Geology Reference
In-Depth Information
maintained (perennial lake). If the evaporation exceeds
inflow for a long time, salinity and the Mg/Ca ratio of
the water will increase, as Ca is depleted by early-stage
precipitates. Finally salts will be precipitated in these
shallow 'salt lakes' and 'soda lakes' which normally
fall dry, with the exception of a central pond contain-
ing highly concentrated brines (ephemeral lakes, playa
lakes: Last 1993). High and extreme salinities can origi-
nate from a separation of former parts of the sea (e.g.
separation of the Caspian Sea from the Mediterranean
Sea), prolonged evaporation of lake waters, or increased
input of dissolved salts.
waters on the playa surfaces cause supersaturation
with respect to calcite, resulting in the formation of
pisoids and carbonate encrustations (Risacher and
Eugster 1979),
•
Hydrothermal fluids emerging from the lake bottom
may produce sinter crusts consisting of aragonite and
magnesium calcite (Lake Tanganyika, East-Central
Africa: Cohen and Thouin 1987, Stoffers and Botz
1994).
Compositional characteristics of lake carbonates
Case studies:
Mono Lake, Death Valley, California;
Great Salt Lake, Utah (Eardly 1966; Halley 1976); sea-
marginal ponds of the eastern Mediterranean e.g. Solar
Lake, Israel (Krumbein and Cohen 1974; Friedman
1978); sea-marginal flats in the Red Sea, Gulf of Elat
and Aqaba (Sneh and Friedman 1985); Australia: (De
Deckker 1988); Lake Clifton (Burne and Moore 1983);
Lake Thetis (Grey et al. 1990); Coorong Lagoon (Rosen
et al. 1988). One of the most intersting saline lakes is
the Dead Sea whose bottom sediments consists of pre-
cipitated aragonite, gypsum and halite, and detrital
grains (Garber et al. 1987).
Many hydrologically closed lakes are characterized
by fringes of supralittoral mudflats (commonly termed
'playa' or 'inland sabkha'), sandflats in the nearshore
zone, and depositions of mixed siliciclastic-evaporitic
sediments as well as organic-rich sediments ('oil
shales') within the lakes. Saline minerals are precipi-
tated in perennial brines, as efflorescent crusts and salt
pans and as cements within the sediments of the mud-
flats.
Climatic changes may turn freshwater lakes into sa-
line water bodies, as exemplified by the history of the
modern Great Salt Lake, Utah, or the great lakes in the
East African rift system.
Similar to marine carbonates, lacustrine carbonates
are composed of allochthonous and autochthonous con-
stituents, the former represented by skeletal and non-
skeletal grains and fine-grained calcite mud caused by
seasonal phytoplankton blooms ('Seekreide': Schäfer
1973), the latter by predominantly biogenic structures
(stromatolites, bioherms and 'reefs') formed or induced
by cyanobacteria and algae. All major grain categories
known from marine carbonates also occur in lacustrine
settings.
Biota:
Differences exist in the inventory of the biota.
Common organisms are (a) calcareous algae (charo-
phycean algae, see Sect. 10.2.1.8 and Pl. 65; plank-
tonic and benthic green algae, see Pl.130/1) and cyano-
bacteria, siliceous algae (diatoms), (b) gastropods and
bivalves, cf. Pl. 130/2, 3, (c) ostracods (see Pl. 130/2),
and (d) arthropods that are responsible for the produc-
tion of abundant peloids. Otherwise marine organisms,
e.g. foraminifera, also may occur in salt lakes. These
marine biota are transported by wind and may thrive in
lakes if environmental conditions are favorable (Lee
and Anderson 1991). Bioturbation and bioerosion are
very important processes modifying the depositional tex-
ture of lacustrine sediments (Schneider 1977, Schröder
et al. 1983).
Peloids
and reworked and redeposited sedimentary
fragments (intraclasts, lithoclasts) are common constitu-
ents. Small ooids and large oncoids (Pl. 131/5) are of
specific interest in microfacies studies of ancient lacus-
trine limestones because these grains may aid in the
differentiation of non-marine and marine settings.
Ooids
occur in marine as well in non-marine set-
tings (Pl. 13/1), they are known from low- and high-
saline environments. Ooids are not common in recent
freshwater lakes. Ooids originating in salt lakes have
been described from the Great Salt Lake, Utah (Eardley
1966; Kahle 1974; Sandberg 1975; Halley 1977; Pl.
13/2) and the Pyramid Lake, Nevada (Popp and Wilkin-
son 1983). Ooids formed in a temperate 'marl' lake were
Carbonates in saltwater lakes are characterized by:
•
Precipitation of High-Mg calcite and aragonite (e.g.
Dead Sea) or Low-Mg calcite and High-Mg calcite
(e.g. Lake Balaton, Hungary) because of the increase
in the Mg/Ca ratio,
•
Formation of carbonate laminites, consisting of Ca-
Mg carbonates, detrital quartz, silicates and organic
layers,
•
Ooid sands are formed in shallow, near-shore areas
of current- or wave-swept lakes, sometimes (e.g.
Great Salt Lake, Utah) associated with
•
Algal bioherms, composed of mm-scale laminated
micritic aragonite or of porous carbonate,
•
Loss of CO
2
by degassing or photosynthesis of spring
