Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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waterlogged setting that histosols (peaty soils) form

in. Other types are indicative of the degree of the

maturity of the soil profile (and hence the time over

which the soil has developed); entisols are very imma-

ture and inceptisols show more development, but are

less mature than the other types lower in the list. The

type of vegetation is an important factor in some

cases: spodosols, alfisols and ultisols are soils formed

in forests, whereas mollisols are grassland soils.

Finally, the formation of andisols is restricted to vol-

canic substrates.

9.7.2 Palaeosols

A palaeosol is a fossil soil. Many of the characteristics

of modern soils noted above can be recognised in soils

that formed in the geological past (Mack et al. 1993;

Retallack 2001). These features include the presence

of fossilised roots, the burrows of soil-modifying

organisms, vertical cracks in the sediment and layers

enriched or depleted in certain minerals. The study of

palaeosols provides important information about

ancient landscapes and in particular they can indicate

the palaeoclimate, the type of vegetation growing and

the time period during which a land surface was

exposed.

The precipitation of calcium carbonate within the

soil is a conspicuous feature of some aridisols that form

in semi-arid to arid climates. These calcrete soils form

by the movement of water through the soil profile

precipitating calcium carbonate as root encrustations

(rhizocretions) and as small soil nodules (glaebules)

(Wright & Tucker 1991). The nodules grow and co-

alesce as precipitation continues to form a fully devel-

oped calcrete, which consists of a dense layer of

calcium carbonate near to the surface with tepee

structures, i.e. domes in the layer formed by the

expansion of the calcium carbonate as it is precipi-

tated (Allen 1974) (Fig. 9.25). The stages in the

development of a calcrete soil profile are easily recog-

nised in palaeosols, so if the rate of development of a

mature profile can be measured, the time over which

an ancient profile formed can be estimated (Leeder

1975).

The passage of time can also be indicated by other

palaeosol types: entisols and inceptisols indicate that

the time available for soil formation on a particular

surface was relatively short, whereas other, more

mature categories of palaeosol require a longer period

of exposure of the surface. These distinctions become

useful when attempting to assess rates of deposition

on, for example, a floodplain surface: entisols would

indicate relatively rapid deposition, with little time for

soil development before flooding deposited more sedi-

ment on the surface, whereas a well-developed spodo-

sol, alfisol or ultisol suggests a much longer period of

time before the surface was covered with younger

sediment. However, it should be noted that the time

taken for any soil profile to develop varies consider-

ably with temperature, rainfall and the availability of

different minerals so time estimates are always rela-

tive, not absolute. Also, soil profiles can become com-

plicated by the superimposition of a younger profile

over an older one (Bown & Kraus 1987).

poorly developed

soil profile

well-developed

soil profile

evaporation at surface

groundwater drawn through soil

carbonate glaebules

coalesced glaebules and rhizoliths

thick calcrete horizon

Fig. 9.25 A calcrete forms by precipitation of calcium

carbonate within a soil in an arid or semi-arid environment.

148 Rivers and Alluvial Fans

Some types of modern and fossil soils have been

given particular names. For example, seatearths are

histisols, argillisols or spodosols that are common in

the coal measures of northwestern Europe and North

America (Percival 1986). They are characterised by a

bed of organic matter underlain by a leached horizon

of white sandstone from which iron has been washed

out. Laterites are oxisols that are the product of

extensive weathering of bedrock to form a soil that

consists mainly of iron and aluminium oxides: exam-

ples of laterites may be found in the stratigraphic

record as strongly reddened layers between basalt

lava flows and provide evidence that the eruption

was subaerial. Iron-rich oxisols that become cemen-

ted are known as ferricretes and they are a type of

hardened soil profile called a duricrust. Duricrusts

are highly resistant surfaces that develop over very

long time periods (e.g. they are found associated with

major unconformities; Retallack 2001); as well as

iron-rich forms there are records of silcretes, which

are silica-rich.

Identification of a palaeosol profile is probably the

most reliable indicator of a terrestrial environment.

Channels are not unique to the fluvial regime because

they also occur in deltas, tidal settings and deep ma-

rine environments, and thin sheets of sandstone are

also common to many other depositional settings.

However, sometimes the recognition of a palaeosol

can be made difficult by diagenetic alteration (18.2 ),

which can destroy the original pedogenic features.

