Page 173 - Sedimentology and Stratigraphy

160 Lakes

sediment is saturated with saline groundwater, which of lakes subject to climatic and lake level fluctuations
evaporates at the ground surface, concentrating the will exhibit frequent vertical changes in facies.
dissolved minerals and leading to the crystallisation Saline and ephemeral lakes are underfilled
of evaporite minerals. These regions are sometimes (Bohacs et al. 2000, 2003). Some saline lakes may
referred to as inland sabkhas (cf. coastal sabkhas: become relatively fresh if there is a change to wetter,
15.2.3) and the most common mineral to be formed is cooler conditions, allowing the formation of a strati-
gypsum, which grows within the sediment in an fied water body and consequently the accumulation of
interconnected mass of bladed crystals known as organic-rich sediment on the lake floor. A return to a
desert rose. drier climate increases evaporation and concentration
of ions leading to evaporite deposition. Cycles of cli-
mate change can be recognised in some lake deposits
10.5 CONTROLS ON LACUSTRINE as alternations between dark, carbonaceous mudrock
DEPOSITION (sometimes oil shales: 3.6.3) and beds of gypsum and
other evaporite minerals. The areas of shallow saline
The characteristics of the deposits of lacustrine envi- and ephemeral lakes can show considerable varia-
ronments are controlled by factors that control the tions through time as a result of changes in climate.
depth and size of the basin (which are largely deter- The rate of sediment supply is significant in all
mined by the tectonic setting), the sediment supply to lacustrine environments. If the rate of deposition of
the lake (which is a function of a combination of clastic, carbonate and evaporite deposits is greater
tectonics and climatic controls on relief and weath- than the rate of basin subsidence (Chapter 24) the
ering) and the balance between water supply and loss lake basin will gradually fill. In overfilled lake settings
through evaporation (which is principally related to this will result in a change from lacustrine to fluvial
the climate). If the climate is humid a lake will be deposition as the river waters no longer pond in
hydrologically open, with water flowing both in and the lake but instead flow straight through the former
out of it. Such lakes can be considered to be overfilled lake area with channel and overbank deposits accu-
(Bohacs et al. 2000, 2003), and their deposits are mulating. Balanced fill and underfilled basins will also
characterised by accumulation both at the margins, gradually fill with sediment, sometimes to the level of
where sediment is supplied to deltas and beaches, and the sill such that they also become areas of fluvial
in the deep water from suspension and turbidity cur- deposition.
rents. The lake level remains constant, so there is no
evidence of fluctuations in water depth under these
conditions. 10.6 LIFE IN LAKES AND FOSSILS
A balanced fill lake is one where the fluvial input IN LACUSTRINE DEPOSITS
is approximately balanced by the loss through eva-
poration. These lakes are sensitive to variations in the Palaeontological evidence is often a critical factor in
climate because a reduction in water input and/or an the recognition of ancient lacustrine facies. Fresh-
increase in evaporation (drier and/or warmer condi- water lakes may be rich in life with a large number
tions) will result in a fall in the water level below the of organisms, but they are of a limited number of
sill and the system becomes hydrologically closed. species and genera when compared with an assem-
The area of the lake will contract, shifting the lake blage from a shallow marine environment. Fauna
shoreline towards the basin centre and leaving a pe- commonly found in lake deposits include gastropods,
ripheral area exposed to subaerial conditions where bivalves, ostracods and arthropods, sometimes occur-
desiccation cracks may form, plants colonise the sur- ring in monospecific assemblages, that is, all organ-
face and pedogenic processes modify the sediment. A isms belong to the same species. Some organisms,
fall in the water level will also bring parts of the lake such as the brine shrimp arthropods, are tolerant of
floor that were previously below the wave base into saline conditions and may flourish in perennial saline
shallower water where wave ripples rework the sedi- lake environments.
ment. These changes will be reversed if the climate Algae and cyanobacteria are an important compo-
reverts to wetter and/or cooler conditions, as the lake nent of the ecology of lakes and also have sedimento-
level rises and the lake margin is reflooded. The deposits logical significance. A common organism found in