11.5 Clastie Sediment, Distribution
The evaporites (including authigenic carbonates)
contribute about one half of the total filling of the
Qinghai basin and about 10% of the sediment fill of
Lake Turkana. In the latter, the precipitates are
strongly diluted as a result of the relatively high mechanical denudation rate (~50 mmlka). Due to the
moderate chemical denudation rates in the drainage
areas, the overall chemical sedimentation rates in the
lakes (around 150 mmlka in Lake Qinghai and
mostly ;:; 100 mmlka in Lake Turkana) are limited.
They cannot compete with the often extremely rapid
11.4.4 Summary (Evaporites in Basin Filling)
- Using the DA concept (Sect. 11.2.1) and known
rates of chemical weathering, the contribution of
evaporites (and biogenie carbonates and silica)
to the filling of closed lake basins can be estimated.
- Closed lake basins need long concentration
times (in the order of 50-200 ka) to achieve saturation with respect to high-soluble salts, such as
halite.
- In contrast, evaporite basins connected with the
open sea have a huge reservoir of salts and can
accurnulate thick evaporite sequences in a very
short time span (cf. Sect. 6.44).
- The accumulation rates of lacustrine evaporites
increase with the growing AJA b ratio (cf. Sect.
11.5 Distribution of Clastic Sediments in
Water-Filled Basins
11.5.1 Introduction
In the previous sections on denudation-accurnulation
systems it was mostly assumed that the incoming
sediment is distributed evenly over the entire basin
area. This assumption is useful for the assessment of
overall sediment mass balances but it is, of course,
unrealistic for studies into sedimentary facies and
stratigraphie architecture of entire or partial basin
fills. One rule is valid in all basins, including fluvial
ones: more sediment tends to accumulate in areas of
relatively strong subsidence than, for example, along
the basin margin.
Distribution of terrestrial sediment in water-filled
basins is more complex and less predictable than that
in fluvial systems (cf. Sect. 2.2). Fundamental differences between these two groups of depositional environments include:
- Fluvial basins never trap all the incoming sediment.
The gradient of the channel system and the turbulent
517
rates of salt deposition known from pre-concentrated
brines of deeper marine basins (cf. Sect. 6.4). When
Ai Ab is much greater than in the lake example (~7
and ~ 16, respectively, Table 11.1), then of course the
rates of chemical sedimentation become higher.
The fate of the dissolved river load entering the
sea and i'iS significance in geochemical cycles is discussed in special volumes, such as Holland (1984)
and Gregor et al. (1988).
11.2.1) and depend on the lithologies in the
drainage areas. With Ad/A b ~ 10, rates in the
order of 100-200 mm!ka are common.
- Long-term, the ratio between clastic and
evaporitic sediments in hydrologically completely closed lakes (no leakage) reflects the
mechanical and chemical denudation rates in
their drainage areas.
- Leakage of lake basins delays or even prevents
the precipitation of well-soluble salts (e.g.
gaylussite, mirabilite, and halite).
- Sulfate reduction and regradation of clay minerals play a significant role in controlling the
types and successions of evaporite minerals.
river flow maintain sediment transport through the
basin, at least intermittently. Generally, the relatively
coarse grained bed load tends to settle whereas the
suspended material is easily exported to areas downstream. The proportion of coarse and fine sediment
deposited in the basin greatly depends on the accommodation space provided by subsidence or change in
base level (cf. Sect. 7.6).
- Water-filled basins norrnally trap both the bed load
and the suspended river load. The modes of sediment
distribution, reworking and redistribution, however,
rnay significantly differ from basin to basin. AIthough one can apply some rules (cf. Chaps. 3
through 5), the current systems operating within the
basin as well as specific bathymetric features of the
basin can greatly modify the "normal" sediment dispers al.
It is mostly assumed that the rate of deposition systematically declines from the mouth of a river (or
several rivers in line) towards the basin center (Fig.
