Handford, R. Marginal marine halite: sabkhas and salinas. In Melvin, J. New York: Elsevier. Developments in Sedimentology , 50, pp. Hardie, L. Evaporites: marine or non-marine. Horita, J. Chemical evolution of seawater during the Phanerozoic: implications from the record of marine evaporites. Acta , 66 , — Kendall, A. Marine evaporites: arid shorelines and basins.
In Reading, H. Oxford: Blackwell, pp. Krijgsman, W. Chronology, causes and progression of the Messinian salinity crisis. Nature , , — Lowenstein, T. Paleoclimate record from death valley salt core. Geology , 27 , 3—6. Oscillations in Phanerozoic seawater chemistry: evidence from fluid inclusions. Science , , — Murray, R. Origin and diagenesis of gypsum and anhydrite. Material salts is eroded from land surfaces and is then carried to the sea by rivers.
When seawater evaporates, the salts precipitate and settle to the bottom. The less-soluble compounds those that dissolve less readily in water are deposited first. Calcium sulfate the compound that forms gypsum and anhydrite is the first to be deposited. Close-up view of twinned Messinian selenite crystals of the Piedmont Basin Italy.
Photo by Marcello Natalicchio. Evaporite rocks Evaporites are layered crystalline sedimentary rocks that form from brines generated in areas where the amount of water lost by evaporation exceeds the total amount of water from rainfall and influx via rivers and streams.
The mineralogy of evaporite rocks is complex, with almost varieties possible, but less than a dozen species are volumetrically important. Minerals in evaporite rocks include carbonates especially calcite, dolomite, magnesite, and aragonite , sulfates anhydrite and gypsum , and chlorides particularly halite, sylvite, and carnallite , as well as various borates, silicates, nitrates, and sulfocarbonates. Evaporite deposits occur in both marine and nonmarine sedimentary successions.
The fundamental controlling factor in the formation of evaporite deposits is climate, because the seawater can become sufficiently concentrated for precipitation to occur only if the rate of loss through evaporation exceeds the input of water. These arid environments are principally found in subtropical regions where the mean annual temperatures are relatively high but the rainfall is low.
Modern marine evaporite deposits are all found in coastal settings where precipitation occurs in semi isolated water bodies such as lagoons or directly within sediments of the coastal plain, places where recharge by seawater is limited. In the past, larger areas of evaporate precipitation resulted from the isolation from the open ocean of epicontinental seas and small ocean basins.
Evaporite minerals are precipitated in order depending on what percentage of the original volume of sea water remains: 1 Calcite CaCO 3 e dolomite CaMg CO 3 2.
If a column of seawater m deep is completely evaporated, only m of evaporites mainly halite are deposited. Some evaporite successions are thick several m , too thick to be simply the result of the evaporation of a single water mass and also their composition would not result from such a single event.
Evaporation of a seawater column and resulting evaporite deposits. Marine Evaporites Evaporite deposits in modern marine environments are largely restricted to coastal regions, such as evaporite lagoons and sabkha mudflats. However, evaporite successions in the stratigraphic record indicate that precipitation of evaporite minerals has at times occurred in more extensive marine settings. Evaporitic basins saline giants Evaporite sedimentation occurs only in situations where a body of water becomes partly isolated from the ocean realm and salinity increases to supersaturation point and there is chemical precipitation of minerals.
This can occur in epicontinental seas or small ocean basins that are connected to the open ocean by a strait that may become blocked by a fall in sea level or by tectonic uplift of a barrier such as a fault block. These are called barred basins and they are distinguished from lagoons in that they are basins capable of accumulating hundreds of metres of evaporite sediment.
To produce just a metre bed of halite a column of seawater over 75m deep must be evaporated, and to generate thick succession of evaporite minerals the seawater must be repeatedly replenished.
Deposition of the thick succession can be produced in three ways each of which will produce characteristic patterns of deposits: 1 A shallow-water to deep-basin setting exists where a basin is well below sea level but is only partly filled with evaporating seawater, which is periodically replenished. Normally graded beds generated by turbidites and poorly sorted deposits resulting from debris flows are evidence of redeposition.
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