evaporites are also different from those of marine
sediments.
By comparison with marine deltas, lacustrine deltas
are among the most constructive of all (e.g. the Gilbert
Delta) because erosion in the basin is so limited.
Nevertheless, wind conditions, the size of the lake
and the composition of the sediments will be crucial
factors. In large lakes we might have relatively high
wave energy which could erode sediments from the
river mouth and deposit them laterally on beaches.
Wind will also cause water to well up from the
bottom, increasing the circulation of oxygen along
the bottom.
In dry regions we may get evaporitic lakes and
lakes which dry up after each flood (ephemeral lakes).
Lake deposits are characterised by:
1. Lack of marine fossils, but diatoms and algae may
be important components.
2. Very fine laminations in clay and silt sediments.
3. Bioturbation structures may be found in lacustrine
sediments, but they are less common than in marine
sediments.
4. Chemical analyses of freshwater sediments will
reveal low concentrations of the trace elements
which are enriched in seawater (e.g. Cl, Br and
B). Minerals formed in freshwater lakes may have
characteristic isotopic compositions. Evaporites in
lakes consist mainly of carbonate minerals;
sulphates and chlorides are normally less common.
5. The content of sulphur-bearing authigenic minerals
must be relatively low since there is little sulphate
in freshwater. However, organic material in lakes
contains a fair amount of sulphur which can be
precipitated as sulphides by sulphate-reducing bacteria. Sulphur may also be derived from volcanic
sources.
6. Lack of tidal structures and similar marine
structures is also an important indicator.
Amongst the most important ancient lacustrine
deposits are the Karoo deposits of South and East
Africa, from the Carboniferous to the Jurassic periods.
These sediments contain important coal deposits. In
eastern China there are large areas of Cretaceous and
Tertiary lacustrine sediments, which are also important petroleum-bearing sediments.
The Green River Formation (Eocene) of Colorado,
Wyoming and Utah, is a lacustrine sediment of great
extent which represents one of the most important
petroleum source rocks in the world.
Rifting associated with the opening of the Atlantic
Ocean in Mesozoic times resulted in a large number of
lacustrine basins, and sediments deposited in such
basins now underlie the continental shelves. In dry
regions these turned into evaporite basins.
2.27 River Deposits
Transport of sediments in rivers depends on the gradient and cross-section of the river, which determine the
flow rate, and on the composition and concentration of
sediment. In rivers which flow only during and directly
after the rains, but are otherwise dry (ephemeral
streams), equilibrium between flow and sediment
load is not achieved, so that even fine-grained sediment is deposited in the channel when it dries up,
leaving a mud-cracked surface. Intensive oxidation
of sediments, including silt and clay beds, is typical
of such fluvial deposits. There is little organic production in areas with a dry climate, and clay and silt
particles will therefore contain little organic material
which could act as a reducing agent. A low water table
during dry periods also contributes to oxidation of
organic matter.
Most rivers have marked seasonal and annual flow
variations. The flow velocity increases appreciably
with flow volume because the friction per unit volume
of water is inversely proportional to the water depth.
When water flow is greater than the capacity of the
channel, the water flows out over the banks. The water
outside the channel will normally be very shallow and
have a low flow velocity and fine-grained sediments,
which have been transported in suspension, are then
deposited as what we call overbank sediments.
Overbank sediment may build up into elevated banks
called levees. The resulting soil along modern rivers is
often very rich and ideal for cultivation. In postDevonian sedimentary rocks we often find traces of
plants, commonly roots, in levees. Levee deposits
have parallel lamination and in some cases also current ripples, particularly climbing ripples, which are
typical of rapid sedimentation. The primary sedimentary structures, however, will often be partly or wholly
destroyed by traces left by plant roots.
Major floods may cover the areas beyond the levees
as well, and clay and silt will be deposited on these
flood plains. Many rivers which carry a large amount
of suspended sediment gradually build up their beds
2 Introduction to Sedimentology
65
sediments.
