oa Evaporites Across Deep Time: Tectonic, Climatic and Rustatic Controls in Marine and Nonmarine Deposits

Plots of the world’s Phanerozoic and Neoproterozoic evaporite deposits, using a GIS base, shows that<br>Quaternary evaporite deposits are poor counterparts to the greater portion of the world’s Phanerozoic<br>evaporite deposits. They are only directly relevant to same-scale continental hydrologies of the past<br>and, as such, can be used to better understand what is needed to create beds rich in salt-cake, sodaash,<br>borate and lithium salts. These deposits tend be Neogene and mostly occurring in suprasealevel<br>hydrographically-isolated (endorheic) continental intermontane and desert margin settings that are<br>subject to the pluvial-interpluvial oscillations of today’s ice-house climate. When compared to ancient<br>marine evaporites, today’s marine-fed subsealevel deposits tend to be small sea-edge deposits, their<br>distribution and extent is limited by the current ice-house driven eustacy and a lack of appropriate<br>hydrographically isolated subsealevel tectonic depressions.<br>For the past forty years, Quaternary continental lacustrine deposit models have been applied to the<br>interpretation of ancient marine evaporite basins without recognition of the time-limited nature of this<br>comparison. Ancient mega-evaporite deposits (platform and/or basinwide deposits) require conditions<br>epeiric seaways (greenhouse climate) and/or continent-continent proximity. Basinwide evaporite<br>deposition is facilitated by continent-continent proximity at tectonic plate margins (Late stage E<br>through stage B in the Wilson cycle). This creates an isostatic response where, in an appropriate arid<br>climate belt, large portions of the collision suture belt or the incipient opening rift can be subsealevel,<br>hydrographically isolated (a marine evaporite drawdown basin) and yet fed seawater by a combination<br>of ongoing seepage and occasional marine overflow. Basinwide evaporite deposits can be classified by<br>tectonic setting into: convergent (collision basin), divergent (rift basin; prerift, synrift and postrift) and<br>intracratonic settings.<br>Ancient platform evaporites can be a subset of basinwide deposits, especially in intracratonic sag<br>basins, or part of a widespread epeiric marine platform fill. The latter tend to be mega-sulphates and<br>are associated with hydrographically isolated marine fed saltern and evaporitic mudflat systems in a<br>greenhouse climatic setting. The lower amplitude 4th and 5th order marine eustatic cycles and the<br>greater magnitude of marine freeboard during greenhouse climatic periods encourages deposition of<br>marine platform mega-sulphates. Platform mega-evaporites in intracratonic settings are typically<br>combinations of halite and sulphate beds. Potash evaporates tend to show a dichotomy of occurrence<br>with Quaternary deposits formed in small scale endorheic basins, while ancient potash deposits formed<br>in basinwide settings in situations that, like all basinwides, have no same-scale Quaternary counterparts.