Page 168 - Sedimentology and Stratigraphy

Freshwater Lakes 155

source rock for oil and gas (18.7.3). The second effect lake margin setting. This lake margin marshy envi-
of anaerobic bottom conditions is that this is an envi- ronment is sometimes referred to as a palustrine
ronment that is unfavourable for life. Stratified lakes environment. Plants and animals living in this setting
therefore have no animals living on the bottom or live in and on the sediment in a wet soil environment
within the surface sediment and hence there is no where sediments will be modified by soil (pedogenic)
bioturbation (11.7.3) to disturb the primary sedimen- processes (9.7), resulting in a nodular texture that
tary layering. may sometimes be calcareous.

10.2.2 Lake margin clastic deposits 10.2.3 Deep lake facies

Where a sediment-laden river enters a lake the water Away from the margins, clastic sedimentation occurs
velocity drops abruptly and a delta forms as coarse in the lake by two main mechanisms: dispersal as
material is deposited at the river mouth (Fig. 10.4). plumes of suspended sediment and transport by den-
The form and processes on a lake delta will be similar sity currents (Fig. 10.3) (Sturm & Matter 1978).
to that seen in river-dominated deltas, with some Plumes of water laden with suspended sediment may
wave reworking of sediment also occurring if the be brought into the lake by rivers: if the sediment–
lake experiences strong winds. The character of the water mixture is a lower density than the hypolim-
delta deposits will be largely controlled by the nature nion the plume will remain above the thermocline
of the sediment supply from the river, and may range and will be distributed around the lake by wind-dri-
from fine-grained deposits to coarse, gravelly fan-del- ven surface currents. The suspended load will even-
tas (12.4.2). tually start to settle out of the epilimnion and fall to
Away from the river mouth the nature of the lake the lake floor to form a layer of mud. Density currents
shore deposits will depend on the strength of winds (4.5) provide a mechanism for transporting coarser
generating waves and currents in the lake basin. If sediment across the lake floor. Mixtures of sediment
winds are not strong, lake shore sediments will tend to and water brought in by a river or reworked from a
be fine-grained but strong, wind-driven currents can lake delta may flow as a turbidity current (4.5.2),
redistribute sandy sediment around the edges of the which can travel across the lake floor. The deposits
lake where it can be reworked by waves into sandy will be layers of sediment that grade from coarse
beach deposits (Reid & Frostick 1985). These mar- material deposited from the current first to finer sedi-
ginal lacustrine facies (Fig. 10.5) will be similar in ment that settles out last. In lakes where sediment
character to beaches developed along marine shore- plumes and turbidity currents are the main transport
lines (13.2). mechanisms the deep lake facies will consist of very
In situations where the slope into the lake is very finely laminated muds deposited from suspension
gentle the edge of the water body is poorly defined as alternating with thin graded turbidites forming
the environment merges from wet alluvial plain into a a characteristic, thinly bedded succession (Fig. 10.5).

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Fig. 10.4 Facies distribution in a
freshwater lake with dominantly clastic
deposition.