9.8 FLUVIAL AND ALLUVIAL FAN

DEPOSITION: SUMMARY

Fluvial environments are characterised by flow and

deposition in river channels and associated overbank

sedimentation. In the stratigraphic record the channel

fills are represented by lenticular to sheet-like bodies

with scoured bases and channel margins, although

these margins are not always seen. The deposits of

gravelly braided rivers are characterised by cross-

bedded conglomerate representing deposition on chan-

nel bars. Both sandy braided river and meandering river

deposits typically consist of fining-upward successions

from a sharp scoured base through beds of trough and

planar cross-bedded, laminated and cross-laminated

sandstone. Lateral accretion surfaces characterise

meandering rivers that are also often associated with a

relatively high proportion of overbank facies. Floodplain

deposits are mainly alternating thin sandstone sheets

and mudstones with palaeosols; small lenticular bodies

of sandstone may represent crevasse splay deposition.

Palaeocurrent data from within channel deposits

are unidirectional, with a wider spread about the

mean in meandering river deposits; palaeocurrents

in overbank facies are highly variable.

Alluvial fan deposits are located near to the mar-

gins of sedimentary basins and are limited in lateral

extent to a few kilometres from the margin. The facies

are dominantly conglomerates, and may include

matrix-supported fabrics deposited by debris flows,

well-stratified gravels and sands deposited by sheet-

flood processes and in channels that migrate laterally

across the fan surface. Alluvial and fluvial deposits

will interfinger with lacustrine and/or aeolian facies,

depending on the palaeoclimate, and many (but not

all) river systems feed into marine environments via

coasts, estuaries and deltas. Other characteristics of

fluvial and alluvial facies include an absence of ma-

rine fauna, the presence of land plant fossils, trace

fossils and palaeosol profiles in alluvial plain deposits.

Characteristics of fluvial and alluvial fan deposits

. lithologies – conglomerate, sandstone and mudstone

. mineralogy – variable, often compositionally imma-

ture

. texture – very poor in debris flows to moderate in

river sands

. bed geometry – sheets on fans, lens shaped river

channel units

. sedimentary structures – cross-bedding and lamina-

tion in channel deposits

. palaeocurrents – indicate direction of flow and

depositional slope

. fossils – fauna uncommon, plant fossils may be

common in floodplain facies

. colour – yellow, red and brown due to oxidising

conditions

. facies associations – alluvial fan deposits may be asso-

ciated with ephemeral lake and aeolian dunes, rivers

may be associated with lake, delta or estuarine facies

Further Reading 149

Bridge, J.S. (2003) Rivers and Floodplains: Forms, Processes,

and Sedimentary Record. Blackwell Science, Oxford.

Bridge, J.S. (2006) Fluvial facies models: recent developments.

In: Facies Models Revisited (Eds Walker, R.G. & Posamen-

tier, H.). Special Publication 84, Society of Economic

Paleontologists and Mineralogists, Tulsa, OK; 85–170.

Collinson, J.D. (1996) Alluvial sediments. In: Sedimentary

Environments: Processes, Facies and Stratigraphy (Ed. Read-

ing, H.G.). Blackwell Science, Oxford; 37–82.

Harvey, A.M., Mather, A.E. & Stokes, M. (Eds) (2005) Alluvial

Fans: Geomorphology, Sedimentology, Dynamics. Special

Publication 251, Geological Society Publishing House,

Bath.

Retallack, G.J. (2001) Soils of the Past: an Introduction to

Paleopedology (2nd edition). Blackwell Science, Oxford.

Smith, N.D. & Rogers, J. (Eds) (1999) Fluvial Sedimentology

VI. Special Publication 28, International Association of

Sedimentologists. Blackwell Science, Oxford.

150 Rivers and Alluvial Fans

Lakes

Lakes form where there is a supply of water to a topographic low on the land surface.

They are fed mainly by rivers and lose water by flow out into a river and/or evaporation

from the surface. The balance between inflow and outflow and the rate at which evapora-

tion occurs control the level of water in the lake and the water chemistry. Under condi-

tions of high inflow the water level in the lake may be constant, governed by the spill point

of the outflow, and the water remains fresh. Low water input coupled with high evapora-

tion rates in an enclosed basin results in the concentration of dissolved ions, which may

be precipitated as evaporites in a perennial saline lake or when an ephemeral lake dries

out. Lakes are therefore very sensitive to climate and climate change. Many of the

processes that occur in seas also occur in lakes: deltas form where rivers enter the

lake, beaches form along the margins, density currents flow down to the water bottom

and waves act on the surface. There are, however, important differences with marine

settings: the fauna and flora are distinct, the chemistry of lake waters varies from lake to

lake and certain physical processes of temperature and density stratification are unique

to lacustrine environments.

