2.2 Fluvial Sediments
We can conclude that floodplain deposits are the most
important facies type in meandering fluvial systems, if
the river system is not constrained by nearby valley
slopes.
Anastomosed and Mud-Dominated Rivers
Anastomosed rivers develop an inter-connected network of straight to sinuous channels, which are relatively narrow and deep (Fig. 2.18). Because their
banks are built up of fine-grained sediment, usually
covered with vegetation, channels and islands are remarkably stable. The channels are filled with sand and
gravel. Since vertical aggradation proceeds comparatively rapidly, rather thick and narrow ribbons of channel sand can form, bounded by the sandy silts of the
levees. Interchannel areas are topographically low relative to the channellevees and fi1led by crevasse splay,
marsh, and lacustrine sediments. On the widely extended river plain or in the wetlands between the channels, laminated silty clays and clayey silts accumulate.
In addition, backswamp deposits and peats containing
large amounts of organic matter and iron sulfides may
develop.
Such systems appear to result from (1) a sediment
source rich in mud, (2) a very low river gradient, and
(3) a seasonal water budget. To accumulate and preserve anastomosed river deposits of some thickness, a
very low gradient in a subsiding basin has to be maintained for considerable time. This mechanism is possible in various tectonic basin settings, but foreland basins seem to be particularly well suited for the formation of anastomosed river sediments.
In contrast to the braided and meandering river systems, anastomosed rivers and their deposits are known
only from a limited number of recent and ancient examples.
Modern examples of anastomosed rivers have been described
from western Canada and central Australia (e.g. Smith 1983;
Rust and Legun 1983; Rust and Nanson 1989; Nadon 1994;
Gibling et al. 1998).
In the ancient record, one can expect that anastomosed
river systems are often associated with coal seams generated
in low-gradient alluvial plains. Dense vegetation favors soi!
formation and hampers erosion. Thus, the supply of sand and
coarser grained material to the river system is reduced.
Mud-dominated low-gradient rivers are, in contrast
to anastomosed rivers, characterized by mud-rich channel fills, apart from muddy overbank fines. These
braided or meandering river systems occur in areas of
very low relief with seasonally or perennially hot, dry
climates and deeply weathered clayey soils. In these
cases, the mud occurs in sand-sized pedogenic aggregates, which originate from desiccation at the land surface and on the channel floors during dry periods.
Bioturbation and vegetation within the channels may
47
aid in the generation of aggregates. The aggregates are
transported by running water like sand or small pebbles as bed (or traction) load over long distances.
Such conditions are found in the present-day arid to semiarid Lake Eyre Basin in Australia, where the deposits ofthe
Cooper and Diamantina Rivers consist predominantly of
muddy material (e.g. Gibling et al. 1998).
The clay content ofthe aggregates may surpass 50%, the
rest being silt and sand-sized particles. As long as the clay
aggregates are moist and exposed to the air, they stick together. After drying, they form micro-cracks and can be
eroded by running water, in which the aggregates are kept
separate and are remarkably durable, even under upper flow
conditions. Finally, they form porous channel fills simi!ar to
sand bars and replace sandy point bars and lateral
accretionary complexes. After burial and compaction, the
aggregate texture of the muds can be obscured or entirely
destroyed. Floodplain muds tend to be converted to vertisols
(cf. Sect. 9.1.2) displaying desiccation cracks and pedogenic
carbonate or gypsum.
Such mud-dominated river systems are probably present
more frequently in the ancient record than identified so far.
It is likely that a number of red and varicolored claystones
were deposited in this manner. To identify them, a careful
study of their texture and sedimentary structures is necessary.
Muddy river bedloads can also originate from the weathering and erosion of over-consolidated claystones and
mudstones in various climates (cf. Sect. 9.1). In these cases,
sand and gravel-sized lithoclasts can constitute a major part
of the bedload, but these are commonly accompanied by hard
coarse-grained material.
2.2.7 Large-Scale Lateral and Vertical Fluvial
Facies Associations
The cross sections of Figures 2.19 show generalized
large-scale vertical and lateral facies associations of
different fluvial systems. All examples represent sediments of wide floodplains which are composed of several contemporaneous, individual fluvial systems. The
axes of these systems coincide with the centers of the
broad sand beIts including levee and crevasse splay
sands, while the regions between the sand bodies are
occupied by the sediments of floodplains, smaller
channels and lakes. The character of the individual
fluvial deposits varies between braided bedload and
meandering mixed-load systems. The core of a sandy
braided river deposit consists of wide, amalgamated,
vertically stacked, sandy and conglomeratic channel
fills and finer-grained, sandy-silty sheet splay units
(Fig. 2.19b). These grade laterally into interbedded
floodplain deposits punctuated locally by isolated
channel fills.
Figure 2.19a,b shows a cross section ofthe Cenozoic alluvial
coastal plain in Texas (Galloway 1981). Fluvial sheet sandstones similar to those in Texas have been described from
many other regions (Miall 1996), e.g. from the Eocene in the
South Pyrenees, Spain (Marzo et al. 1988).
