GEOL 342 Sedimentation and StratigraphyLecture 10: Lacustrine environments3 March 2005Assoc. Prof. A. Jay KaufmanLacustrine environmentsLakes are a very important terrestrial depositional environment. About 60% of lakes arefreshwater; the rest are saline. They are dominated by low energy silt and mud with occasionalcarbonate. Because lakes typically fill with time they show a shallowing and coarsening upwardtrend. Unlike oceans, lakes tend not to be dominated by wave or tidal processes, so theirassociated sedimentary structures are limited. They do normally have periods of overturn, whenthe entire lake circulates, and density stratification, when the lake is stagnant.Lake level is very sensitive to climatic and tectonic conditions, and as such lake depositsare terrific paleotectonic and paleoclimatic records. Many lakes stratify their water columns, thusbecoming 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 theyhave become important sources of oil shale (e.g. Eocene-aged Green River Formation), uranium,and coal. 1Different 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.RecognitionIn the modern, weak currents characterize most lakes. As such, laminated fine-graineddeposits dominate all but the margins of ancient lake deposits. They are commonly characterizedby a low abundance, low diversity fauna. They are commonly sandwiched between other types ofdeposits (the lake sandwich), like fluvial or aeolian successions. ClassificationLakes can be subdivided into two main categories based on their water level. The nextsubdivision involves lake chemistry. Importantly, lake character can vary a lot over short timescales. As such, rather different kinds of lake deposit can occur in close succession (e.g.,interbedded fresh vs. saline).Name Water character Special depositsOpen LakeStratified Organic-rich shales(Water & sediment flux high, water can leave)Mixed ----Closed LakeFresh Organic-rich shales(Water & sediment flux lower, water cannot leave)Saline Carbonates, evaporitesLakes are most common in areas of internal drainage, where a closed basin accumulateswater. Thhey are found in regions of 1) tectonic depression (e.g. rift grabens), 2) volcaniccalderas, 3) glacial depressions, 4) karst sinkholes, 5) meteorite craters, or 5) impounded waterbehind some geological or man-made barrier. 2WaterWater can enter a lake through three means: rain, groundwater, and rivers. Usually, it onlyleaves by evaporation, but may also have rivers flowing out as well. Thus, lakes have a waterbudget as to whether or not the lake is receiving more water than it loses. If a lake is open (water leaves by surface flow), the lake height will be fixed by atopographic 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 floodevents.ChemistryWhere the clastic input to the lake is limited, chemical sedimentation may predominate,including evaporites and carbonates. Where evaporation exceeds inflow the salinity can becomegreater than the oceans. As evaporation proceeds and lake level falls carbonate, gypsum, and haliteare sequentially precipitated, and these may be followed by potassium and magnesium salts. Tufaspires are common in closed lakes where evaporation is great.However, most lacustrine carbonates form in environments where there is not greatevaporation, but fluctuations in pH. They typically contain ample carbonate ion from thedissolution of surrounding bedrock, and are in general equilibrium with atmospheric CO2. Changesin pH induced by plant growth and warming, which affect the CO2 concentration in the lakes. Themixture of siliciclastic mud and carbonate in lakes produce the sediment type known as marl.Characteristic carbonate accumulations include oncolites or algal biscuits formed by freshwatercalcareous algae.3associated deposits/environmentsAlluvial fans/Fan Deltas: These will be found right at the edge of the lake against the activerangefront. Fan deltas are alluvial fans that build into standing water and are very similar incharacterDeltas: These can be found building at the edge of the lakes, often in an axial position. These canbe coarse or fine grained. As lake-level varies quickly, deltas can step forwards, backwards, orbe abandoned very quickly. As such, deltaic deposits can interfinger with lake center sort ofsediments“ Gilbert Deltas”: These are very steep, coarse-grained deltas that buildincrementally into the lake. These deltas exist at the angle of repose forcoarse sediment, often 10º-20º. Gilbert deltas form where glacialstreams meet the deep proglacial lake. They have horizontal topsetbeds and south-dipping foreset beds. In front of the foreset bedsare shallow horizontal bottomset beds. Rivers: As lake level drops, various kinds of rivers can occupy the old lake beds. These aretypically flashy, shallow channels with small loads that make a kind of “sand flat”environment.Lake-floor fans: In lakes of good depth (>200 m), lakes may behave a bit like deep-sea systemsand develop basinal turbidite fans along the lake floor. These are similar to deep-water fans,but are commonly more thinly bedded and a bit