10.1 LAKES AND LACUSTRINE

ENVIRONMENTS

A lake is an inland body of water. Although some

modern lakes may be referred to as ‘inland seas’, it is

useful to draw a distinction between water bodies that

have some exchange of water with the open ocean

(such as lagoons – 13.3.2) and those that do not,

which are true lakes. Lakes form where there is a

depression on the land surface which is bounded by

a sill such that water accumulating in the depression

is retained. Lakes are typically fed by one or more

streams that supply water and sediment from the

surrounding hinterland. Groundwater may also feed

water into a lake. The amount of sediment accumu-

lated in lakes is small compared with marine basins,

but they may be locally significant, resulting in strata

hundreds of metres thick and covering hundreds to

thousands of square kilometres. Sand and mud are the

most common components of lake deposits, although

almost any other type of sediment can accumulate in

lacustrine (lake) environments, including limestones,

evaporites and organic material. Plants and animals

living in a lake may be preserved as fossils in lacus-

trine deposits, and concentrations of organic material

can form beds of coal (18.7.1) or oil and gas source

rocks (18.7.3). The study of modern lakes is referred

to as limnology.

10.1.1 Lake formation

Large inland depressions that allow the accumulation

of water to form a lake are usually the result of

tectonic forces creating a sedimentary basin. The for-

mation of different sedimentary basins is discussed in

Chapter 24, and the most important processes for

the creation of lake basins are those of continental

extension to generate rifts (24.2.1), basins related

to strike-slip within continental crust (24.5.1) and

intracontinental sag basins (24.2.3). Rift and strike-

slip basins are bounded by faults that cause parts

of the land surface to subside relative to the surround-

ing area. Drainage will always follow a course to the

lowest level, so rivers will feed into a subsiding area

and may form a lake. With continued movement

on the faults and hence continued subsidence, the

lake may become hundreds of metres deep, and

through time may accumulate hundreds or even

thousands of metres of sediment. Depressions that

are related to broad subsidence of the crust

(sag basins) tend to be larger and shallower; lacus-

trine deposits in these settings are likely to be rela-

tively thin (tens to hundreds of metres) but may be

spread over a very large area. Lakes can also be

created where thrust faults (24.4) locally uplift part

of the land surface and create a dam across the path of

a river.

A depression on the land surface can also form

by erosion, but the erosive agent cannot be water

alone because a stream will always follow a path

down hill. Glaciers, on the other hand, can scour

more deeply into a valley. Provided that the top sur-

face of the glacier has an overall slope down-flow,

the base of the ice flow can move down and up creat-

ing depressions in the valley floor. When the ice

retreats these overdeepened parts of the valley floor

will become areas where lakes form. Glacial proces-

ses can also create lakes by building up a natural

dam of detritus across a valley floor through the

formation of a terminal moraine (7.4.1). Lakes formed

in glacial areas tend to be relatively small and the

chances of long-term preservation of deposits in

glacial lakes is lower as they are typically in areas

undergoing erosion (cf. continental glacial environ-

ments: 7.4).

Other processes of dam building are by landslides

(6.5.1) that block the path of a stream in a valley

and large volumes of volcanic ash or lava that can

create topography on the land surface and result

in the formation of a lake. Volcanic activity can also

create large lakes by caldera collapse and explosive

eruptions that remove large quantities of material

from the centre of a volcanic edifice, leaving a rem-

nant rim within which a crater lake can form

(17.4.3).

10.1.2 Lake hydrology

The supply of water to a lake is through streams,

groundwater and by direct rainfall on the lake sur-

face. If there is no loss of water from the lake, the level

will rise through time until it reaches the spill point,

which is the top of the sill or barrier around the lake

basin (Figs 10.1 & 10.2). A lake is considered to be

hydrologically open if it is filled to the spill point and

there is a balance of water supply into and out of the

basin. Under these circumstances the level of the

water in the lake will be constant, and the constant

supply from rivers will mean that the water in the

lake will be fresh (i.e. with a low concentration of

dissolved salts and hence low salinity).

