Page 165 - Sedimentology and Stratigraphy

152 Lakes

evaporites and organic material. Plants and animals chances of long-term preservation of deposits in
living in a lake may be preserved as fossils in lacus- glacial lakes is lower as they are typically in areas
trine deposits, and concentrations of organic material undergoing erosion (cf. continental glacial environ-
can form beds of coal (18.7.1) or oil and gas source ments: 7.4).
rocks (18.7.3). The study of modern lakes is referred Other processes of dam building are by landslides
to as limnology. (6.5.1) that block the path of a stream in a valley
and large volumes of volcanic ash or lava that can
create topography on the land surface and result
10.1.1 Lake formation in the formation of a lake. Volcanic activity can also
create large lakes by caldera collapse and explosive
Large inland depressions that allow the accumulation eruptions that remove large quantities of material
of water to form a lake are usually the result of from the centre of a volcanic edifice, leaving a rem-
tectonic forces creating a sedimentary basin. The for- nant rim within which a crater lake can form
mation of different sedimentary basins is discussed in (17.4.3).
Chapter 24, and the most important processes for
the creation of lake basins are those of continental
extension to generate rifts (24.2.1), basins related 10.1.2 Lake hydrology
to strike-slip within continental crust (24.5.1) and
intracontinental sag basins (24.2.3). Rift and strike- The supply of water to a lake is through streams,
slip basins are bounded by faults that cause parts groundwater and by direct rainfall on the lake sur-
of the land surface to subside relative to the surround- face. If there is no loss of water from the lake, the level
ing area. Drainage will always follow a course to the will rise through time until it reaches the spill point,
lowest level, so rivers will feed into a subsiding area which is the top of the sill or barrier around the lake
and may form a lake. With continued movement basin (Figs 10.1 & 10.2). A lake is considered to be
on the faults and hence continued subsidence, the hydrologically open if it is filled to the spill point and
lake may become hundreds of metres deep, and there is a balance of water supply into and out of the
through time may accumulate hundreds or even basin. Under these circumstances the level of the
thousands of metres of sediment. Depressions that water in the lake will be constant, and the constant
are related to broad subsidence of the crust supply from rivers will mean that the water in the
(sag basins) tend to be larger and shallower; lacus- lake will be fresh (i.e. with a low concentration of
trine deposits in these settings are likely to be rela- dissolved salts and hence low salinity).
tively thin (tens to hundreds of metres) but may be The surface of a lake will be subject to evaporation
spread over a very large area. Lakes can also be of water vapour into the atmosphere, a process that
created where thrust faults (24.4) locally uplift part becomes increasingly important at higher tempera-
of the land surface and create a dam across the path of tures and where the air is dry. If the rate of evapora-
a river. tion exceeds or balances the rate of water supply there
A depression on the land surface can also form is no outflow from the lake and it is considered to be
by erosion, but the erosive agent cannot be water hydrologically closed. These types of lake basin
alone because a stream will always follow a path are also sometimes referred to as endorheic and are
down hill. Glaciers, on the other hand, can scour basins of internal drainage. Soluble ions chemically
more deeply into a valley. Provided that the top sur- weathered from bedrock are carried in solution in
face of the glacier has an overall slope down-flow, rivers to the lake. If the supply of dissolved ions is
the base of the ice flow can move down and up creat- low the evaporation will have little effect on the con-
ing depressions in the valley floor. When the ice centration of ions in the lake water, but more com-
retreats these overdeepened parts of the valley floor monly dissolved ions become concentrated by
will become areas where lakes form. Glacial proces- evaporation to make the waters saline. With sufficient
ses can also create lakes by building up a natural evaporation and concentration evaporite minerals
dam of detritus across a valley floor through the may precipitate (3.2) and under conditions of low
formation of a terminal moraine (7.4.1). Lakes formed water supply and high evaporation rate the lake
in glacial areas tend to be relatively small and the may dry up completely.