Lake Environments: Types, Processes, and Sedimentation

Perennial Saline Lakes

Perennial saline lakes are common in tropical and subtropical regions with relatively low rainfall. They include:

• Soda lakes (trona: Na₂CO₃.HNaCO₃.2H₂O)
• Sulphate lakes (mirabilite: Na₂SO₄.10H₂O)
• Chloride lakes (halite: NaCl)
• Nitrate lakes (Chile salpeter: NaNO₃)

Ephemeral Lakes

Ephemeral lakes form in dry regions where seasonal effects and evaporation play an important role in the sedimentation pattern. They include:

• Playa lakes
• Salt pans
• Oxbow lakes

Sub-Environments and Processes

Stratification

As the upper water layer is heated by the sun, an exponential temperature curve is expected in lakes, but this is seldom the case due to various factors. Evaporation at the surface cools the uppermost layer and produces convection currents, radiation takes place at night, and wind causes turbulence on the surface that disperses the heat to deeper levels. The temperature curve therefore shows an upper layer of warm water (the epilimnion), which is separated by a thermocline (rapid change in temperature) from the hypolimnion of cold water. Seasonal changes in this thermal stratification may in some cases cause water inversion, which can be monomictic (once a year) or dimictic (twice a year).

Chemical stratification can prevent the inversion of the deepest parts of lakes in cold regions. Groundwater seeping into the lake at the bottom may have a higher salinity and thus increased density compared to the water at higher levels. If the chemical stratification is stronger than the thermal stratification, inversion can only occur in the upper lake water, the so-called mixolimnion. The latter can thus develop its own thermal stratification and is holomictic, i.e., free circulating.

Circulation

Circulation in lakes is primarily caused by the influx of river water, which produces density differences and therefore currents. Where the river water has the same density as the lake water, three-dimensional mixing takes place and the sediment is distributed evenly. Stratified inflow takes place where the densities of the river and lake water differ.

Overflow occurs where the river water is less dense, which is caused by a low sediment load. The sediment is distributed by wind-driven currents. Interflow is when the river water is more dense than the upper lake water but less dense than the lower layer, which can only happen in deep, density-stratified lakes. Underflow, where the river water is denser, can be due to cold temperatures or a high sediment load. The sediment is distributed by density currents in this case. Wind and the air pressure gradient are also responsible for lake circulation.

Sedimentation

Clastic deposition in lakes takes place from suspension during all types of inflow and from traction during underflow. Only in the last case will bedforms be prominent on the lake bottom. In shallow water close to the shore, large areas are commonly covered with straight-crested small ripples, which are oriented parallel to the coastline. Sand volcanoes and other fluid escape structures are encountered on the beaches. Low energy conditions are indicated by horizontally laminated shales, which often form varves due to changing sediment influx during summer and winter. Glacial varves consist of light (clastic) and dark (sapropel) laminae, which form during summer, when the ice melts, and during winter, respectively. Other types of varves are thinner (fractions of a mm) and consist of intercalated fine-grained carbonate and sapropel. Both types of varves can also develop due to variations in sediment influx over shorter periods than seasons (e.g., storms).

Clastic deposits in lakes are usually characterized by well-defined belts, with a decrease in grain size towards the center of the lake. Oil shale commonly develops here due to the high percentage of organic matter derived from algae, plankton, and floating fragments of land plants. The filling of lakes cause coarsening-upward cycles, e.g., laminated shale, turbidite sand, beach sand, and fluvio-deltaic sand.

Chemical deposition in lakes is dominated by limestone with a low Mg content, although saline lakes can also contain dolomite. The carbonates are commonly bio-induced; i.e., they precipitate due to CO₂ released during photosynthesis of algae and water plants. As the latter are more abundant close to the shore, the carbonate content of lake deposits commonly increases towards the shore, except opposite river mouths where clastic [sediment input is high].