The surface of a lake will be subject to evaporation

of water vapour into the atmosphere, a process that

becomes increasingly important at higher tempera-

tures and where the air is dry. If the rate of evapora-

tion exceeds or balances the rate of water supply there

is no outflow from the lake and it is considered to be

hydrologically closed. These types of lake basin

are also sometimes referred to as endorheic and are

basins of internal drainage. Soluble ions chemically

weathered from bedrock are carried in solution in

rivers to the lake. If the supply of dissolved ions is

low the evaporation will have little effect on the con-

centration of ions in the lake water, but more com-

monly dissolved ions become concentrated by

evaporation to make the waters saline. With sufficient

evaporation and concentration evaporite minerals

may precipitate (3.2) and under conditions of low

water supply and high evaporation rate the lake

may dry up completely.

152 Lakes

From a sedimentological point of view, three types

of lake can be considered, irrespective of their mode

of formation or hydrology. Freshwater lakes have

low salinity waters and are either hydrologically

open, or are hydrologically closed with a low supply

of dissolved ions allowing the water to remain fresh.

Saline lakes are hydrologically closed and are pe-

rennial water bodies in which dissolved ions have

become concentrated by evaporation. Ephemeral

lakes mainly occur in arid climatic settings and are

temporary bodies of water that exist for a few months

or years after large rainstorms in the catchment area,

but are otherwise dry.

10.2 FRESHWATER LAKES

The majority of large modern lakes are freshwater:

they occur at latitudes ranging from the Equator to

the polar regions (Bohacs et al. 2003) and include

some of the largest and deepest in the world today.

Lacustrine deposits from lakes of similar scales are

known from the stratigraphic record, mainly from

Devonian through to Neogene strata.

Fig. 10.1 Hydrological regimes of lakes.

hydrologically

closed lake

basin

hydrologically

open lake

basin

lake filled to top of sill

- outflow from the lake

into a river

lake level below sill

- no outflow

sill

Fig. 10.2 A lake basin supplied by a river in the foreground,

with outflow through a sill to the sea in the distance.

Freshwater Lakes 153

10.2.1 Hydrology of freshwater lakes

Lakes are relatively static bodies of water, with no

currents driven by tidal processes or oceanic circu-

lation (cf. seas). Waves form when a wind blows

over the s urface of the water, but the limited size of

any lake means that there is not a large fetch (4.4)

and hence the waves cannot grow to the sizes seen in

the world’s oceans. Wind-driven surface currents may

reach velocities up to 30 cm s

1

(Talbot & Allen

1996), especially in narrow valleys where the

wind is funnelled by the topography. However,

currents driven by the wind in lakes are too weak

to move anything more than silt and fine sand and

will not redistribute c oarser sediment. These cur-

rents and the relatively small waves form ed on a

lake influence the upper part of the water body, and

the effects of the water oscillation decrease with

depth (4.4.1). Therefore, below about 10 or 20 m

depth the lake waters are unaffected by any wave or

current activity. This allows for the development

of lake water stratification, which is seen as a

contrast in the temperature, density and the chemis-

try of the waters in the upper and lower parts of the

water body.

The surface of the lake is warmed by the Sun

and the water retains the heat to acquire a steady

temperature that varies gradually with the seasons.

Due to the lack of circulation the water in the lower

part of the lake remains at a constant, cooler tempera-

ture. These two divisions of the lake water are known

as the epilimnion, which is the upper, warmer lake

water, and the hypolimnion, the lower, colder part:

they are separated by a surface across which the

temperature changes, the thermocline (Fig. 10.3).

The density of pure water is determined by the tem-

perature and, above 48C, the density decreases as it

becomes warmer. The stratification is therefore one

of density as well as temperature, and, because

the lower density warm water is above the higher

density cold water, the situation is stable (Talbot &

Allen 1996).

Agitation of the lake surface by waves and circula-

tion in the epilimnion means that this part of the

water body is oxygenated by contact with the air. In

the hypolimnion, any oxygen is quickly used up

by aerobic bacterial activity and, due to the lack

of circulation, is not replenished. The bottom of

the lake therefore becomes anaerobic (without air,

and therefore oxygen) and this has two important

consequences. First, any organic material that falls

through the water column to the lake floor will not

be subject to breakdown by the activity of the aerobic

processes that normally cause decomposition of plant

and animal tissue. If there is abundant plant material

being swept into the lake, this has the potential to

form a detrital coal layer (18.7.1), and the remains

of algal or bacterial life within the lake may also

accumulate to form a bed rich in organic matter,

which may ultimately form a sapropelic coal or a

Lake surface

Epilimnion - Warm

Anoxic

Oxic

Hypolimnion - Cold

Underflow (high density)

Sediment deposited as turbidite

Sediment input

from river

Cold, dense water

Overflow (low density)

Fine sediment deposited

from suspension

Warm, buoyant

water

Thermocline

Fig. 10.3 The thermal stratification of fresh lake waters results in a more oxic, upper layer, the epilimnion, and a colder,

anoxic lower layer, the hypolimnion. Sedimentation in the lake is controlled by this density stratification above and

below the thermocline.