11.23, broken lines).
The evaporites (including authigenic carbonates)
contribute about one half of the total filling of the
Qinghai basin and about 10% of the sediment fill of
Lake Turkana. In the latter, the precipitates are
strongly diluted as a result of the relatively high mechanical denudation rate (~50 mmlka). Due to the
moderate chemical denudation rates in the drainage
areas, the overall chemical sedimentation rates in the
lakes (around 150 mmlka in Lake Qinghai and
mostly ;:; 100 mmlka in Lake Turkana) are limited.
They cannot compete with the often extremely rapid
11.4.4 Summary (Evaporites in Basin Filling)
- Using the DA concept (Sect. 11.2.1) and known
rates of chemical weathering, the contribution of
evaporites (and biogenie carbonates and silica)
to the filling of closed lake basins can be estimated.
- Closed lake basins need long concentration
times (in the order of 50-200 ka) to achieve saturation with respect to high-soluble salts, such as
halite.
- In contrast, evaporite basins connected with the
open sea have a huge reservoir of salts and can
accurnulate thick evaporite sequences in a very
short time span (cf. Sect. 6.44).
- The accumulation rates of lacustrine evaporites
increase with the growing AJA b ratio (cf. Sect.
11.5 Distribution of Clastic Sediments in
Water-Filled Basins
11.5.1 Introduction
In the previous sections on denudation-accurnulation
systems it was mostly assumed that the incoming
sediment is distributed evenly over the entire basin
area. This assumption is useful for the assessment of
overall sediment mass balances but it is, of course,
unrealistic for studies into sedimentary facies and
stratigraphie architecture of entire or partial basin
fills. One rule is valid in all basins, including fluvial
ones: more sediment tends to accumulate in areas of
relatively strong subsidence than, for example, along
the basin margin.
Distribution of terrestrial sediment in water-filled
basins is more complex and less predictable than that
in fluvial systems (cf. Sect. 2.2). Fundamental differences between these two groups of depositional environments include:
- Fluvial basins never trap all the incoming sediment.
The gradient of the channel system and the turbulent
517
rates of salt deposition known from pre-concentrated
brines of deeper marine basins (cf. Sect. 6.4). When
Ai Ab is much greater than in the lake example (~7
and ~ 16, respectively, Table 11.1), then of course the
rates of chemical sedimentation become higher.
The fate of the dissolved river load entering the
sea and i'iS significance in geochemical cycles is discussed in special volumes, such as Holland (1984)
and Gregor et al. (1988).
11.2.1) and depend on the lithologies in the
drainage areas. With Ad/A b ~ 10, rates in the
order of 100-200 mm!ka are common.
- Long-term, the ratio between clastic and
evaporitic sediments in hydrologically completely closed lakes (no leakage) reflects the
mechanical and chemical denudation rates in
their drainage areas.
- Leakage of lake basins delays or even prevents
the precipitation of well-soluble salts (e.g.
gaylussite, mirabilite, and halite).
- Sulfate reduction and regradation of clay minerals play a significant role in controlling the
types and successions of evaporite minerals.
river flow maintain sediment transport through the
basin, at least intermittently. Generally, the relatively
coarse grained bed load tends to settle whereas the
suspended material is easily exported to areas downstream. The proportion of coarse and fine sediment
deposited in the basin greatly depends on the accommodation space provided by subsidence or change in
base level (cf. Sect. 7.6).
- Water-filled basins norrnally trap both the bed load
and the suspended river load. The modes of sediment
distribution, reworking and redistribution, however,
rnay significantly differ from basin to basin. AIthough one can apply some rules (cf. Chaps. 3
through 5), the current systems operating within the
basin as well as specific bathymetric features of the
basin can greatly modify the "normal" sediment dispers al.
It is mostly assumed that the rate of deposition systematically declines from the mouth of a river (or
several rivers in line) towards the basin center (Fig.
11.23, broken lines).