Page 174 - Sedimentology and Stratigraphy

Further Reading 161

lake deposits are charophytes, algae belonging to the lacustrine environments becomes more difficult in
Chlorophyta (3.1.3), which are seen in many ancient rocks that are from further back in the stratigraphic
lacustrine sediments in the form of calcareous record, and in Precambrian strata it may be almost
encrusted stems and spherical reproductive bodies. impossible to be sure.
Charophytes are considered to be intolerant of high A feature of lakes that is much less commonly
salinities and the recognition of these millimetre- found in marine settings is the stratification of the
scale, often dark, spherical bodies in fine-grained water body (Fig. 10.3). The lack of mixing of the
sediment is a good indicator of fresh or possibly brack- oxygenated surface water with the lower part of
ish water conditions. the water column results in anaerobic conditions at
Cold, sediment-starved lakes in mountainous or polar the bottom of a deep, stratified lake. Animals are
environments may be sites of deposition of siliceous unable to tolerate the anaerobic conditions so the
oozes (3.3). The origin of the silica is diatom phyto- lake floor is devoid of life, and therefore there is no
plankton, which can be very abundant in glacial bioturbation (11.7). Deep lake deposits may therefore
lakes. These deposits are typically bright white cherty preserve primary sedimentary lamination that in
beds that are called diatomites, and they are basi- marine environments is typically destroyed by bur-
cally made up entirely of the silica from diatoms. rowing organisms. The anoxia also prevents the aero-
bic breakdown of organic material that settles on the
lake floor, allowing the accumulation of organic-rich
10.7 RECOGNITION OF LACUSTRINE sediments. The deposits of saline and ephemeral lakes
FACIES usually can be distinguished from marine facies by the
chemistry of the evaporite minerals.
If the succession is entirely terrigenous clastic mate-
Characteristics of lake deposits
rial, it is not always easy to distinguish between the
. lithologies – sandstone, mudstone, fine-grained
deposits of a lake and those of a low energy marine
limestones and evaporites
environment such as a lagoon (13.3.2), the outer part
. mineralogy – variable
of a shelf (Chapter 14) or even the deep sea (Chapter
. texture – sands moderately well sorted
16). Shallow lake facies will have similar character-
. bed geometry – often very thin-bedded
istics to lagoonal deposits, with wave ripple sands
. sedimentary structures – wave ripples and very fine
interbedded with muds deposited from suspension,
parallel lamination
while the deeper environments of a lake resemble
. palaeocurrents – few with palaeoenvironmental sig-
those of seas with similar or greater depths, as they
nificance
include deposits from suspension and turbidites. The
main criteria for distinguishing between lacustrine . fossils – algal and microbial plus uncommon shells
and marine facies are often the differences between . colour – variable, but may be dark grey in deep lake
the organisms and habitats that exist in these envi- deposits
ronments. . facies associations – commonly occur with
There are a number of groups of organisms that are fluvial deposits, evaporites and associated with aeo-
found only in fully marine environments: these lian facies
include corals, echinoids, brachiopods, cephalopods,
graptolites and foraminifers, amongst others (3.1.3).
The occurrence of fossils of members of these groups FURTHER READING
therefore provides evidence of marine deposition.
Anado ´n, P., Cabrera, L. & Kelts, K. (Eds) (1991) Lacustrine
There are many genera of other phyla that can be
Facies Analysis. Special Publication 13, International
used as indicators if found as fossils, in particular
Association of Sedimentologists. Blackwell Science,
there are groups of bivalve and gastropod molluscs
Oxford, 318 pp.
that are considered to be freshwater forms, and fish Bohacs, K.M., Carroll, A.R., Neal, J.E. & Mankiewicz, P.J.
that are thought to be exclusively lake-dwellers. (2000) Lake-basin type, source potential, and hydro-
Some of the more reliable indicators of freshwater carbon character: an integrated-sequence-stratigraphic–
conditions are algal and bacterial (3.1.3) fossil geochemical framework. In: Lake Basins through Space
forms. Reliance on fossils to provide indicators of and Time (Eds Gierlowski-Kordesch, E.H. & Kelts, K.R.).