Sedimentologi Kamal Roslan Mohamed LAKE
INTRODUCTION A lake is an inland body of water. Lakes form where there is a depression on the land surface which is bounded by a sill such that water accumulating in the depression is retained. A lake basin supplied by a river in the foreground, with outflow through a sill to the sea in the distance. Lakes form where there is a supply of water to a topographic low on the land surface. They are fed mainly by rivers and lose water by flow out into a river and/or evaporation from the surface. The balance between inflow and outflow and the rate at which evaporation occurs control the level of water in the lake and the water chemistry. Under conditions of high inflow the water level in the lake may be constant, governed by the spill point of the outflow, and the water remains fresh.
INTRODUCTION Low water input coupled with high evaporation rates in an enclosed basin results in the concentration of dissolved ions, which may be precipitated as evaporites in a perennial saline lake or when an ephemeral lake dries out. Lakes are therefore very sensitive to climate and climate change. Many of the processes that occur in seas also occur in lakes: deltas form where rivers enter the lake, beaches form along the margins, density currents flow down to the water bottom and waves act on the surface. There are, however, important differences with marine settings: the fauna and flora are distinct, the chemistry of lake waters varies from lake to lake and certain physical processes of temperature and density stratification are unique to lacustrine environments.
Lake hydrology A lake is considered to be hydrologically open if it is filled to the spill point and there is a balance of water supply into and out of the basin. Under these circumstances the level of the water in the lake will be constant, and the constant supply from rivers will mean that the water in the lake will be fresh. If the rate of evaporation exceeds or balances the rate of water supply there is no outflow from the lake and it is considered to be hydrologically closed.
Hydrology of freshwater lakes Below about 10 or 20m depth the lake waters are unaffected by any wave or current activity. This allows for the development of lake water stratification, which is seen as a contrast in the temperature, density and the chemistry of the waters in the upper and lower parts of the water body. The epilimnion, which is the upper, warmer lake water, and the hypolimnion, the lower, colder part: they are separated by a surface across which the temperature changes, the thermocline.
Hydrology of freshwater lakes The bottom of the lake therefore becomes anaerobic (without air, and therefore oxygen) and this has two important consequences. Any organic material that falls through the water column to the lake floor will not be subject to breakdown by the activity of the aerobic processes that normally cause decomposition of plant and animal tissue. If there is abundant plant material being swept into the lake, this has the potential to form a detrital coal layer. The second effect of anaerobic bottom conditions is that this is an environment that is unfavourable for life.
Lake clastic deposits Where a sediment-laden river enters a lake the water velocity drops abruptly and a delta forms as coarse material is deposited at the river mouth. The form and processes on a lake delta will be similar to that seen in river-dominated deltas, with some wave reworking of sediment also occurring if the lake experiences strong winds. Facies distribution in a freshwater lake with dominantly clastic deposition. The character of the delta deposits will be largely controlled by the nature of the sediment supply from the river, and may range from fine-grained deposits to coarse, gravelly fan-deltas.
Lake clastic deposits Mixtures of sediment and water brought in by a river or reworked from a lake delta may flow as a turbidity current, which can travel across the lake floor. The deposits will be layers of sediment that grade from coarse material deposited from the current first to finer sediment that settles out last. In lakes where sediment plumes and turbidity currents are the main transport mechanisms the deep lake facies will consist of very finely laminated muds deposited from suspension alternating with thin graded turbidites forming a characteristic, thinly bedded succession. A schematic graphic sedimentary log through clastic deposits in a freshwater lake.
Lake clastic deposits In lakes formed in regions where there is an annual thaw of winter snow a distinctive stratification may develop due to seasonal variations in the sediment supply. The spring thaw will result in an influx of sediment-laden cold water, which will form a layer of sediment on the lake floor. During the summer months organic productivity in and around the lake provides a supply of organic material that settles on the lake floor where it is preserved in the anaerobic conditions. This alternation of dark, organic-rich deposits formed in the summer months and paler, clastic sediment brought in by the spring thaw is a distinctive feature of many temperate lakes. The millimetre-scale laminae formed in this way are known as varves and they have been used in chronostratigraphy of Holocene deposits
Lacustrine carbonates Carbonates can form a significant proportion of the succession in any lake setting only if the terrigenous clastic input is reduced. Direct chemical precipitation of carbonate minerals occurs in lakes with raised salinity, but in freshwater lakes the formation of calcium carbonates is predominantly associated with biological activity. Facies distributions in a freshwater lake with carbonate deposition. The hard shells of animals such as bivalve molluscs, gastropods and ostracods can contribute some material to lake sediments, and this coarse skeletal material may be deposited in shallow water or redistributed around the lake by wave-driven currents. However, the most abundant carbonate material in lakes is usually from algal and microbial sources.
EPHEMERAL LAKES Large bodies of water that periodically dry out are probably best described as ephemeral lakes, although the term playa lake is also commonly Used. A salt crust of minerals formed by evaporation in an ephemeral lake. Once the lake has formed, particles suspended in the water will start to deposit and form a layer of fine-grained muddy sediment. Evaporation of the water body gradually reduces its volume and the area of the lake starts to shrink, leaving areas of margin exposed where desiccation cracks may form in the mud as it dries out. With further evaporation the ion concentration in the water starts to increase to the point where precipitation of minerals occurs.
EPHEMERAL LAKES When an ephemeral lake receives an influx of water and sediment, mud is deposited from suspension to form a thin bed that is overlain by evaporite minerals as the water evaporates. Repetitions of this process create a series of couplets of mudstone and evaporite.
Characteristics of lake deposits . lithologies – sandstone, mudstone, fine-grained limestones and evaporites . mineralogy – variable . texture – sands moderately well sorted . bed geometry – often very thin-bedded . sedimentary structures – wave ripples and very fine parallel lamination . palaeocurrents – few with palaeoenvironmental significance . fossils – algal and microbial plus uncommon shells . colour – variable, but may be dark grey in deep lake deposits . facies associations – commonly occur with fluvial deposits, evaporites and associated with aeolian facies