Sedimentologi
Kamal Roslan Mohamed
LAKE
INTRODUCTION
A lake is an inland body of water.
Lakes form where there is a
depression on the land surface
which is bounded by a sill such that
water accumulating in the
depression is retained.
A lake basin supplied by a river in the
foreground, with outflow through a sill to the
sea in the distance.
Lakes form where there is a supply of water to a topographic low on the land
surface. They are fed mainly by rivers and lose water by flow out into a river
and/or evaporation from the surface.
The balance between inflow and outflow and the rate at which evaporation
occurs control the level of water in the lake and the water chemistry.
Under conditions of high inflow the water level in the lake may be constant,
governed by the spill point of the outflow, and the water remains fresh.
INTRODUCTION
Low water input coupled with high evaporation rates in an enclosed
basin results in the concentration of dissolved ions, which may be
precipitated as evaporites in a perennial saline lake or when an
ephemeral lake dries out.
Lakes are therefore very sensitive to climate and climate change.
Many of the processes that occur in seas also occur in lakes: deltas
form where rivers enter the lake, beaches form along the margins,
density currents flow down to the water bottom and waves act on the
surface.
There are, however, important differences with marine settings: the
fauna and flora are distinct, the chemistry of lake waters varies from
lake to lake and certain physical processes of temperature and
density stratification are unique to lacustrine environments.
Lake hydrology
A lake is considered to be
hydrologically open if it is filled to
the spill point and there is a balance
of water supply into and out of the
basin. Under these circumstances
the level of the water in the lake will
be constant, and the constant
supply from rivers will mean that the
water in the lake will be fresh.
If the rate of evaporation exceeds or
balances the rate of water supply
there is no outflow from the lake
and it is considered to be
hydrologically closed.
Hydrology of freshwater lakes
Below about 10 or 20m depth the lake waters are unaffected by any
wave or current activity. This allows for the development of lake water
stratification, which is seen as a contrast in the temperature, density
and the chemistry of the waters in the upper and lower parts of the water
body.
The epilimnion, which is the upper, warmer lake water, and the
hypolimnion, the lower, colder part: they are separated by a surface
across which the temperature changes, the thermocline.
Hydrology of freshwater lakes
The bottom of the lake therefore becomes anaerobic (without air, and
therefore oxygen) and this has two important consequences.
Any organic material that falls through the water column to the lake floor
will not be subject to breakdown by the activity of the aerobic processes
that normally cause decomposition of plant and animal tissue. If there is
abundant plant material being swept into the lake, this has the potential
to form a detrital coal layer.
The second effect of anaerobic bottom conditions is that this is an
environment that is unfavourable for life.
Lake clastic deposits
Where a sediment-laden river
enters a lake the water velocity
drops abruptly and a delta forms as
coarse material is deposited at the
river mouth.
The form and processes on a lake
delta will be similar to that seen in
river-dominated deltas, with some
wave reworking of sediment also
occurring if the lake experiences
strong winds.
Facies distribution in a freshwater lake with
dominantly clastic deposition.
The character of the delta deposits will be largely controlled by the nature
of the sediment supply from the river, and may range from fine-grained
deposits to coarse, gravelly fan-deltas.
Lake clastic deposits
Mixtures of sediment and water brought in
by a river or reworked from a lake delta
may flow as a turbidity current, which can
travel across the lake floor.
The deposits will be layers of sediment
that grade from coarse material deposited
from the current first to finer sediment that
settles out last.
In lakes where sediment plumes and
turbidity currents are the main transport
mechanisms the deep lake facies will
consist of very finely laminated muds
deposited from suspension alternating
with thin graded turbidites forming a
characteristic, thinly bedded succession.
A schematic graphic sedimentary log through
clastic deposits in a freshwater lake.
Lake clastic deposits
In lakes formed in regions where there is an annual thaw of
winter snow a distinctive stratification may develop due to
seasonal variations in the sediment 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 millimetre-scale laminae formed in this way are known as
varves and they have been used in chronostratigraphy of
Holocene deposits
Lacustrine carbonates
Carbonates can form a significant
proportion of the succession in any
lake setting only if the terrigenous
clastic input is reduced.
Direct chemical precipitation of
carbonate minerals occurs in lakes
with raised salinity, but in freshwater
lakes the formation of calcium
carbonates is predominantly
associated with biological activity.
Facies distributions
in a freshwater lake with carbonate deposition.
The hard shells of animals such as bivalve molluscs, gastropods and
ostracods can contribute some material to lake sediments, and this coarse
skeletal material may be deposited in shallow water or redistributed around
the lake by wave-driven currents.
However, the most abundant carbonate material in lakes is usually from algal
and microbial sources.
EPHEMERAL LAKES
Large bodies of water that
periodically dry out are
probably best described as
ephemeral lakes,
although the term playa lake is also
commonly
Used.
A salt crust of minerals formed by evaporation in
an ephemeral lake.
Once the lake has formed, particles suspended in the water will start to
deposit and form a layer of fine-grained muddy sediment.
Evaporation of the water body gradually reduces its volume and the area of
the lake starts to shrink, leaving areas of margin exposed where desiccation
cracks may form in the mud as it dries out.
With further evaporation the ion concentration in the water starts to increase
to the point where precipitation of minerals occurs.
EPHEMERAL LAKES
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.
Characteristics of lake deposits
. lithologies – sandstone, mudstone, fine-grained limestones and
evaporites
. mineralogy – variable
. texture – sands moderately well sorted
. bed geometry – often very thin-bedded
. sedimentary structures – wave ripples and very fine parallel lamination
. palaeocurrents – few with palaeoenvironmental significance
. fossils – algal and microbial plus uncommon shells
. colour – variable, but may be dark grey in deep lake deposits
. facies associations – commonly occur with fluvial deposits, evaporites
and associated with aeolian facies
SEKIAN