Earth-Science Reviews 98 (2010) 217–268 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits John K. Warren Petroleum Geoscience Program, Department of Geology, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand article info abstract Article history: Throughout geological time, evaporite sediments form by solar-driven concentration of a surface or Received 25 February 2009 nearsurface brine. Large, thick and extensive deposits dominated by rock-salt (mega-halite) or anhydrite Accepted 10 November 2009 (mega-sulfate) deposits tend to be marine evaporites and can be associated with extensive deposits of Available online 22 November 2009 potash salts (mega-potash). Ancient marine evaporite deposition required particular climatic, eustatic or tectonic juxtapositions that have occurred a number of times in the past and will so again in the future. Keywords: Ancient marine evaporites typically have poorly developed Quaternary counterparts in scale, thickness, evaporite deposition tectonics and hydrology. When mega-evaporite settings were active within appropriate arid climatic and marine hydrological settings then huge volumes of seawater were drawn into the subsealevel evaporitic nonmarine depressions. These systems were typical of regions where the evaporation rates of ocean waters were at plate tectonics their maximum, and so were centred on the past latitudinal equivalents of today's horse latitudes. But, like economic geology today's nonmarine evaporites, the location of marine Phanerozoic evaporites in zones of appropriate classification adiabatic aridity and continentality extended well into the equatorial belts. Exploited deposits of borate, sodium carbonate (soda-ash) and sodium sulfate (salt-cake) salts, along with evaporitic sediments hosting lithium-rich brines require continental–meteoric not marine-fed hydrologies. Plots of the world's Phanerozoic and Neoproterozoic evaporite deposits, using a GIS base, shows that Quaternary evaporite deposits are poor counterparts to the greater part of the world's Phanerozoic evaporite deposits. They are only directly relevant to same-scale continental hydrologies of the past and, as such, are used in this paper to better understand what is needed to create beds rich in salt-cake, soda-ash, borate and lithium salts. These deposits tend be Neogene and mostly occur in suprasealevel hydrographically-isolated (endorheic) continental intermontane and desert margin settings that are subject to the pluvial–interpluvial oscillations of Neogene ice-house climates. When compared to ancient marine evaporites, today's marine-fed subsealevel deposits tend to be small sea-edge deposits, their distribution and extent is limited by the current ice-house driven eustasy and a lack of appropriate hydrographically isolated subsealevel tectonic depressions. For the past forty years, Quaternary continental lacustrine deposit models have been applied to the interpretation of ancient marine evaporite basins without recognition of the time-limited nature of this type of comparison. Ancient mega-evaporite deposits (platform and/or basinwide deposits) require conditions of epeiric seaways (greenhouse climate) and/or continent–continent proximity. Basinwide evaporite deposi- tion is facilitated by continent–continent proximity at the plate tectonic scale (Late stage E through stage B in the Wilson cycle). This creates an isostatic response where, in the appropriate arid climate belt, large portions of the collision suture belt or the incipient opening rift can be subsealevel, hydrographically isolated (a marine evaporite drawdown basin) and yet fed seawater by a combination of ongoing seepage and occasional marine overflow. Basinwide evaporite deposits can be classified by their tectonic setting into: convergent (collision basin), divergent (rift basin; prerift, synrift and postrift) and intracratonic settings. Ancient platform evaporites can be a subset of basinwide deposits, especially in intracratonic sag basins, or part of a widespread epeiric marine platform fill. In the latter case they tend to form mega-sulfate deposits and are associated with hydrographically isolated marine fed saltern and evaporitic mudflat systems in a greenhouse climatic setting. The lower amplitude 4 and 5th order marine eustatic cycles and the greater magnitude of marine freeboard during greenhouse climatic periods encourages deposition of marine platform mega-sulfates. Platform mega-evaporites in intracratonic settings are typically combinations of halite and sulfate beds. © 2009 Elsevier B.V. All rights reserved. E-mail address: [email protected]. 0012-8252/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2009.11.004 218 J.K. Warren / Earth-Science Reviews 98 (2010) 217–268 Contents 1. Introduction .............................................................. 218 2. Depositional parity across time and space ................................................ 220 3. Nonmarine evaporites ......................................................... 222 3.1. Nonmarine hydrogeochemistry .................................................. 222 3.2. Climatic framework ....................................................... 223 4. Nonmarine salts across time and tectonics ................................................ 224 4.1. Borate evaporites ........................................................ 226 4.2. Sodium sulfate evaporites (salt-cake) ............................................... 230 4.3. Sodium carbonate (soda-ash) ................................................... 236 5. Marine evaporites ........................................................... 240 5.1. Marine hydrogeochemistry .................................................... 242 5.2. The now and then of marine climates ............................................... 244 6. Marine evaporites across time and tectonics ............................................... 246 6.1. Platform evaporites ....................................................... 246 6.2. Basinwide evaporites....................................................... 248 6.3. Tectonic settings for marine-fed basinwide evaporites ....................................... 249 6.4. Potash salts ........................................................... 255 7. Evaporite brines (lithium and CaCl2)................................................... 258 7.1. CaCl2 brines and minerals .................................................... 258 7.2. Lithium brines .......................................................... 261 8. Summary ............................................................... 265 References ................................................................. 265 1. Introduction Most exploited evaporite salts and their associated brines are used in the manufacture of various chemical feedstocks, especially in the By definition, for an evaporite salt to precipitate, liquid water loss from chloralkali industries, with lesser volumes supplying feedstock to the a brine mass must exceed inflow and the brine concentration process fertiliser industry (Table 1). Volumetrically, halite (rock-salt) is the should be driven by solar evaporation (Warren, 2006). Throughout time, most utilised evaporite salt, principally as feedstock to the chemical evaporite salts have accumulated on the floors of standing brine lakes or industry for the manufacture of a wide range of industrial chemicals seaways, in nearsurface pore throats, and as capillary efflorescences atop (Table 2). In cold climates substantial volumes of crushed rock-salt are exposed arid landsurfaces. Variation in the resultant textures (subaque- used for road de-icing. Next after rock-salt is gypsum, which is chiefly ousversussabkhaversusburialsalts)indicatebothhydrologicalstability used in the manufacture of wallboard, plaster and cement for the and mechanical energy level in the brine at the time of precipitation. The world's construction industries. Next in terms of extracted volume is diversity and extent of ancient evaporite deposits is far greater than in the soda-ash, which is used in the manufacture of glass and detergent; eustatically and time limited sample available for study in Quaternary then the potash salts, primarily used to manufacture agricultural evaporite deposits (Fig. 1). fertilisers; then the borates and the sodium sulfates, all with a variety All evaporites are ionic salts containing the major ions Na, Ca, Mg, K, of industrial usages. Lithium (as lithium carbonate) is increasingly Cl, SO4,andCO3 in varying proportions, along with other less common extracted from a brine rather than a solid mineral source. ionic constituents such as B, Ba, Sr, Br, Li and I and varying amounts of Throughout the Phanerozoic the largest nonmarine evaporite bound or structural water (Appendix A lists chemical formulae). deposits typically accrued at depositional scales that were more than Mineralogy and order of salts precipitating in a concentrating brine is an order of magnitude smaller than the larger of the ancient marine- controlled by the ionic make-up of the parent water (Hardie and fed evaporite (Fig. 2). This dichotomy in scale reflects the simple Eugster, 1970). Deposits of various evaporite salts can be classified aphorism that the hydrology needed to form a huge mass of halite- according to the source of the