8.5.2: Saline Lakes

Introduction

Saline lakes occur in the hydrographically lowest areas of arid environments within closed-drainage basins (Hardie et al, 1978). They can be categorized into several categories: ephemeral, perennial, acidic, and alkaline (Hardie et al, 1978). Moreover, they have several depositional sub-environments: alluvial fans, sandflats, dry mudflats, dune fields, perennial and ephemeral stream floodplains, perennial and ephemeral lakes, springs – travertine and tufa flats and mounds, and shoreline features (Hardie et al, 1978).

(Above, there is a picture of the Great Salt Lake in Utah, USA. It is one of the largest lakes in the United States aside from the Great Lakes and is largely considered a saline lake. Various portions of the lake are too salty for anything other than a few highly-specialized species.)

Fluids

A lake is called a saline lake if the water in it is made up of more than 5000 ppm of dissolved solute (Hardie et al, 1978). The amount of precipitation/influx of water into saline lakes is less than the amount that is evaporating and water flow into and out of the saline lake is non-existent or at least restricted (Hardie et al, 1978). Considering these circumstances, saline lakes are very low energy depositional environments. Also, the influx into this environment may come from perennial or ephemeral streams, springs, groundwater, or un-channeled sheet flow from storms (Hardie et al, 1978).

Sediment Transport Processes

Sediment transport into saline lakes is typically driven by fluid flow from storm flooding (Hardie et al, 1978). And so fluid flow is high in the alluvial fan sub-environment and slows as it passes through sandflats, then mudflats, and then into the saline lakes. Once storm floodwaters reach the mudflat environment, they have moderate velocities, are shallow, and un-channeled (Hardie et al, 1978). Evaporative pumping is another major way sediment (in the form of salts) is transported within saline lakes, particularly in the mudflat facies/sub-environment where this process creates surface efflorescence (Hardie et al, 1978). The dune field facies is associated with the bottom of the alluvial fan, sandflat, and both of the inflow stream floodplain sub-environments of saline lakes in that when any of these areas lack efflorescent salt crusts, they can be eroded and have sediment deposited on them by wind (Hardie et al, 1978).

(Above, is a figure from Benison and Goldstein 2001. It illustrates a cross-section of a salt lake depositional environment [this is the bottom cross-section] and a cross-section of a coastal marina [the cross-section on top]. Notice that the pan above occupies the area where the lake would normally be after it is recharged with water.)

Blue-green algae are associated with the precipitation of tufas and travertines in both spring orifices and inflow streams; however, the cause of this association may be due to the metabolisms of the algae or inorganic precipitation of carbonates that trap the algae (Hardie et al, 1978).

Characteristics of Deposited Sediment

Saline lakes are subdivided into several sub-environments that can be used as facies: alluvial fans, sandflats, dry mudflats, dune fields, perennial and ephemeral stream floodplains, perennial and ephemeral lakes, springs – travertine and tufa flats and mounds, and shoreline features (Hardie et al, 1978).

Alluvial Fans and Sandflats

Saline lakes are surrounded by alluvial fans that are typically created by stream point flows and have gravelly wedges that then fan out with very coarse sediments (Hardie et al, 1978). Alluvial fans are typically the products of storm-flooding and so this type of flow creates four kinds of deposits: shallow braid channel deposits, fills inside deeper incised channels, sieve deposits, and debris flow deposits (Hardie et al, 1978). On the surface of the alluvial fan, the braid channel deposits appear as coarse gravel bars that are isolated by braided shallow stream channels (Hardie et al, 1978). In vertical sections, the braid stream deposits in alluvial fans appear as cross-cutting lenses that are made up of framework-supported boulders and pebbles that may be imbricated and may have gaps in the framework filled in by sand (Hardie et al, 1978). Incised channel fill deposits can be several meters thick and have sediment that fines upward in grain size (Hardie et al, 1978). Evaporative pumping of groundwater beneath an alluvial fan’s surface can also create calcite pore cement, caliche crusts, coatings, root moulds, and nodules within the fan’s sediment (Hardie et al, 1978). Within ephemeral and perennial saline lakes, the water has very little velocity (Hardie et al, 1978).

