『Abstract
Throughout geological time, evaporite sediments form by solar-driven
concentration of a surface or nearsurface brine. Large, thick
and extensive deposits dominated by rock-salt (mega-halite) or
anhydrite (mega-sulfate) deposits tend to be marine evaporites
and can be associated with extensive deposits of 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.
Ancient marine evaporites typically have poorly developed Quaternary
counterparts in scale, thickness, tectonics and hydrology. When
mega-evaporite settings were active within appropriate arid climatic
and hydrological settings then huge volumes of seawater were drawn
into the subsealevel evaporitic depressions. These systems were
typical of regions where the evaporation rates of ocean waters
were at their maximum, and so were centered on the past latitudinal
equivalents of today's horse latitudes. But, like today's nonmarine
evaporites, the location of marine Phanerozoic evaporites in zones
of appropriate 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 Neooproterozoic
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
hydrographycally-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 hydrographycally 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 deposition 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,
hydrographycally 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 hydrographycally
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.
Keywords: evaporite; deposition; marine; nonmarine; plate tectonics;
economic geology; classification』
Contents
1. Introduction
2. Depositional parity across time and space
3. Nonmarine evaporites
3.1. Nonmarine hydrogeochemistry
3.2. Climatic framework
4. Nonmarine salts across time and tectonics
4.1. Borate evaporites
4.2. Sodium sulfate evaporites (salt-cake)
4.3. Sodium carbonate (soda-ash)
5. Marine evaporites
5.1. Marine hydrogeochemistry
5.2. The now and then of marine climates
6. Marine evaporites across time and tectonics
6.1. Platform evaporites
6.2. Basinwide evaporites
6.3. Tectonic settings for marine-fed basinwide evaporites
6.4. Potash salts
7. Evaporite brines (lithium and CaCl2)
7.1. CaCl2 brines and minerals
7.2. Lithium brines
8. Summary
Appendix A
References