Page 170 - Sedimentology and Stratigraphy

Saline Lakes 157

Fig. 10.6 Facies distributions
in a freshwater lake with carbonate
deposition.

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.
Fig. 10.7 A saline lake, Mono Lake, California: the mineral The balance between the concentrations of different
deposit mounds are associated with underground spring ions determines the minerals formed (Fig. 10.8) and
waters. three main saline lake types are recognised according
to the composition of the brines (ion-rich waters) in
Tufa (travertine) is an inorganic precipitate of them (Eugster & Hardie 1978). Soda lakes have
calcium carbonate, which may form sheets or brines with high concentrations of bicarbonate ions
mounds in lakes. Springs along the margins or in and sodium carbonate minerals such as trona and
the floor of the lake can be sites of quite spectacular natron: these minerals are not precipitated from ma-
build-ups of tufa (Fig. 10.7).
rine waters and are therefore exclusive indicators of
non-marine evaporite deposition. Sulphate lake
10.3 SALINE LAKES brines have lower concentrations of bicarbonate but
are relatively enriched in magnesium and calcium:
Saline lakes are perennial, supplied by rivers contain- they precipitate mainly sulphate minerals such as
ing dissolved ions weathered from bedrock and in a gypsum and mirabilite (a sodium sulphate). Salt
climatic setting where there are relatively high rates lakes or chloride lakes such as the Dead Sea are
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of evaporation. The salinity may vary from 5 g L of similar in mineral composition to marine evaporites.
solutes, which is brackish water, to saline, close to Organisms in saline lakes are very restricted in
the concentration of salts in marine waters (3.2), to variety but large quantities of blue-green algae and
hypersaline waters, which have values well in excess bacteria may bloom in the warm conditions. These
of the concentrations in seawater. From a sedimento- form part of a food chain that includes higher plants,
logical point of view, brackish water lakes are similar worms, specialised crustaceans and birds such as fla-
to freshwater lakes because it is the high concentra- mingoes which feed on them. Organic productivity
tions of salts that provide saline lakes with their dis- may be high enough to result in sedimentary succes-
tinctive character. The chemistry of saline lake waters sions that contain both evaporite minerals and black,