By comparison with marine deltas, lacustrine deltas
are among the most constructive of all (e.g. the Gilbert
Delta) because erosion in the basin is so limited.
Nevertheless, wind conditions, the size of the lake
and the composition of the sediments will be crucial
factors. In large lakes we might have relatively high
wave energy which could erode sediments from the
river mouth and deposit them laterally on beaches.
Wind will also cause water to well up from the
bottom, increasing the circulation of oxygen along
the bottom.
In dry regions we may get evaporitic lakes and
lakes which dry up after each flood (ephemeral lakes).
Lake deposits are characterised by:
1. Lack of marine fossils, but diatoms and algae may
be important components.
2. Very fine laminations in clay and silt sediments.
3. Bioturbation structures may be found in lacustrine
sediments, but they are less common than in marine
sediments.
4. Chemical analyses of freshwater sediments will
reveal low concentrations of the trace elements
which are enriched in seawater (e.g. Cl, Br and
B). Minerals formed in freshwater lakes may have
characteristic isotopic compositions. Evaporites in
lakes consist mainly of carbonate minerals;
sulphates and chlorides are normally less common.
5. The content of sulphur-bearing authigenic minerals
must be relatively low since there is little sulphate
in freshwater. However, organic material in lakes
contains a fair amount of sulphur which can be
precipitated as sulphides by sulphate-reducing bacteria. Sulphur may also be derived from volcanic
sources.
6. Lack of tidal structures and similar marine
structures is also an important indicator.
Amongst the most important ancient lacustrine
deposits are the Karoo deposits of South and East
Africa, from the Carboniferous to the Jurassic periods.
These sediments contain important coal deposits. In
eastern China there are large areas of Cretaceous and
Tertiary lacustrine sediments, which are also important petroleum-bearing sediments.
The Green River Formation (Eocene) of Colorado,
Wyoming and Utah, is a lacustrine sediment of great
extent which represents one of the most important
petroleum source rocks in the world.
Rifting associated with the opening of the Atlantic
Ocean in Mesozoic times resulted in a large number of
lacustrine basins, and sediments deposited in such
basins now underlie the continental shelves. In dry
regions these turned into evaporite basins.
2.27 River Deposits
Transport of sediments in rivers depends on the gradient and cross-section of the river, which determine the
flow rate, and on the composition and concentration of
sediment. In rivers which flow only during and directly
after the rains, but are otherwise dry (ephemeral
streams), equilibrium between flow and sediment
load is not achieved, so that even fine-grained sediment is deposited in the channel when it dries up,
leaving a mud-cracked surface. Intensive oxidation
of sediments, including silt and clay beds, is typical
of such fluvial deposits. There is little organic production in areas with a dry climate, and clay and silt
particles will therefore contain little organic material
which could act as a reducing agent. A low water table
during dry periods also contributes to oxidation of
organic matter.
Most rivers have marked seasonal and annual flow
variations. The flow velocity increases appreciably
with flow volume because the friction per unit volume
of water is inversely proportional to the water depth.
When water flow is greater than the capacity of the
channel, the water flows out over the banks. The water
outside the channel will normally be very shallow and
have a low flow velocity and fine-grained sediments,
which have been transported in suspension, are then
deposited as what we call overbank sediments.
Overbank sediment may build up into elevated banks
called levees. The resulting soil along modern rivers is
often very rich and ideal for cultivation. In postDevonian sedimentary rocks we often find traces of
plants, commonly roots, in levees. Levee deposits
have parallel lamination and in some cases also current ripples, particularly climbing ripples, which are
typical of rapid sedimentation. The primary sedimentary structures, however, will often be partly or wholly
destroyed by traces left by plant roots.
Major floods may cover the areas beyond the levees
as well, and clay and silt will be deposited on these
flood plains. Many rivers which carry a large amount
of suspended sediment gradually build up their beds
2 Introduction to Sedimentology
65