lake sediments

lake sediments (lacustrine) Lakes show a great range in size and depth, from small local ponds to bodies of water covering thousands of square kilometres. Many large lakes originate in tectonic depressions (rift basins) (for example, Lake Tanganyika and Lake Baikal) or major crustal sags (for example, Lake Chad and Lake Eyre), whereas others may form in extinct volcanoes, be produced by lava flows, or are the results of glaciation. Many small lakes are produced by changes in river courses or landslips. Lakes show a bewildering variety of sediments, reflecting local topography, climate, and availability of sediment.

Sediment may be supplied to lakes by rivers, wind, and ice. Wave- and wind-driven currents distribute the incoming sediment load in a similar way to that along marine coastlines. When surface winds drive water to one side of a lake they produce return flows along the bottom of the lake. When the water of lakes is significantly disturbed it oscillates to produce a slopping motion, the phenomenon being known as a seiche. Incoming surface water, depending upon its temperature and/or sediment load may spread over the surface, sink and flow at a level of similar density, or sink to the bottom to produce bottom currents (turbidity currents) which can deposit graded sands far from the point of entry (Fig. 1a).

Lake waters show a well-marked density stratification. The upper, warm waters are usually well mixed (the epilimnion) and these overlie denser, colder waters (the hypolimnion). The upper water is usually oxygenated, whereas the lower layer becomes anoxic due to the exhaustion of oxygen by the oxidation of organic matter descending from the surface. Whereas in temperate lakes, surface cooling in winter leads to overturn of the waters and oxygenation of the bottom waters (oligotrophic lakes); in low-latitude lakes, where there is little variation in seasonal temperatures, the bottom waters remain without any replenishment of oxygen (eutrophic lakes). Lakes receive water by precipitation, surface flow, and, particularly in arid areas, by subsurface flow from surrounding rocks. Also, in some areas (rift valleys) they receive hydrothermal magmatic waters from the subsurface. Lake water has a much greater variation in composition than sea water, and hence there is a greater variation in the resulting chemical precipitates.

Lakes experience much more frequent and faster changes in water level than seas. As a consequence, their nearshore and marginal environments suffer exposure and fast transgressions and regressions, and have a complicated stratigraphy when preserved in the geological record.

Fluviolacustrine deltas (Fig. 1b), constructed by incoming rivers are very similar in structure and stratigraphy to fluviomarine deltas. Sands pass lakewards into fine-grained sediments. Turbid, cold waters are often sufficiently dense to plunge beneath the surface waters and flow down the slope to form subaqueous fans with channels and levées as seen in fluviomarine deltas. The resulting sediments are composed of graded and plane-bedded sands, sometimes with cross-stratification in the channels, wavy laminated silts in the levées, and graded silts and laminated muds on the more distal (i.e. distant), deeper, lake floor. Subaqueous slumps occur on deltas (e.g. Lake Brienz, Switzerland) and they may also occur around the margins, to produce gravity-flow deposits. Seasonal floods frequently produce graded sands and silts over lake floors. In glacial terrains coarse-grained, so-called Gilbert-type deltas of gravels and sands are common, and these pass in deeper water into varved sediments.

A great variety of carbonate sediments are present in lakes draining carbonate-rich terrains (Fig. 1c). Laminated, fine-grained carbonate mud and marl, commonly interlaminated with fine-grained organic layers produced by planktonic remains descending from the surface, are common bottom sediments of temperate lakes.

In shallow water in arid areas, oolitic sands are common along the shorelines of some lakes (e.g. Great Salt Lake, Utah). Elsewhere, plants such as charophytes coat their stems with calcium carbonate and produce calcified oogonia which accumulate on the lake floor to produce chara marls when mixed with siliciclastic detritus. Various particles become coated with films of cyanobacteria which trap and precipitate calcium carbonate to produce coated grains (oncoids) as found in marine lagoons. Also, cyanobacterial mats cloak sediment and rocks to produce stromatolitic coatings. Large mounds, often several metres high and tens of square metres in area (tufa mounds) are produced by bacteria that trap or precipitate calcium carbonate or gypsum issuing from springs. Dolomite is also precipitated in lake sediments in Lake Baikal, Deep Spring Lake, California; and other lakes in Texas.

Lake deposits are often very rich in organic matter. It may be derived from surrounding vegetation and be composed of the debris of land plants. Also, diatoms and dinoflagellates living in lake waters produce vast quantities of organic matter in large temperate lakes, while green algae produce most of the organic matter in hypersaline lakes. Large amounts of organic matter are found in low-latitude lakes that do not experience a seasonal overturn and remain eutrophic. The organic-rich bottom deposits (sapropels or gyttja) are rich in oil-producing organic compounds. In vegetated areas, lakes are often colonized and invaded by marginal plants and become filled with peat.

In arid areas a rich variety of chemical deposits is formed by subsurface water seeping from adjacent rocks into the depression (playa). Many of these lakes are ephemeral and dry out in summer to produce bare salt-covered flats or sabkhas. In some lakes in western USA, carbonate occurs with gypsum and halite. Sulphate-rich waters in bitter lakes produce mirabilite (Na₂SO₄.10H₂O) which alters on exposure to thenardite (Na₂SO₄) both along the margins of the lake and in its deeper parts. In alkaline lakes, natron (Na₂CO₃.10H₂)) and trona (NaHCO₃.2H₂O) are found in the United States, Africa, and South America. In lakes in the East African rift valley, volcanic emanations supply high concentrations of sodium carbonate, and reaction between these products and volcanic detritus produces unusual mineral suites. Trona (Na₂CO₃.HCO₃.2H₂O) forms layers interbedded with detrital clays and volcanic detritus in which there is formation in situ of zeolites. The rare minerals magadiite (NaSi₇O₁₃(OH)₃.3H₂O) and kenyaite (NaS₁₁O_20.5(OH)₄.3H₂O) and associated cherts are thought to form by leaching of the latter. In other lakes, borates are deposited as the rare mineral borax (Na₂B₄O₇H₂O). Searles Lake in California produces 90 per cent of the world's boron from such a deposit. Other more unusual deposits are the diatomaceous-rich sediments found in temperate and high-latitude lakes, and iron and manganese crusts and iron hydroxides and oxides of some high-latitude lakes (e.g. Scandinavia).

Thick deposits of ancient lake sediments are known in many parts of the geological column. The Eocene Green River Formation is one of the most famous, but ancient lake deposits are known in the Devonian and Triassic of Europe and North America. They are extremely valuable economically, both for their rare evaporitic salts and as important sources of hydrocarbons.

G. Evans

Bibliography

Galloway, W. E. and and Hobday, D. K. (1983) Terrigenous clastic deposition systems, applications to petroleum, coal, and uranium exploration, pp. 185–201. Springer-Verlag, New York.
Matter, A. and Tucker, M. (eds) (1978) Modern and ancient lake sediments. International Association of Sedimentologists, Special Publication 2.
Reeves, C. C. (1968) Introduction to paleolimnology. Elsevier, Amsterdam.