Page 171 - Sedimentology and Stratigraphy

158 Lakes

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Fig. 10.8 Three general types of saline lake can be distinguished on the basis of their chemistry.

organic-rich shales. Seasonal temperature variations
in permanent saline lakes result in a fine layering.
This is due to direct precipitation of calcium carbonate
as aragonite needles in the hot summer to form white
laminae, which alternate with darker laminae formed
by clastic input when sediment influx is greater in the
winter.

10.4 EPHEMERAL LAKES

Large bodies of water that periodically dry out are
probably best described as ephemeral lakes, Fig. 10.9 A salt crust of minerals formed by evaporation in
although the term playa lake is also commonly an ephemeral lake.
used (Briere 2000). Terms such as ‘saline pan’ are
also sometimes used to describe these temporary lake floods supply water and sediment to the basin centre
environments. It is perhaps simpler to just use the where it ponds to form a lake. Once the lake has
term ‘ephemeral lakes’ as this unambiguously implies formed, particles suspended in the water will start to
that the water body is temporary. They occur in semi- deposit and form a layer of fine-grained muddy sedi-
arid and arid environments where the rainfall is low ment (Lowenstein & Hardie 1985). Evaporation of the
and the rate of evaporation is high. water body gradually reduces its volume and the area
Many desert areas are subject to highly irregular of the lake starts to shrink, leaving areas of margin
rainfall with long periods of dry conditions inter- exposed where desiccation cracks (4.6) may form in
rupted by intense rainfall that may occur only every the mud as it dries out. With further evaporation the
few years or tens of years. After rainfall in the catch- ion concentration in the water starts to increase to
ment area, the rivers become active (9.2.3) and flash the point where precipitation of minerals occurs. The