Page 172 - Sedimentology and Stratigraphy

Ephemeral Lakes 159

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Fig. 10.10 When an ephemeral lake receives an influx of water and sediment, mud is deposited from suspension to form a thin
bed that is overlain by evaporite minerals as the water evaporates. Repetitions of this process create a series of couplets of
mudstone and evaporite.

least soluble minerals will precipitate first, followed by lain by a layer of evaporite minerals: these couplets
other evaporite minerals until the lake has dried are typically a few millimetres to centimetres thick
up completely (Fig. 10.9). The resulting deposit con- and are a characteristic signature of ephemeral lake
sists of a layer of mud overlain by a layer of evaporite deposition (Lowenstein & Hardie 1985). Ephemeral
minerals (Fig. 10.10) The minerals formed by eva- lake deposits occur in arid environments and
poration will be determined by the chemistry of the are therefore likely to be associated with other facies
waters and show the same ranges of composition as formed in these settings: these will include aeolian
is found in saline lakes. Subsequent flooding of the sandflat and dune deposits, alluvial fan facies and
lake floor following another flood event does not material deposited by flash floods from ephemeral riv-
necessarily result in solution of the evaporite minerals ers. These facies are likely to be found interfingering
on the surface as they may become quickly blanketed with ephemeral lake deposits to form a facies associa-
by mud. tion characteristic of arid depositional environments.
Repeated flooding and evaporation results in a ser- Evaporite minerals also form within the sediments
ies of depositional couplets of a layer of mud over- surrounding ephemeral lakes. In these areas the