2.2 Fluvial Sediments
Many of the common minor bed forms and corresponding
internal structures occur in all fluvial environments, but in
varying proportions. This is so, because all river systems are
characterized by frequently changing water stages and a wide
range of current velocities. For example, small sand ripples
of various shape, larger dunes, various types of cross-bedding, and flat lamination with or without gravel are observed
in nearly all river systems. Consequently, these small-scale
structures alone have limited diagnostic value. Rather, it is
the vertical or lateral succession of such structures, or the
proportion of specific structures in the total sediment body,
which can be used to identifY the overall depositional environment.
The grain size distribution of fluvial deposits does not
solely depend on the type of river system or its flow
regime. Both small and large streams rnay, for exampIe, develop a braided river system, whether they are
sand or gravel-dominated. In both cases, the transport
capacity of channel flow is intermittently sufficient to
move gravel as bedload. Hence, the proportion of
gravel in such fluvial deposits reflects rock types, relief, and weathering conditions in the source area
rather than the depositional environment. On the other
hand, the grain size distribution available in the source
area, including jointed hard rocks, has limited influence on the grain size distribution of river deposits
(Walger 1964; Ibbeken 1983). Even after a short transport distance, river sediments show a characteristic
frequency curve resembling a log-normal distribution,
while the material in the source area may significantly
deviate from this distribution.
The processes generating medium-sized sand bars or
gravel bars are more difficult to study than those responsible of the transport of individual particles and
the formation of small-scale sedimentary structures. In
present-day rivers we observe that sand and gravel bars
move during floods, have different shapes (transverse,
longitudinal, linguoid bars), and are modified and reorganized by repeated changes between lower and higher
water stages in the channel system. As a result, the
bars frequently show composite internal structures
(Fig. 2.9), which may be associated with both upper
and lower flow conditions. The bars often contain lag
deposits, gravel beds and sand beds.
The most important lithofacies types of fluvial systems and their terminology are listed in Table 2.1 (cf.
Figs. 2.8 and 2.10).
Clast or Jramework-supported gravel (Gm, Gt, and
partially Gp) originates from high energy flow transporting and accumulating coarse bedload and keeping
sand and finer material in suspension. The remaining
pore space between the gravel is usually infiltrated
later by sand, when the flow velocity has decreased.
The matrix-supported gravel results either from debris .
flows (Gms), or from the simultaneous transport of
sand and gravel in a river with a high flow regime
(Gp).
35
2.2.4 Basic Faeies Elements and Architecture of
Fluvial Systems
In order to describe and reconstruct modem and ancient fluvial systems in more detail than that above,
Allen (1983) and Miall (1985) have proposed subdivi ding fluvial deposits into eight basic architectural
elements (Fig. 2.10). These elements differ, however,
in dimension and rank as structural units forming the
total fluvial sediment body. Single elements are
bounded by bed contacts of different order, the most
prominent ones being erosion surfaces or bounding
surfaces, such as those at the bases of channels. Large
architectural elements may be composed of several
smaller elements. For example, channel fills can consist ofup to five subordinate units. It is, however, the
smaller elements which we can recognize in outcrops
of limited size. Therefore, an approach from smaller to
larger architectural elements is recommended for the
study of such compound structures seen in many fluvial sediments.
Since the introduction of the eight basic architectural elements, some modifications supplements, and subdivisions of
the major elements have been proposed (see, e.g., Miall
1996). A new element is the fill of scour hollows. Subdivisions and refinements mainly deal with different types of
channel fills, lateral accretion complexes, and overbank deposits.
Fluvial deposits mayaiso alternate with eolian sediments
when sand is blown out from dry, neighboring alluvial
plains, channels, or land surfaces (Sect. 2.3). Commonly,
such eolian intercalations only form thin beds, frequentlyon
top of coarser lag deposits, because they are partially eroded
by subsequent floods. Modern examples of intercalated fluvialleolian deposits have been described from several regions, for example from the Great Sand Dunes in Colorado
(Fryberger et al. 1979); ancient examples include parts ofthe
predominantly fluvial Triassie Bunter sandstone in Europe
(e.g., Mader 1985; Marzo 1986; see also Miall 1996).
2.2.5 Alluvial Fans and Fan Deltas
Alluvial Fans
Alluvial fans are cone-shaped piles of sediment formed
at the foot of high lands where streams confined by
narrow valleys emerge into an adjacent lowland (Figs.
2.7 and 2.11). Aseries of overlapping alluvial fans
generates a clastic wedge. Fan deltas are alluvial fans
that have built into a lake or the sea. The proximal faeies of alluvial fans and the subaerial part of fan deltas
are essentially the same, but the subaqueous sediments
of fan deltas differ strongly from those of their
subaerial counterparts. Alluvial fans in arid to semiarid regions have been frequently deseribed, whereas
fans in humid regions have reeeived less attention.
