78
land sabkhas) are shallow and fall dry most of the
time, apart from a central pond containing highly
concentrated brines (Fig. 2.28b3). By "evaporative
pumping", i.e., ascending capillary water from a shallow groundwater table, they can precipitate salts at
the surface. If the groundwater table drops too deep
for capillary water to reach the surface, salts of the
playa or salt pan are leached and transported into the
underground (Fig. 2.28b4, see also below).
Finally, several open lake systems may be combined to form a lake chain (Fig. 2.28a2), where the
last (lowermost) lake has either an outflow or is
closed. In arid to semi-arid regions, such lake chains
may display a systematic change in water chemistry
and thus also in their precipitated salts. Similarly, the
chemistry of lakes near the ocean may be influenced
by both seaward flowing groundwater and landward
intruding sea water (Fig. 2.28c). This situation can
create special chemical environments for the formation of coastal carbonates including dolomite (Sect.
3.4).
2.5.3 Sediments of Open Lake Systems
Lakes in regions of temperate, humid climate can
accumulate four main types of sediments (Dean
1981 ):
~ Detrital clastic material (primarily siliciclastics
and reworked carbonates).
~ Autochthonous biogenic and bio-induced carbonate.
~ Autochthonous biogenic silica.
~ Sediments rich in organic matter.
Lakes Dominated by Detrital Clastics
The detrital clastic components of lake sediments
reflect the relief, climate, and rock types present in
the drainage area of the lake (Sect. 9.3). Proglacial
lakes (Sect. 2.1) and many lakes in mountainous areas are dominated by terrigenous clastic sediments
including detrital carbonate. Due to a high sedimentation rate, the autochthonous sediment components
are strongly diluted, or they are produced in quantities too low to gain a significant influence. Sedimentation in such lakes is largely controlled by physical
processes distributing both the bedload and suspended load ofthe entering river(s) over the lake.
The river bedload is in motion during floods and
builds a delta out into the lake. This usually has a
lobate or birds-foot form and typically consists of
fluvial topsets and lake fore sets which together form
a classical "Gilbert-type" delta (Fig. 2.29a). The
foresets pass into finer grained bottomsets. However,
Chapter 2 Continental Sediments
where rivers discharge into very shallow lakes, the
architecture of the deltaic sediments is less regular.
In deeper lakes, a great part of the sandy river
bedload is transported from oversteepened delta
slopes into deeper water, either directly by
underflows during river floods, or by gravity mass
flows (evolving into turbidity currents; cf. Sect. 5.4.).
Thus, the principal sediment texture on the bottom of
such lakes is silty-sandy, i.e. comparatively thin,
sandy or silty turbidites alternate with fine-grained
mud which reflects slow settling from suspension.
The sediment-laden flows of large rivers can also
form subaqueous channels with levees on the
prodelta slope extending into the deeper lake (Fig.
2.29a) as, for example, observed in Lake Geneva,
Switzerland.
If muddy river water is less dense than the bottom
water of the lake, the suspended matter is distributed
by overflow (surface currents) or interflow over the
lake (Fig. 2.28a). Rhythmic sequences resulting from
these processes range from sandy, mostly thin-bedded proximal turbidites to varve-type distal successions. The recurrence time of turbidite events is seldom controlled by the seasons, but rather by irregular
storms occurring several times per year and/or rare
larger events.
Lake slopes at some distance away from river deltas receive only fine-grained material from overflows
and intermediate flows (apart from autochthonous
sediments, see below). Part of these fine-grained
slope sediments is transported by slumps and mud
flows into the deeper lake.
The sedimentation rates of small to medium-sized
lakes receiving water from large to medium-sized
rivers draining nearby mountainous regions are very
high. The following orders of magnitude are characteristic for the different zones of a lake dominated by
detrital clastics (see also Wright et al. 1980; Hsü and
Kelts 1984; Sun Shuncai 1988):
~ Prodelta area: several tens to hundreds of meters
per thousand years (ka).
~ Lake center: 3 to 10 mika.
~ Lake slopes: 1 to 3 mika.
These values demonstrate that such lakes have a
short life time and that prodelta and proximal bottom
sediments make up the bulk of their sediment fill.
This typically shows a distinct coarsening-upward
sequence from silty, varve-type bottom sets to sandy
prodelta foresets and fluvial sands and gravel (Fig.
