period, runoff was uninhibited by plants, their roots, and thick humus-rich soil. Except in arid climates, runoff has been much slower in post-Devonian times, due to the presence of vegetation.
This is demonstrated by the fact that all Pre-Cambrian and pre-Devonian Phanerozoic
alluvium shows the characteristics of deposition from ephemeral braided channel systems. Characteristic features of deposition from meandering channel systems occur only
in post-Silurian alluvium (refer back to Section 6.3.2.2.3).
The chemical composition and physical parameters of the rock in the source area will
also play an important part in controlling both the volume and mineralogy of the sediment supplied to a basin. For instance, a hinterland of igneous and metamorphic rocks
will supply a diverse range of detrital minerals to a basin, together with sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, and carbonate ions, carried in
solution in groundwater. By contrast, a hinterland of pure quartz sand can only contribute a detrital load of the same, or, to take another extreme situation, a limestone terrain
will contribute little detritus to a basin, though the runoff will be rich in calcium, magnesium, and carbonate ions.
Figure 10.1 attemps to display the various factors just outlined that control the generation of sediment in a catchment area, and its dispersal within the accommodation space
of a basin.
10.1.2 Basic Concepts and Terminology
The geographic distribution and organization of sediments is now described. This topic
can be loosely referred to as basin analysis (Miall, 1984; Klein, 1987; Leeder, 1999). Sedimentary rocks cover a large part of the earth's surface, including some 75% of the land
areas. Yet sedimentary rocks make up only 5 % of the lithosphere (data from Pettijohn,
1957, p. 7). From this it follows that sedimentary rocks cover the earth only as a thin and
superficial veneer. This cover is not evenly distributed. Thick sedimentary deposits were
laid down in localized areas, loosely termed "sedimentary basins." Large areas of the
continents lack a thick sedimentary cover; Pre-Cambrian igneous and metamorphic
rocks crop out at or near the surface. These stable continental cores are termed "cratons." Sedimentary basins are of many types, ranging from small alluvial intermontane
valleys to vast mountain ranges of contorted sediment, kilometers thick. Before proceeding to describe these various basins, some terms and concepts are first defined.
Basins are of three types: topographic, structural, and sedimentary. Topographic basins are low-lying areas of the earth's surface naturally surrounded by higher areas. Topographic basins are both subaerial and subaqueous. Subaerial topographic basins
range from bolsons (intermontane plains) to transcontinental alluvial valleys such as the