Many of the common minor bed forms and corresponding
internal structures occur in all fluvial environments, but in
varying proportions. This is so, because all river systems are
characterized by frequently changing water stages and a wide
range of current velocities. For example, small sand ripples
of various shape, larger dunes, various types of cross-bedding, and flat lamination with or without gravel are observed
in nearly all river systems. Consequently, these small-scale
structures alone have limited diagnostic value. Rather, it is
the vertical or lateral succession of such structures, or the
proportion of specific structures in the total sediment body,
which can be used to identifY the overall depositional environment.
The grain size distribution of fluvial deposits does not
solely depend on the type of river system or its flow
regime. Both small and large streams rnay, for exampIe, develop a braided river system, whether they are
sand or gravel-dominated. In both cases, the transport
capacity of channel flow is intermittently sufficient to
move gravel as bedload. Hence, the proportion of
gravel in such fluvial deposits reflects rock types, relief, and weathering conditions in the source area
rather than the depositional environment. On the other
hand, the grain size distribution available in the source
area, including jointed hard rocks, has limited influence on the grain size distribution of river deposits
(Walger 1964; Ibbeken 1983). Even after a short transport distance, river sediments show a characteristic
frequency curve resembling a log-normal distribution,
while the material in the source area may significantly
deviate from this distribution.
The processes generating medium-sized sand bars or
gravel bars are more difficult to study than those responsible of the transport of individual particles and
the formation of small-scale sedimentary structures. In
present-day rivers we observe that sand and gravel bars
move during floods, have different shapes (transverse,
longitudinal, linguoid bars), and are modified and reorganized by repeated changes between lower and higher
water stages in the channel system. As a result, the
bars frequently show composite internal structures
(Fig. 2.9), which may be associated with both upper
and lower flow conditions. The bars often contain lag
deposits, gravel beds and sand beds.
The most important lithofacies types of fluvial systems and their terminology are listed in Table 2.1 (cf.
Figs. 2.8 and 2.10).
Clast or Jramework-supported gravel (Gm, Gt, and
partially Gp) originates from high energy flow transporting and accumulating coarse bedload and keeping
sand and finer material in suspension. The remaining
pore space between the gravel is usually infiltrated
later by sand, when the flow velocity has decreased.
The matrix-supported gravel results either from debris .
flows (Gms), or from the simultaneous transport of
sand and gravel in a river with a high flow regime
(Gp).
35
2.2.4 Basic Faeies Elements and Architecture of
Fluvial Systems
In order to describe and reconstruct modem and ancient fluvial systems in more detail than that above,
Allen (1983) and Miall (1985) have proposed subdivi ding fluvial deposits into eight basic architectural
elements (Fig. 2.10). These elements differ, however,
in dimension and rank as structural units forming the
total fluvial sediment body. Single elements are
bounded by bed contacts of different order, the most
prominent ones being erosion surfaces or bounding
surfaces, such as those at the bases of channels. Large
architectural elements may be composed of several
smaller elements. For example, channel fills can consist ofup to five subordinate units. It is, however, the
smaller elements which we can recognize in outcrops
of limited size. Therefore, an approach from smaller to
larger architectural elements is recommended for the
study of such compound structures seen in many fluvial sediments.
Since the introduction of the eight basic architectural elements, some modifications supplements, and subdivisions of
the major elements have been proposed (see, e.g., Miall
1996). A new element is the fill of scour hollows. Subdivisions and refinements mainly deal with different types of
channel fills, lateral accretion complexes, and overbank deposits.
Fluvial deposits mayaiso alternate with eolian sediments
when sand is blown out from dry, neighboring alluvial
plains, channels, or land surfaces (Sect. 2.3). Commonly,
such eolian intercalations only form thin beds, frequentlyon
top of coarser lag deposits, because they are partially eroded
by subsequent floods. Modern examples of intercalated fluvialleolian deposits have been described from several regions, for example from the Great Sand Dunes in Colorado
(Fryberger et al. 1979); ancient examples include parts ofthe
predominantly fluvial Triassie Bunter sandstone in Europe
(e.g., Mader 1985; Marzo 1986; see also Miall 1996).
2.2.5 Alluvial Fans and Fan Deltas
Alluvial Fans
Alluvial fans are cone-shaped piles of sediment formed
at the foot of high lands where streams confined by
narrow valleys emerge into an adjacent lowland (Figs.
2.7 and 2.11). Aseries of overlapping alluvial fans
generates a clastic wedge. Fan deltas are alluvial fans
that have built into a lake or the sea. The proximal faeies of alluvial fans and the subaerial part of fan deltas
are essentially the same, but the subaqueous sediments
of fan deltas differ strongly from those of their
subaerial counterparts. Alluvial fans in arid to semiarid regions have been frequently deseribed, whereas
fans in humid regions have reeeived less attention.