Evaporite Deposition across Time; Why the Past is the Key to the Present?

Quaternary evaporite deposits are poor counterparts to most of the world’s ancient evaporites. Tectonics, marine drawdown and basin-scale elevation changes control the style and distribution of all ancient large halite-dominant (megahalite) and sulphate-dominant (megasulphate) evaporites. Eustacy, especially greenhouse eustacy, exerts a more significant influence over the deposition of ancient marine-fed megasulphate evaporites. Neither megahalite or megasulphate deposits have same-scale modern counterparts and intracratonic, rather than plate-edge, basinwide evaporites host the larger of the world’s potash salts. The paper focuses on potash controls, but illustrates a more general principal, namely; the breadth of ancient evaporite depositional styles underlines the time-limited sample of evaporite styles available for study in the world’s Quaternary-age saline depositional systems.

Quaternary-age potash deposits are small and continental. Ancient potash deposits, like the greater volume of ancient halite and anhydrite, are marine salts deposited across areas and thicknesses for which there is no modern same-scale counterpart. In mined Quaternary potash examples, muriate of potash (KCl) is the dominant phase produced, or expected to be produced, in the hot arid desert setting of Dead Sea and the Danakil depressions, while combination potash salts (KCl-K2SO4) are manufactured from brines in the modern salt pans in cold arid desert or steppe climates in Salar de Atacama, Lake Dabuxum and Lop Nur. All these modern potash deposits are forming in extreme endoheic lows or highs on the Earth’s surface (some 415 and 115m metres below sea level or more than 2500m above sealevel), all are in adiabatic deserts in regions of drastic local subsidence, with brine inflows, chemistries and ionic proportions substantially influenced by the dissolution of older marine evaporites.

Most all pre-Quaternary potash mines extract product from marine-fed basinwide ore hosts. In terms of ease of extraction and preserved lateral bedded extent the largest of the world’s examples come from intracratonic potash deposits formed during times of MgSO4-depleted and to a lesser extent MgSO4-enriched oceans. This time-based dichotomy reflects a simple lack of basinwide marine deposits since the late Miocene. The same dichotomy explains why megasulphates are seals to most oil fields in the Middle East and why the Circum-Atlantic megahalites are associated with most giant oil fields there.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California