6.3 SEDIMENTARY MODELS
193
munications, and pollution. Important accounts of modern river systems have been given
by Marzo and Puigdefabregas (1993), Collinson (1996), and Smith and Rogers (1999).
6.3.2.2.1 Fluvial processes and modern alluvium
Piedmont fans pass gradationally downslope into the alluvial environment. Alluvial systems may be arbitrarily divided into two main types according to channel morphology,
though there are gradations between the two. Alluvial fans are characterized by ephemeral flood discharge that generates low sinuosity braided channels.
Alluvial fans often grade into, or occasionally flank, alluvial floodplains. These normally contain a single sinuous river channel with continuous discharge. The lateral
changes from piedmont to alluvial fan to floodplain are associated with changes in gradient, grain size, and sorting (Fig. 6.5). The factors that determine whether a channel meanders or braids appear to include gradient, grain size, and discharge. Braided channels
are normally associated with steep slopes, coarse, often gravelly, sediment, and ephemeral discharge. Meandering channels are normally associated with fine-grained sediment, gentle gradient, and steady discharge. There are, however, exceptions to these
generalizations, and the relationships between these factors and channel morphology
are complex. Analyses of these problems will be found in Allen (1964), Leopold et al.
(1964), Miall (1978), Collinson and Lewin (1983), and Collinson (1996). Many different
fluvial models have been defined; see, for example, Cant (1982) and Miall (1981). All
pre-Silurian alluvial deposits appear to be of braided channel type. Alluvium attributable to meandering channels is absent. This may be because the land was not colonized
by plants, whose roots and stems would slow down runoff, until this time. Thus in PreCambrian and Early Paleozoic time rainwater would have drained quickly off the land
Fig. 6.5. Diagram illustrating the transitional relationship between piedmont fans, braided and meandering
alluvial systems, and facies.
193
munications, and pollution. Important accounts of modern river systems have been given
by Marzo and Puigdefabregas (1993), Collinson (1996), and Smith and Rogers (1999).
6.3.2.2.1 Fluvial processes and modern alluvium
Piedmont fans pass gradationally downslope into the alluvial environment. Alluvial systems may be arbitrarily divided into two main types according to channel morphology,
though there are gradations between the two. Alluvial fans are characterized by ephemeral flood discharge that generates low sinuosity braided channels.
Alluvial fans often grade into, or occasionally flank, alluvial floodplains. These normally contain a single sinuous river channel with continuous discharge. The lateral
changes from piedmont to alluvial fan to floodplain are associated with changes in gradient, grain size, and sorting (Fig. 6.5). The factors that determine whether a channel meanders or braids appear to include gradient, grain size, and discharge. Braided channels
are normally associated with steep slopes, coarse, often gravelly, sediment, and ephemeral discharge. Meandering channels are normally associated with fine-grained sediment, gentle gradient, and steady discharge. There are, however, exceptions to these
generalizations, and the relationships between these factors and channel morphology
are complex. Analyses of these problems will be found in Allen (1964), Leopold et al.
(1964), Miall (1978), Collinson and Lewin (1983), and Collinson (1996). Many different
fluvial models have been defined; see, for example, Cant (1982) and Miall (1981). All
pre-Silurian alluvial deposits appear to be of braided channel type. Alluvium attributable to meandering channels is absent. This may be because the land was not colonized
by plants, whose roots and stems would slow down runoff, until this time. Thus in PreCambrian and Early Paleozoic time rainwater would have drained quickly off the land
Fig. 6.5. Diagram illustrating the transitional relationship between piedmont fans, braided and meandering
alluvial systems, and facies.