INTERPRETING ANCIENT EVAPORITES: QUATERNARY SABKHAS AND SALINAS ARE NOT THE "HOLE" STORY

Our largest and thickest Quaternary sabkhas, saline pans and salinas are continental. They define same-scale analogs only for ancient continental lacustrine deposition, not for ancient marine-fed salt basins, such as the Hith, Zechstein and Messinian deposits. No widespread marine platform evaporites have accumulated since the Eocene and the Mediterranean Messinian (5.5 mya) is the latest basinwide evaporite event. Most ancient marine fed ancient evaporites are a order of magnitude larger than today’s systems and accumulated in large marine-seepage fed subsealevel depressions with no surface connection to the ocean. These platform and basinwide evaporites have no Quaternary counterpart in terms of scale or thickness but can be interpreted using models that are combinations of the hydrologies of sabkhas, saline pans and salinas. The hydrological position of the active top of the aggrading brine curtain, with respect to the evaporite depositional surface, in both platform and basinwide settings, defines the dominant textural signature of the resulting salt sequence (saline-pan, evaporitic mudflat (sabkha), saltern, deeper slope and basin).

Once we realize that thick evaporite accumulations (continental or marine) require a stable long-term brine curtain (100,000-1,000,000 years) to accumulate to substantial thicknesses and lateral extents, we reach an understanding of why the present is not a good time to study the scale and diversity of possible marine-fed platform or basinwide evaporite systems. High amplitude, high frequency 4th-order sea level oscillations of our current “icehouse” climate do not allow the set up of stable brine curtains behind laterally-continuous seepage shoals in present-day carbonate platforms and so there are no Neogene examples of platform evaporites. Platform evaporites require greenhouse eustacy to form. Nor are there suitable subsealevel rift-induced intracratonic sags or soft-collision belts in arid marine-fed settings where conditions are suitable for the creation of basinwide marine drawdown deposits. The need for subsealevel tectonically-induced basins explains why times of worldwide “zip then split” tectonics, which define supercontinent accretion and disaggregation (e.g. Pangea or Rodinia), also encourage the accumulation of substantial volumes of salts (independent climate modes)