The deposits of the alluvial fans then lead into sandy aprons, which are referred to as the sandflats facies. The sediment in sandflats are finer than those in the alluvial fans and have pore and vug fillings that are likely to be gypsum, which can dehydrate into anhydrite, or high-Mg calcite (Hardie et al, 1978). The sandflats can also have wind ripples and longitudinal trains (Hardie et al, 1978).

Dry Mudflats

The dry mudflat facies is commonly associated with ephemeral saline lakes and are in general differentiated from the ephemeral saline lake facies in that they preserve depositional structures. At the surface, the mudflats will usually have polygonal mudcracks which are desiccation cracks and a thin layer of saline crusts which are surface efflorescences (Hardie et al 1978). Two types of saline crusts can form: thin, brittle, and dense crust made up of micritic alkaline earth carbonates; and puffy, porous, and crystalline crusts that are made up of soluble minerals like halite, trona, thermonatrite, or thenardite (Hardie et al, 1978). In addition, the soluble minerals in the second type of saline crust are usually dissolved by storm flooding and so their presence is recorded via the disruption and destruction of the top part of each depositional unit (Hardie et al, 1978). However, the sedimentary features of dry mudflats are not well known, but it is hypothesized that they could have small, millimeter-scale laminae and crystals of gypsum, glauberite, etc. (Hardie et al, 1978). The flow velocity, shallowing storm flood water, and traction deposition in dry mudflats could also produce sedimentary structures that are fine grained but also upper flow regime bedforms (Hardie et al, 1978).

Ephemeral Lakes

Ephemeral lakes are a type of saline lake that is shallow and every few years dries up leaving behind a layer of salt(s) precipitated by the evaporating brine (Hardie et al, 1978). These lakes are cyclically recharged via storm water runoff and then gradually recede as the water evaporates, which leads to the formation of two sub-facies – a salt pan facies and a saline mudflat facies (Hardie et al, 1978). The salt pan facies occurs in the lowest part of the lake bed and it is characterized by layered salts, while the saline mudflat facies occurs around the salt pan and is characterized by muddy clastic sediment that contains salt mineral crystals (Hardie et al, 1978). Also, the saline mudflat sub-facies may have the following features: root casts,  When the turbulence from the storm flooding ceases, silt and clay-sized grains settle from suspension they create a thin lamina that extends across the recharged saline lake (Hardie et al, 1978). Wind-induced waves may then cause the formation of wave ripples on the mud lamina or rework the mud lamina into a silt clay lenticular lamination (Hardie et al, 1978). In the salt pan facies, this thin mud lamina can become the site of bacterial reduction of sulfate and creates iron sulfides and sulfuric acid, which give the sediment a black coloring (Hardie et al, 1978). Additionally, the salt pan facies is characterized by couplets of layers following a storm flooding of the lake – the thin mud lamina that is black in color and has intergrown salt crystals and an overlying, thick layer of crystalized salts (Hardie et al, 1978).

Perennial Lakes

Perennial saline lakes are a type of saline lake that remains deeply water-filled to some extent for most years. The strata of perennial lakes reflect the fact that they are constantly depositing saline material onto the lake bottom and so a major structure within these strata are crystalized salt laminae (Hardie et al, 1978). Clastic mud laminae can also be deposited at irregular points throughout the strata of a perennial lake as a result of different storm flood events (Hardie et al, 1978). The salt crystal laminae can also be altered diagenetically as a result of bacterial reduction into a different material, such as sulfide, or through recrystallization which would erase the original depositional structure (Hardie et al, 1978).

Dune Fields

Erosion from wind action on the bottom of the alluvial fan, sandflat, and both of the inflow stream floodplains sub-environments creates the dune fields, deflation flats, and hollows that are characteristic of the dune fields facies (Hardie et al, 1978). In some cases where the sediments of a dry saline lake bed are eroded, the sand making up the dunes in this facies can be made up of salt mineral grains like gypsum (Hardie et al, 1978).