154 Lakes

source rock for oil and gas (18.7.3). The second effect

of anaerobic bottom conditions is that this is an envi-

ronment that is unfavourable for life. Stratified lakes

therefore have no animals living on the bottom or

within the surface sediment and hence there is no

bioturbation (11.7.3) to disturb the primary sedimen-

tary layering.

10.2.2 Lake margin clastic deposits

Where a sediment-laden river enters a lake the water

velocity drops abruptly and a delta forms as coarse

material is deposited at the river mouth (Fig. 10.4).

The form and processes on a lake delta will be similar

to that seen in river-dominated deltas, with some

wave reworking of sediment also occurring if the

lake experiences strong winds. The character of the

delta deposits will be largely controlled by the nature

of the sediment supply from the river, and may range

from fine-grained deposits to coarse, gravelly fan-del-

tas (12.4.2).

Away from the river mouth the nature of the lake

shore deposits will depend on the strength of winds

generating waves and currents in the lake basin. If

winds are not strong, lake shore sediments will tend to

be fine-grained but strong, wind-driven currents can

redistribute sandy sediment around the edges of the

lake where it can be reworked by waves into sandy

beach deposits (Reid & Frostick 1985). These mar-

ginal lacustrine facies (Fig. 10.5) will be similar in

character to beaches developed along marine shore-

lines (13.2).

In situations where the slope into the lake is very

gentle the edge of the water body is poorly defined as

the environment merges from wet alluvial plain into a

lake margin setting. This lake margin marshy envi-

ronment is sometimes referred to as a palustrine

environment. Plants and animals living in this setting

live in and on the sediment in a wet soil environment

where sediments will be modified by soil (pedogenic)

processes (9.7), resulting in a nodular texture that

may sometimes be calcareous.

10.2.3 Deep lake facies

Away from the margins, clastic sedimentation occurs

in the lake by two main mechanisms: dispersal as

plumes of suspended sediment and transport by den-

sity currents (Fig. 10.3) (Sturm & Matter 1978).

Plumes of water laden with suspended sediment may

be brought into the lake by rivers: if the sediment–

water mixture is a lower density than the hypolim-

nion the plume will remain above the thermocline

and will be distributed around the lake by wind-dri-

ven surface currents. The suspended load will even-

tually start to settle out of the epilimnion and fall to

the lake floor to form a layer of mud. Density currents

(4.5) provide a mechanism for transporting coarser

sediment across the lake floor. Mixtures of sediment

and water brought in by a river or reworked from a

lake delta may flow as a turbidity current (4.5.2),

which can travel across the lake floor. The deposits

will be layers of sediment that grade from coarse

material deposited from the current first to finer sedi-

ment that settles out last. In lakes where sediment

plumes and turbidity currents are the main transport

mechanisms the deep lake facies will consist of very

finely laminated muds deposited from suspension

alternating with thin graded turbidites forming

a characteristic, thinly bedded succession (Fig. 10.5).

Fig. 10.4 Facies distribution in a

freshwater lake with dominantly clastic

deposition.

lake delta

river

lake shoreline

marsh

(palustrine)

environment

sandy lake

floor deposits

anoxic deep

water muds

Freshwater Lakes 155

The absence of organisms living in deep lake environ-

ments means that the fine lamination is preserved

because it is not disrupted by biogenic activity.

In lakes formed in regions where there is an annual

thaw of winter snow a distinctive stratification may

develop due to seasonal variations in the sediment

supply. The spring thaw will result in an influx of

sediment-laden cold water, which will form a layer

of sediment on the lake floor. During the summer

months organic productivity in and around the lake

provides a supply of organic material that settles on

the lake floor where it is preserved in the anaerobic

conditions. This alternation of dark, organic-rich

deposits formed in the summer months and paler,

clastic sediment brought in by the spring thaw is a

distinctive feature of many temperate lakes. The milli-

metre-scale laminae formed in this way are known as

varves and they have been used in chronostratigra-

phy of Holocene deposits (21.5.7).

