Terrestrial sedimentary environments VI - Lakes:

Lakes are a very important terrestrial depositional environment. About 60% of lakes are freshwater; the rest are saline. They are dominated by low energy silt and mud with occasional carbonate. Because lakes typically fill with time they show the following trends:

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.

Lake level is very sensitive to climatic and tectonic conditions, and as such lake deposits are terrific paleotectonic and paleoclimatic records. Many lakes stratify their water columns, thus becoming anoxic in their bottom waters and preserving large volumes of organic matter (kerogen) at their base. Ancient lacustrine deposits have received little attention in the past, but recently they have become important sources of oil shale (e.g. Eocene-aged Green River Formation, right), uranium, and coal.

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.

Recognition

As subaqueous basin deposits, lake deposits resemble those of oceans, but are different in distinctive ways. We distinguish lake deposits form those of oceans by what they lack that oceans have and by what they have that oceans lack.

What they generally lack that oceans have:

As a result, wave sculpted shore deposits, intratidal environments, and associated bedforms are absent. Instead, lake deposits interfinger directly with those of adjacent subaerial environments.

What they generally have that oceans lack:

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.

Classification

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).

Open Lake

Closed Lake

Lakes are most common in areas of internal drainage, where a closed basin accumulates water. They are found in regions of:

Water can enter a lake through three means:

Usually, it only leaves by evaporation, but may also have rivers flowing out as well. Thus, lakes have a water budget.

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)

Chemistry

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.

However, most lacustrine carbonates form in environments where there is not great evaporation, but fluctuations in pH. They typically contain ample carbonate ion from the dissolution of surrounding bedrock, and are in general equilibrium with atmospheric CO2. Changes in pH induced by plant growth and warming, which affect the CO2 concentration in the lakes. Characteristic carbonate accumulations include oncolites (a.k.a. algal biscuits - right) formed by/from freshwater calcareous algae.

Associated deposits/environments: Can include:


  • Rivers: As lake level drops, various kinds of rivers can occupy the old lake beds. These are typically flashy, shallow channels with small loads that make a kind of "sand flat" environment. (Note: the Triassic New Oxford formation seen on field trip 1 is a good example.)

    Lake-floor fans: In lakes of good depth (>200 m), lakes may behave a bit like deep-sea systems and develop basinal turbidite fans along the lake floor. These are similar to deep-water fans, but are commonly more thinly bedded and a bit muddier.