We can conclude that floodplain deposits are the most
important facies type in meandering fluvial systems, if
the river system is not constrained by nearby valley
slopes.
Anastomosed and Mud-Dominated Rivers
Anastomosed rivers develop an inter-connected network of straight to sinuous channels, which are relatively narrow and deep (Fig. 2.18). Because their
banks are built up of fine-grained sediment, usually
covered with vegetation, channels and islands are remarkably stable. The channels are filled with sand and
gravel. Since vertical aggradation proceeds comparatively rapidly, rather thick and narrow ribbons of channel sand can form, bounded by the sandy silts of the
levees. Interchannel areas are topographically low relative to the channellevees and fi1led by crevasse splay,
marsh, and lacustrine sediments. On the widely extended river plain or in the wetlands between the channels, laminated silty clays and clayey silts accumulate.
In addition, backswamp deposits and peats containing
large amounts of organic matter and iron sulfides may
develop.
Such systems appear to result from (1) a sediment
source rich in mud, (2) a very low river gradient, and
(3) a seasonal water budget. To accumulate and preserve anastomosed river deposits of some thickness, a
very low gradient in a subsiding basin has to be maintained for considerable time. This mechanism is possible in various tectonic basin settings, but foreland basins seem to be particularly well suited for the formation of anastomosed river sediments.
In contrast to the braided and meandering river systems, anastomosed rivers and their deposits are known
only from a limited number of recent and ancient examples.
Modern examples of anastomosed rivers have been described
from western Canada and central Australia (e.g. Smith 1983;
Rust and Legun 1983; Rust and Nanson 1989; Nadon 1994;
Gibling et al. 1998).
In the ancient record, one can expect that anastomosed
river systems are often associated with coal seams generated
in low-gradient alluvial plains. Dense vegetation favors soi!
formation and hampers erosion. Thus, the supply of sand and
coarser grained material to the river system is reduced.
Mud-dominated low-gradient rivers are, in contrast
to anastomosed rivers, characterized by mud-rich channel fills, apart from muddy overbank fines. These
braided or meandering river systems occur in areas of
very low relief with seasonally or perennially hot, dry
climates and deeply weathered clayey soils. In these
cases, the mud occurs in sand-sized pedogenic aggregates, which originate from desiccation at the land surface and on the channel floors during dry periods.
Bioturbation and vegetation within the channels may
47
aid in the generation of aggregates. The aggregates are
transported by running water like sand or small pebbles as bed (or traction) load over long distances.
Such conditions are found in the present-day arid to semiarid Lake Eyre Basin in Australia, where the deposits ofthe
Cooper and Diamantina Rivers consist predominantly of
muddy material (e.g. Gibling et al. 1998).
The clay content ofthe aggregates may surpass 50%, the
rest being silt and sand-sized particles. As long as the clay
aggregates are moist and exposed to the air, they stick together. After drying, they form micro-cracks and can be
eroded by running water, in which the aggregates are kept
separate and are remarkably durable, even under upper flow
conditions. Finally, they form porous channel fills simi!ar to
sand bars and replace sandy point bars and lateral
accretionary complexes. After burial and compaction, the
aggregate texture of the muds can be obscured or entirely
destroyed. Floodplain muds tend to be converted to vertisols
(cf. Sect. 9.1.2) displaying desiccation cracks and pedogenic
carbonate or gypsum.
Such mud-dominated river systems are probably present
more frequently in the ancient record than identified so far.
It is likely that a number of red and varicolored claystones
were deposited in this manner. To identify them, a careful
study of their texture and sedimentary structures is necessary.
Muddy river bedloads can also originate from the weathering and erosion of over-consolidated claystones and
mudstones in various climates (cf. Sect. 9.1). In these cases,
sand and gravel-sized lithoclasts can constitute a major part
of the bedload, but these are commonly accompanied by hard
coarse-grained material.
2.2.7 Large-Scale Lateral and Vertical Fluvial
Facies Associations
The cross sections of Figures 2.19 show generalized
large-scale vertical and lateral facies associations of
different fluvial systems. All examples represent sediments of wide floodplains which are composed of several contemporaneous, individual fluvial systems. The
axes of these systems coincide with the centers of the
broad sand beIts including levee and crevasse splay
sands, while the regions between the sand bodies are
occupied by the sediments of floodplains, smaller
channels and lakes. The character of the individual
fluvial deposits varies between braided bedload and
meandering mixed-load systems. The core of a sandy
braided river deposit consists of wide, amalgamated,
vertically stacked, sandy and conglomeratic channel
fills and finer-grained, sandy-silty sheet splay units
(Fig. 2.19b). These grade laterally into interbedded
floodplain deposits punctuated locally by isolated
channel fills.
Figure 2.19a,b shows a cross section ofthe Cenozoic alluvial
coastal plain in Texas (Galloway 1981). Fluvial sheet sandstones similar to those in Texas have been described from
many other regions (Miall 1996), e.g. from the Eocene in the
South Pyrenees, Spain (Marzo et al. 1988).