Terrestrial sedimentary environments VI - Lakes:
- a shallowing upward, as the lake basin is filled with sediment
- coarsening upward, as shoreline progrades into the lake.
Unlike oceans, lakes tend not to be dominated by wave or tidal processes, so their associated sedimentary structures are limited. Rather, they often experience alternating periods of overturn, when the entire lake circulates, and density stratification, when the lake is stagnant, resulting in varves - seasonally alternating light and dark deposits.
Different kinds of lakes are found in different climates, but desert, temperate, Mediterranean, and even arctic zones have large lakes. This discussion will focus on large, permanent lakes, as opposed to small ephemeral lakes or ponds.
What they generally lack that oceans have:
- Significant wave energy
- Significant tides
- Organisms with calcareous skeletons
- Seasonal alternation of turnover and stagnation.
- Because lakes tend, geologically speaking, to be ephemeral, their deposits are often sandwiched between adjacent continental deposits.
- Because they are comparatively deficient in critters with CaCO3 skeletons, carbonate deposits are likely to occur as direct precipitates.
As such, laminated fine-grained deposits dominate all but the margins of ancient lake deposits (right). If the laminations show alternating seasonal variation, they are called varves. They are commonly characterized by a low abundance, low diversity fauna.
Lakes can be subdivided into two main categories based on fluxes in and out. Beyond this, they are classified by chemistry. Note, lake character can vary a lot over short time scales. As such, rather different kinds of lake deposit can occur in close succession (e.g., interbedded fresh vs. saline).
- Water and sediment flux: High. Water enters and exits
- Water character: Stratified or mixed
- Special deposits: Organic-rich shales
- Water and sediment flux: Lower. Water cannot leave.
- Water character: Fresh or saline
- Special deposits: Organic-rich shales, carbonates, or evaporites
- tectonic depression (E.G. rift grabens - Lake Turkana, Rt.)
- volcanic calderas (E.G. Crater Lake)
- glacial depressions (E.G. Great Lakes)
- karst sinkholes (E.G. Cenote Xlacah)
- meteorite craters (E.G. Lake Manicouagan)
- impounded water behind some geological or man-made barrier. (E.G. Tibetan landslide lake.)
Water can enter a lake through three means:
If a lake is open (water leaves by surface flow), the lake height will be fixed by a topographic saddle called a sill. Sills usually maintain a relatively constant height over 100,000-1,000,000 yr. time scales, but they may also spontaneously fail, resulting in very large flood events. (E.G. Drumheller Scablands, WA)
Where the clastic input to the lake is limited, chemical sedimentation may predominate, including evaporites and carbonates. Where evaporation exceeds inflow the salinity can become greater than the oceans. As evaporation proceeds and lake level falls carbonate, gypsum, and halite are sequentially precipitated, and these may be followed by potassium and magnesium salts. Spires of tufa - direct precipitates of CaCO3 - (E.G. Mono Lake, Ca, right) are common in closed lakes where evaporation is great.
Associated deposits/environments: Can include:
- Alluvial fans/Fan Deltas: These can be found right at the edge of the lake against the active rangefront.
- Deltas: These can be found building at the edge of the lakes, often in an axial position. These can be coarse or fine grained. As lake-level varies quickly, deltas can step forwards, backwards, or be abandoned very quickly, in contrast to marginal marine deltas. As such, deltaic deposits can interfinger with lake center sort of sediments
- "Gilbert Deltas:" These are very steep, coarse-grained deltas that build incrementally into the lake. Delta fronts assume the angle of repose for coarse sediment, often 10 deg.-20 deg. Gilbert deltas form where glacial streams meet the deep proglacial lake. Characterized by:
- Horizontal topset beds
- Steeply-dipping foreset beds
- Shallow horizontal bottomset beds sharply set off from the foreset.