Lacustrine Environments
Introduction
Lacustrine depositional systems, or lakes, form in topographic lows and are generally low energy systems. Lacustrine systems display diversity among each other, as they vary in size, water chemistry, formation, water sources, morphology, and seasonal variances among other factors. Lacustrine environments are great sources of paleoenvironmental indicators for both the lake itself, the greater watershed they are a part of, and have implications on global conditions during deposition. Several classification systems for modern lacustrine environments exist, such as if it is an open or closed system, how the lake was formed, if the lake dries up during dry periods, and if the lake is stratified and experiences seasonal turnover. These various types of lakes can often be deciphered from the deposits of paleo lacustrine environments.
Lacustrine Environment Formation and Geomorphology
- 2.1) Lake Formation
Lacustrine environments form where basins or other topographies have created accommodation space. The morphology of the lake is highly dependent on how it was formed, along with the original topography prior to formation of the accommodation space. Examples of lake formation will include the modern Lake Michigan-Huron, the Dead Sea, and Lake Chelan. These modern lakes help to show the morphological differences between fault-driven, subsidence-driven, and scour-driven lakes.
- 2.1.1) Sag Basin Lakes
Isostatic adjustment of the crust, due to high loads such as glaciers, can create sag basins. During the last several glacial periods, the Laurentide and parent glaciers that covered the modern day great lakes had ice masses up to a mile thick in the region (Simon, James, Henton, and Dyke, 2016). This large ice load caused the crust to buckle in the region and subside over time. When the overlying glaciers melted, the depressions filled with water and the modern day Lake Michigan-Huron took form. Lakes that formed in sag basins such as Lake Michigan-Huron usually have large surface areas, but are generally shallow relative to the size of their surface areas.
- 2.1.3) Glacial Scour Lakes
Aside from being the main driver of lakes formed in sag basins, glaciers can also form other types of lakes. As glaciers advance and retreat across landscapes, they can carve out new topographic lows (Nichols, 2009). Upon creating new topographic lows, the glaciers may melt in place and fill the topographic low creating a new lake. Lakes formed by glacial scour display a wide range of depths. Kettle lakes can be extremely shallow, whereas valleys carved by glaciers can create some of the world's deepest lakes such as Lake Chelan (Whetten, 1967).
Depositional Processes and Depositional Facies
- 3.1) Varves
Lacustrine facies are highly dependent on the type of lake the facies formed in. The basic building block of lacustrine facies is the varve. Varves are the seasonal or annual laminae, and are often characterized as being clastic, biogenic, or endogenic (Zolitschka, Francus, Ojala, and Schimmelmann, 2015). If preserved, lacustrine varves are able to show a calendar of the region that depicts precipitation patterns and other climatic conditions and disturbances.
- 3.1.1) Clastic Varves
Clastic varves form from the input of siliciclastic material. This siliciclastic material is most often transported through fluvial systems during higher energy seasonal runoff and high precipitation events (Zolitschka et al, 2015). Clastic varves will often see a change in grain size across the lake profile. Larger grains are often deposited near the edges of lakes, especially near the mouths of feeder rivers. Due to the low energy of most lacustrine environments larger sand grains will often immediately settle at the mouth of rivers, whereas finer silt sized grains will stay in suspension and settle in the deeper and more distal areas of the lake (Zolitschka et al, 2015). Clastic varves will often cap the end of the year with clay grains. Despite the low energy of many lakes, clay will often stay in suspension throughout the year and only settle to the bottom of the lake when the lake is frozen over (Zolitschka et al, 2015). The amount of sediment available to be driven to a lake is constrained by the rigidity of the surrounding sediment in the watershed, factors such as rock strength and vegetation cover can affect the weathering and erosion rates of sediment into a lake (Zolitschka et al, 2015).
- 3.1.2) Biogenic Varves
Biogenic varves are able to show seasonal variations in organic matter that are transported to, or formed in, a lake. In highly vegetated regions, organic detritus is transported to the lake and will eventually settle out within the lake. In watersheds that are not highly vegetated, dissolved minerals from runoff can fuel eutrophic conditions that can make lakes highly productive in terms of organic matter growth (Zolitschka et al, 2015). This organic matter growth is often expressed as large algal blooms. A modern example of highly eutrophic lakes is Lake Erie; which has seen an excess of nutrients which leads to algal blooms. Regardless of the origin of the organic matter, biogenic varves will often form dark laminae that become the biogenic varves.
- 3.1.3) Endogenic Varves
Endogenic varves, also called evaporite varves, form in more extreme lacustrine environments. These varves depict extreme drops in lake level that increase salinity, sometimes endogenic varves can show a complete drying up of a lake during an arid summer (Zolitschka et al, 2015). When lakes experience evaporation that outweighs the precipitation in the watershed, halite and gypsum may start forming these endogenic varves.
Controls on Depositional System Evolution
The evolution of lacustrine depositional systems is often dependent on tectonics in the basin, and also the relationship between precipitation and evaporation in the region. In order to see continued expansion of a lacustrine depositional system, accommodation space must be greater than the sediment supply going into and out of the basin. Likewise, precipitation must be greater than evaporation or the lake will just dry up.
- 4.1) Accommodation Space and Tectonics
The Dead Sea is a modern example of how continued accommodation space allows for a long continued lacustrine history. The previously mentioned Dead Sea pull-apart basin has been creating accommodation space since 15 Ma (Brink and Ben‐Avraham, 1989). This increasing accommodation space has allowed for continued deposition of lacustrine deposits.
- 4.2) Effects of Precipitation vs Evaporation on Water Chemistry
The balance or imbalance between evaporation and precipitation in a lake has a great effect on the water chemistry and life of the lake. Due to low rainfall and high evaporation rates, regions such as Western Australia have a high number of ephemeral lakes. Ephemeral lakes have differing water chemistries based on the time of year and seasonal climates. During dry months, but before the lake dries up completely, they can become hypersaline and highly acidic and precipitate large amounts of salt and sulfate minerals (Benison and Bowen, 2013). During wetter months, the pH of the lakes become less saline and more neutral.
- 4.3) Open vs Closed Lakes
Lakes can also be classified by whether they are open or closed systems. Closed lakes are the terminus of any sediment transport in the area. If a lake is closed, then there are no rivers flowing out of the lake to further transport material. Closed lakes often have more extreme water chemistry, with the Dead Sea and the Great Salt Lake being prime examples (Huybers, Rupper, and Roe, 2015).
Open lakes differ in that they have outlets that allow for the outflow of material. Because of water and sediment being able to flow out of the lake, open lakes generally have more moderate water chemistry. Lake Michigan-Huron is a prime example as the waters eventually flow out to the Atlantic Ocean.
Facies Model
Lacustrine environments can undergo severe changes in water chemistry. Salinity of the environment is one such possible change and is reflected in the following facies model of a paleo lacustrine environment in the Wessex Basin in Dorset, UK. The environment became increasingly hypersaline due to it likely being a closed lake and the region becoming more arid (Gallois, Bosence, and Burgess, 2018).
The wackestone and grainstones in the lower section of the facies sequence were likely deposited in deep portions of the lake and when waters were brackish (Gallois et al, 2018). The above microbialite deposits were also likely formed when the waters were brackish, but likely in a shallower part of the lake where there was more sunlight. Gypsiferous peloidal packstone and other evaporite deposits show the shift from the originally brackish waters to a hypersaline environment (Gallois et al, 2018).
Reference
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