2.29b).
land sabkhas) are shallow and fall dry most of the
time, apart from a central pond containing highly
concentrated brines (Fig. 2.28b3). By "evaporative
pumping", i.e., ascending capillary water from a shallow groundwater table, they can precipitate salts at
the surface. If the groundwater table drops too deep
for capillary water to reach the surface, salts of the
playa or salt pan are leached and transported into the
underground (Fig. 2.28b4, see also below).
Finally, several open lake systems may be combined to form a lake chain (Fig. 2.28a2), where the
last (lowermost) lake has either an outflow or is
closed. In arid to semi-arid regions, such lake chains
may display a systematic change in water chemistry
and thus also in their precipitated salts. Similarly, the
chemistry of lakes near the ocean may be influenced
by both seaward flowing groundwater and landward
intruding sea water (Fig. 2.28c). This situation can
create special chemical environments for the formation of coastal carbonates including dolomite (Sect.
3.4).
2.5.3 Sediments of Open Lake Systems
Lakes in regions of temperate, humid climate can
accumulate four main types of sediments (Dean
1981 ):
~ Detrital clastic material (primarily siliciclastics
and reworked carbonates).
~ Autochthonous biogenic and bio-induced carbonate.
~ Autochthonous biogenic silica.
~ Sediments rich in organic matter.
Lakes Dominated by Detrital Clastics
The detrital clastic components of lake sediments
reflect the relief, climate, and rock types present in
the drainage area of the lake (Sect. 9.3). Proglacial
lakes (Sect. 2.1) and many lakes in mountainous areas are dominated by terrigenous clastic sediments
including detrital carbonate. Due to a high sedimentation rate, the autochthonous sediment components
are strongly diluted, or they are produced in quantities too low to gain a significant influence. Sedimentation in such lakes is largely controlled by physical
processes distributing both the bedload and suspended load ofthe entering river(s) over the lake.
The river bedload is in motion during floods and
builds a delta out into the lake. This usually has a
lobate or birds-foot form and typically consists of
fluvial topsets and lake fore sets which together form
a classical "Gilbert-type" delta (Fig. 2.29a). The
foresets pass into finer grained bottomsets. However,
Chapter 2 Continental Sediments
where rivers discharge into very shallow lakes, the
architecture of the deltaic sediments is less regular.
In deeper lakes, a great part of the sandy river
bedload is transported from oversteepened delta
slopes into deeper water, either directly by
underflows during river floods, or by gravity mass
flows (evolving into turbidity currents; cf. Sect. 5.4.).
Thus, the principal sediment texture on the bottom of
such lakes is silty-sandy, i.e. comparatively thin,
sandy or silty turbidites alternate with fine-grained
mud which reflects slow settling from suspension.
The sediment-laden flows of large rivers can also
form subaqueous channels with levees on the
prodelta slope extending into the deeper lake (Fig.
2.29a) as, for example, observed in Lake Geneva,
Switzerland.
If muddy river water is less dense than the bottom
water of the lake, the suspended matter is distributed
by overflow (surface currents) or interflow over the
lake (Fig. 2.28a). Rhythmic sequences resulting from
these processes range from sandy, mostly thin-bedded proximal turbidites to varve-type distal successions. The recurrence time of turbidite events is seldom controlled by the seasons, but rather by irregular
storms occurring several times per year and/or rare
larger events.
Lake slopes at some distance away from river deltas receive only fine-grained material from overflows
and intermediate flows (apart from autochthonous
sediments, see below). Part of these fine-grained
slope sediments is transported by slumps and mud
flows into the deeper lake.
The sedimentation rates of small to medium-sized
lakes receiving water from large to medium-sized
rivers draining nearby mountainous regions are very
high. The following orders of magnitude are characteristic for the different zones of a lake dominated by
detrital clastics (see also Wright et al. 1980; Hsü and
Kelts 1984; Sun Shuncai 1988):
~ Prodelta area: several tens to hundreds of meters
per thousand years (ka).
~ Lake center: 3 to 10 mika.
~ Lake slopes: 1 to 3 mika.
These values demonstrate that such lakes have a
short life time and that prodelta and proximal bottom
sediments make up the bulk of their sediment fill.
This typically shows a distinct coarsening-upward
sequence from silty, varve-type bottom sets to sandy
prodelta foresets and fluvial sands and gravel (Fig.
2.29b).