Evaporite Depositional Systems: Is the Past the Key to the Present?
Warren, John K.*1
(1) Petroleum Geoscience, Chulalongkorn University, Bangkok, Thailand.
Area 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 seals. They are only directly relevant to same-scale same-climate continental lacustrine hydrologies of the past and, as such, can be used to better understand what is needed to create widespread salt beds rich in Na2SO4, NaCO3, borate and lithium salts. Exploited examples of these deposits tend be Neogene and mostly occur in suprasealevel hydrographically-isolated (endoheic) continental intermontane and desert margin settings, which were subject to the pluvial-interpluvial climatic oscillations, also seen in today’s ice-house climate. When compared to ancient marine evaporites, today’s marine-fed 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. Ancient platform evaporites that typify the seals to many petroleum systems in the Middle East 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 salts tend to show a dichotomy of occurrence with Quaternary deposits formed in small-scale endoheic basins, while ancient potash deposits formed in basinwide settings in situations that, like all basinwides, have no same-scale Quaternary counterparts. Larger, laterally continuous examples of potash as KCl tend to be found in ancient marine intracratonic evaporite basins in compressional flexure settings, while potash as K2SO4 salts are derived by processing of modern saline lacustrine brines in cool arid settings, or via the solution processing of diapiric marine-evaporites that precipitated at times of MgSO4 oceans.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain