2 Continental Sediments
2.2
Fluvial Sediments,
Alluvial Fans and Fan Deltas
2.2.1 Introduction
2.2.2 Types ofF1uvial Systems (Overview)
2.2.3 Minor Bed Forms and Sedimentary Structures
2.2.4 Basic Facies Elements and Architecture of
F1uvia1 Systems
2.2.5 Alluvial Fans and Fan Deltas
Alluvial Fans
Fan Deltas
2.2.6 Various River Systems and Their Sediments
Overview
Braided Rivers and Braidp1ains
Meandering Rivers
Anastomosed and Mud-Dominated Rivers
2.2.7 Large-Sca1e Lateral and Vertica1 F1uvia1 Facies
Associations
2.2.8 Reservoir Properties ofF1uvial Sediments
2.2.9 Summary (F1uvia1 Sediments etc.)
2.2.1 Introduction
Fluvial sediments constitute a significant part of the
depositional record of the Earth's his tory. At first
glance this seems to be surprising, because rivers act as
transport systems deli vering weathering products from
mountain ranges to lakes and the sea. However, subsiding areas, i.e. sedimentary basins, enable the accumulati on and preservation of fluvial deposits of considerable thicknesses.
For many past decades, the origin and interpretation
of fluvial sediments appeared essentially c1ear in
comparison with the deposits of many other environments. For this reason, fluvial sediments did not
receive much attention from sedimentologists. In the
last 10 to 20 years, however, a considerable number of
geologists and sedimentologists have viewed both
recent and ancient fluvial sediments as interesting
objects of study. This new impetus was initiated not
only by academic scientists, but also by many workers
interested in fluvial deposits as reservoirs for hydrocarbons and groundwater.
The body ofnew, detai1ed know1edge gained in this fie1d has
been summarized in severa1 specia1ized vo1umes (e.g., Miall
1978, 1985, 1996; Scholle and Spearing 1982; Galloway and
Hobday 1983; Flores et al. 1985; Ethridge et aI. 1987;
Fielding 1993; Best and Bristol 1993; Marzo and
Puigdefabregas 1993; Heller and Paola 1996; Collinson
1996; Shanley and McCabe 1998).
Partic1e transport, minor bed forms and sedimentary
structures, as described in several text books (e.g. Allen
1982; Collinson and Thompson 1989; Reid and Frostick
1994; Pye 1994), are on1y briefly mentioned here. The
various factors controlling downstream fining trends have
been studied, e.g., by Robinson and Slingerland (1998).
Although much is known about the physics of sediment
transport in rivers and the flow conditions underwhich minor
sedimentary structures are formed, the large-seale, threedimensional depositional struetures of fluvial systems are
difficult to describe by simple laws. Likewise, mineralogical
and petrographie aspects of fluvial deposits, inc1uding their
provenance from various rock types, cannot be treated in any
detail (see, e.g., Pettijohn et aI. 1987; Füchtbauer 1988;
Bosellini et aI. 1989; Potter 1994; Bahlburg and Floyd 1999).
In the context of this book, only abrief and therefore
incomplete overview can be inc1uded.
2.2.2 Types of Fluvial Systems (Overview)
The most distinctive features of fluvial systems are
large-scale phenomena, such as the size and geometry
of channels, their sinuosity and ability to migrate,
associated compound bars, and the occurrence of more
or less extended overbank deposits, naturallevees, etc.
In present-day fluvial environments, the channel
system and its sinuosity are the most striking, diagnostic features. With the aid of these criteria, one can
distinguish between several types of fluvial systems,
although there are no sharp boundaries between these
depositional environments (Fig. 2.7):
- Alluvial fans and fan deltas (bedload channels).
- Braided rivers and braidplains (bedload channels).
- Meandering river systems (mixed-load and suspended-load channels).
- Anastomosed river systems (predominantly suspended-load channels).
The factors controlling these large-scale structures are
more complex than those generating small- and
medium-scale phenomena (see below). In the case of
meandering channels, for example, it is a general rule
that the wave length and width of the meanders grow
with increasing river discharge and slope of the river
valley or fluvial plain. Braided stream systems with
channels of low sinuosity tend to form in environments
oflarge sediment supply and relatively steep gradients.
2.2
Fluvial Sediments,
Alluvial Fans and Fan Deltas
2.2.1 Introduction
2.2.2 Types ofF1uvial Systems (Overview)
2.2.3 Minor Bed Forms and Sedimentary Structures
2.2.4 Basic Facies Elements and Architecture of
F1uvia1 Systems
2.2.5 Alluvial Fans and Fan Deltas
Alluvial Fans
Fan Deltas
2.2.6 Various River Systems and Their Sediments
Overview
Braided Rivers and Braidp1ains
Meandering Rivers
Anastomosed and Mud-Dominated Rivers
2.2.7 Large-Sca1e Lateral and Vertica1 F1uvia1 Facies
Associations
2.2.8 Reservoir Properties ofF1uvial Sediments
2.2.9 Summary (F1uvia1 Sediments etc.)
2.2.1 Introduction
Fluvial sediments constitute a significant part of the
depositional record of the Earth's his tory. At first
glance this seems to be surprising, because rivers act as
transport systems deli vering weathering products from
mountain ranges to lakes and the sea. However, subsiding areas, i.e. sedimentary basins, enable the accumulati on and preservation of fluvial deposits of considerable thicknesses.
For many past decades, the origin and interpretation
of fluvial sediments appeared essentially c1ear in
comparison with the deposits of many other environments. For this reason, fluvial sediments did not
receive much attention from sedimentologists. In the
last 10 to 20 years, however, a considerable number of
geologists and sedimentologists have viewed both
recent and ancient fluvial sediments as interesting
objects of study. This new impetus was initiated not
only by academic scientists, but also by many workers
interested in fluvial deposits as reservoirs for hydrocarbons and groundwater.
The body ofnew, detai1ed know1edge gained in this fie1d has
been summarized in severa1 specia1ized vo1umes (e.g., Miall
1978, 1985, 1996; Scholle and Spearing 1982; Galloway and
Hobday 1983; Flores et al. 1985; Ethridge et aI. 1987;
Fielding 1993; Best and Bristol 1993; Marzo and
Puigdefabregas 1993; Heller and Paola 1996; Collinson
1996; Shanley and McCabe 1998).
Partic1e transport, minor bed forms and sedimentary
structures, as described in several text books (e.g. Allen
1982; Collinson and Thompson 1989; Reid and Frostick
1994; Pye 1994), are on1y briefly mentioned here. The
various factors controlling downstream fining trends have
been studied, e.g., by Robinson and Slingerland (1998).
Although much is known about the physics of sediment
transport in rivers and the flow conditions underwhich minor
sedimentary structures are formed, the large-seale, threedimensional depositional struetures of fluvial systems are
difficult to describe by simple laws. Likewise, mineralogical
and petrographie aspects of fluvial deposits, inc1uding their
provenance from various rock types, cannot be treated in any
detail (see, e.g., Pettijohn et aI. 1987; Füchtbauer 1988;
Bosellini et aI. 1989; Potter 1994; Bahlburg and Floyd 1999).
In the context of this book, only abrief and therefore
incomplete overview can be inc1uded.
2.2.2 Types of Fluvial Systems (Overview)
The most distinctive features of fluvial systems are
large-scale phenomena, such as the size and geometry
of channels, their sinuosity and ability to migrate,
associated compound bars, and the occurrence of more
or less extended overbank deposits, naturallevees, etc.
In present-day fluvial environments, the channel
system and its sinuosity are the most striking, diagnostic features. With the aid of these criteria, one can
distinguish between several types of fluvial systems,
although there are no sharp boundaries between these
depositional environments (Fig. 2.7):
- Alluvial fans and fan deltas (bedload channels).
- Braided rivers and braidplains (bedload channels).
- Meandering river systems (mixed-load and suspended-load channels).
- Anastomosed river systems (predominantly suspended-load channels).
The factors controlling these large-scale structures are
more complex than those generating small- and
medium-scale phenomena (see below). In the case of
meandering channels, for example, it is a general rule
that the wave length and width of the meanders grow
with increasing river discharge and slope of the river
valley or fluvial plain. Braided stream systems with
channels of low sinuosity tend to form in environments
oflarge sediment supply and relatively steep gradients.