Page 169 - Sedimentology and Stratigraphy

156 Lakes

supply. The spring thaw will result in an influx of
sediment-laden cold water, which will form a layer
of sediment on the lake floor. During the summer
months organic productivity in and around the lake
provides a supply of organic material that settles on
the lake floor where it is preserved in the anaerobic
conditions. This alternation of dark, organic-rich
deposits formed in the summer months and paler,
clastic sediment brought in by the spring thaw is a
distinctive feature of many temperate lakes. The milli-
metre-scale laminae formed in this way are known as
varves and they have been used in chronostratigra-
phy of Holocene deposits (21.5.7).

10.2.4 Lacustrine carbonates

Carbonates can form a significant proportion of the
succession in any lake setting only if the terrigenous
clastic input is reduced (Fig. 10.6). Direct chemical
precipitation of carbonate minerals occurs in lakes
with raised salinity, but in freshwater lakes the for-
mation of calcium carbonates is predominantly asso-
ciated with biological activity. The hard shells of
animals such as bivalve molluscs, gastropods and
ostracods can contribute some material to lake sedi-
ments, and this coarse skeletal material may be depos-
ited in shallow water or redistributed around the lake
by wave-driven currents. However, the most abun-
dant carbonate material in lakes is usually from algal
and microbial sources.
The breakdown of calcareous algal filaments is
an important source of lime mud, which may be
deposited in shallow lake waters or redeposited by
density currents into deeper parts of the lake. Cyano-
bacteria and green algae form stromatolite bioherms
and biostromes (15.3.2) in shallow (less than 10 m)
lake waters: these carbonate build-ups may form mats
centimetres to metres thick or form thick coatings
of bedrock near lake margins. They form by the
microbial and algal filaments trapping and binding
Fig. 10.5 A schematic graphic sedimentary log through
clastic deposits in a freshwater lake. carbonate (see also marine stromatolites, 3.1.3). A
common feature of lakes with areas of active carbo-
nate deposition is coated grains. Green algae and
The absence of organisms living in deep lake environ- cyanobacteria form oncoids, irregularly shaped, con-
ments means that the fine lamination is preserved centrically layered bodies of calcium carbonate sev-
because it is not disrupted by biogenic activity. eral millimetres or more across, formed around
In lakes formed in regions where there is an annual a nucleus. Oncoids form in shallow, gently wave-
thaw of winter snow a distinctive stratification may agitated zones: in these settings ooids may also form
develop due to seasonal variations in the sediment and build up oolite shoals in shallow water.