Shoreline

The shoreline facies is partly influenced by groundwater discharge from springs that may be located in them (Renaut and Long 1989). As a result, travertine deposits may be found on the shoreline along with spring mounds (Renaut and Long 1989). Typically, shoreline deposits vary based on the saline lake's water level and this leads to the deposition of sparingly-soluble salts during high stands and deposition of more readily soluble salts during low stands (Renaut and Long 1989). In some parts of the shoreline, wind-action can cause certain areas of the lakeshore to have more salts deposited on them - resulting in a thicker salt layer. Deposits on the shoreline may consist of carbonate muds and their deposits are typically laminated and/or display mottling (Renaut and Long 1989). 

Biological Deposition

In the saline lake sub-environments, bioturbation may come in the form of halophyte plant roots, invertebrate burrows, bird feeding pits, or terrestrial animal burrows (Hardie et al, 1978). The alluvial fan, sandflats, dunes, and mudflats facies tend to have these features, which interrupt or destroy depositional layers (Hardie et al, 1978). Blue-green algal mats can accumulate on the bottom of saline lakes and it is diagenetically possible for them to be converted into kerogen, which has been associated with oil shales (Hardie et al, 1978). Additionally, the shoreline facies is commonly occupied by small cyanobacterial mats (Renaut and Long 1989).

Vertical Sequence of Facies

(The above was taken from Renaut and Long 1989. It depicts stratigraphic columns of Carbonate-dominated and Siliclastic-carbonate mudflat facies within the saline lakes of the Cariboo Plateau in British Columbia, Canada. The mudflat facies and shoreline facies are interlinked in that depending on the water level of the saline lake the shoreline can move further up a mudflat or down to its lowest extent.)

(Source. The above illustrates a stratigraphic section of Bristol Dry Lake, USA. Notice that alluvial fan facies starts off with the coarsest grains and that the sediments closer to the saline lake have finer sediment. Also, note that the bottom saline mudflat section illustrates a periodic increase in grain size due to faster water flow from storm flooding.)

Saline lake deposits can be differentiated from marine deposits in that they are halite dominated, have many soil features, lack marine fossils, and the scarcity of carbonates (Benison and Goldstein, 2001). Additionally, the strata associated with saline lake deposits have a combination of aeolian, flood, desiccation, and evaporation -related features (Benison and Goldstein, 2001).

Summary

Saline lakes are a unique depositional environment whose deposits are influenced by largely periodic water flow from storm flooding, but they are also shaped by aeolian processes and the surfacing of groundwater through springs. Along the cross-section of this environment towards the lake-itself, water flow is expected to decrease into a near or complete standstill. The deposits as a result decrease in grain size towards the lake - largely forming mud layers at the perimeter of and within the saline lake. As the lake level changes, the type of salts deposited change and the level change defines the saline mudflat subfacies and dry mudflat facies as the border between these is the shoreline. Sedimentary structures are highly deformed within the saline lake due to disturbance from salt crystal growth and many of the facies along the shoreline and within the lake are deposited upon by microorganisms including cyanobacteria. Lastly, the habitat surround saline lakes is in danger of disappearing putting a variety of organisms that rely on it in danger (Further Reading).

References

Benison, K.C., and Goldstein R.H. “Evaporites and Siliciclastics of the Permian Nippewalla Group of Kansas, USA: a Case for Non-Marine Deposition in Saline Lakes and Saline Pans.” Sedimentology, vol. 48, no. 1, 2001, pp. 165–188., doi:10.1046/j.1365-3091.2001.00362.x.

Hardie, L.A., Smoot, J.P., Eugster, H.P. “Evaporites and Siliciclastics of the Permian Nippewalla Group of Kansas, USA: a Case for Non-Marine Deposition in Saline Lakes and Saline Pans.” Modern and Ancient Lake Sediments. Special Publications of the International Association of Sedimentologists, no. 2, 1978, pp. 7-41.

Renaut, R.W., and Long, P.R. "Sedimentology of the saline lakes of the Cariboo Plateau, Interior British Columbia, Canada." Sedimentary Geology, vol. 64, 1989, pp. 239-264.