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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Evaporites Across Deep Time: Tectonic, Climatic and Rustatic Controls in Marine and Nonmarine Deposits
(1) Dept. Geology, Chulalongkorn University, Chevron-PTTEP International MSc Program Petroleum Geoscience, Bangkok, Thailand.
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 portion of the world’s Phanerozoic evaporite deposits. They are only directly relevant to same-scale continental hydrologies of the past and, as such, can be used 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 occurring in suprasealevel hydrographically-isolated (endorheic) continental intermontane and desert margin settings that are subject to the pluvial-interpluvial oscillations of today’s ice-house climate. 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 eustacy 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 comparison. Ancient mega-evaporite deposits (platform and/or basinwide deposits) require conditions epeiric seaways (greenhouse climate) and/or continent-continent proximity. Basinwide evaporite deposition is facilitated by continent-continent proximity at tectonic plate margins (Late stage E through stage B in the Wilson cycle). This creates an isostatic response where, in an 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 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. The latter tend to be mega-sulphates and are associated with hydrographically isolated marine fed saltern and evaporitic mudflat systems in a greenhouse climatic setting. The lower amplitude 4th and 5th order marine eustatic cycles and the greater magnitude of marine freeboard during greenhouse climatic periods encourages deposition of marine platform mega-sulphates. Platform mega-evaporites in intracratonic settings are typically combinations of halite and sulphate beds. Potash evaporates tend to show a dichotomy of occurrence with Quaternary deposits formed in small scale endorheic basins, while ancient potash deposits formed in basinwide settings in situations that, like all basinwides, have no same-scale Quaternary counterparts.
Distribution or the world’s major halite-rich salt basins classified according to plate tectonic association at time of salt accumulation. Many of these basins experienced subsequent polyphase tectonic deformation histories.