10.2.4 Lacustrine carbonates

Carbonates can form a significant proportion of the

succession in any lake setting only if the terrigenous

clastic input is reduced (Fig. 10.6). Direct chemical

precipitation of carbonate minerals occurs in lakes

with raised salinity, but in freshwater lakes the for-

mation of calcium carbonates is predominantly asso-

ciated with biological activity. The hard shells of

animals such as bivalve molluscs, gastropods and

ostracods can contribute some material to lake sedi-

ments, and this coarse skeletal material may be depos-

ited in shallow water or redistributed around the lake

by wave-driven currents. However, the most abun-

dant carbonate material in lakes is usually from algal

and microbial sources.

The breakdown of calcareous algal filaments is

an important source of lime mud, which may be

deposited in shallow lake waters or redeposited by

density currents into deeper parts of the lake. Cyano-

bacteria and green algae form stromatolite bioherms

and biostromes (15.3.2) in shallow (less than 10 m)

lake waters: these carbonate build-ups may form mats

centimetres to metres thick or form thick coatings

of bedrock near lake margins. They form by the

microbial and algal filaments trapping and binding

carbonate (see also marine stromatolites, 3.1.3). A

common feature of lakes with areas of active carbo-

nate deposition is coated grains. Green algae and

cyanobacteria form oncoids, irregularly shaped, con-

centrically layered bodies of calcium carbonate sev-

eral millimetres or more across, formed around

a nucleus. Oncoids form in shallow, gently wave-

agitated zones: in these settings ooids may also form

and build up oolite shoals in shallow water.

Fig. 10.5 A schematic graphic sedimentary log through

clastic deposits in a freshwater lake.

156 Lakes

Tufa (travertine) is an inorganic precipitate of

calcium carbonate, which may form sheets or

mounds in lakes. Springs along the margins or in

the floor of the lake can be sites of quite spectacular

build-ups of tufa (Fig. 10.7).

10.3 SALINE LAKES

Saline lakes are perennial, supplied by rivers contain-

ing dissolved ions weathered from bedrock and in a

climatic setting where there are relatively high rates

of evaporation. The salinity may vary from 5 g L

1

solutes, which is brackish water, to saline, close to

the concentration of salts in marine waters (3.2), to

hypersaline waters, which have values well in excess

of the concentrations in seawater. From a sedimento-

logical point of view, brackish water lakes are similar

to freshwater lakes because it is the high concentra-

tions of salts that provide saline lakes with their dis-

tinctive character. The chemistry of saline lake waters

is determined by the nature of the salts dissolved from

the bedrock of the catchment area of the river systems

that supply the lake. The bedrock geology varies from

place to place, so the chemical composition of every

lake is therefore unique, unlike marine waters, which

all have the same composition of salts. The types and

proportions of evaporite minerals formed in saline

lakes are therefore variable, and include minerals

not found in marine evaporite successions.

The main ions present in modern saline lake waters

are the cations sodium, calcium and magnesium

and the carbonate, chloride and sulphate anions.

The balance between the concentrations of different

ions determines the minerals formed (Fig. 10.8) and

three main saline lake types are recognised according

to the composition of the brines (ion-rich waters) in

them (Eugster & Hardie 1978). Soda lakes have

brines with high concentrations of bicarbonate ions

and sodium carbonate minerals such as trona and

natron: these minerals are not precipitated from ma-

rine waters and are therefore exclusive indicators of

non-marine evaporite deposition. Sulphate lake

brines have lower concentrations of bicarbonate but

are relatively enriched in magnesium and calcium:

they precipitate mainly sulphate minerals such as

gypsum and mirabilite (a sodium sulphate). Salt

lakes or chloride lakes such as the Dead Sea are

similar in mineral composition to marine evaporites.

Organisms in saline lakes are very restricted in

variety but large quantities of blue-green algae and

bacteria may bloom in the warm conditions. These

form part of a food chain that includes higher plants,

worms, specialised crustaceans and birds such as fla-

mingoes which feed on them. Organic productivity

may be high enough to result in sedimentary succes-

sions that contain both evaporite minerals and black,

Fig. 10.6 Facies distributions

in a freshwater lake with carbonate

deposition.

shallow water

bioherms

lake margin

carbonate shoals

marsh

(palustrine)

environment

Fig. 10.7 A saline lake, Mono Lake, California: the mineral

deposit mounds are associated with underground spring

waters.

Saline Lakes 157