The depth of the Miocene Mediterranean and other evaporite basins

Marine evaporites such as the Permian Zechstein and the Miocene Mediterranean evaporite, are characterized by thick successions (~250-2500 m) that consist of stacked sulphate-halite-potash cycles. The number of cycles is small, typically no more than 6-7, and in most cases cycle thickness decreases rapidly up the succession. It has been shown for the Dutch Zechstein that each cycle is approximately half as thick as the underlying cycle, which was attributed to a mechanism of repeated salt loading and isostatic compensation (Van den Belt & De Boer, 2014). When a 500 metres deep basin is filled with halite this causes a subsidence correction of ~250 m to regain isostatic equilibrium. This new basin may subsequently be filled with halite and trigger the formation of a 125 m deep basin, and so on. The ultimate succession reaches a thickness of ~1000 m and is capped by a compound top cycle that represents an infinite number of thin cycles. The ideal cycle-thickness pattern for a complete succession (100%) can be written as 50%, 25%, 12.5%, 6.25% and 6.25% (compound top cycle). An inventory has shown that the mechanism applies to other marine-evaporite basins as well. This includes the Miocene Mediterranean Sea, the depth of which has been debated for decades. With cycle thicknesses of ~395 m (cycle 2), ~ 215 m (cycle 3) and ~ 95 m (cycle 4), and a total thickness of ~1.6 km, the Western Mediterranean seems to have been about 800 m deep. For the Sicilian succession the cycle pattern is very close to predicted: 445 m (54%, initial basin depth) , 226 m (25%), 107 m (13%), 56 m (7%) and 28 m (3 %). Another example is the thin Devonian Muskeg evaporite succession, which comprises four cycles. Its thickness is ~270 m indicating an initial basin depth of 135 m. The first three cycles are well developed anhydrite-halite cycles, but the top cycle is composed of anhydritic red beds. In other succession the top cycle consists of evaporitic claystones as well, e.g. the Upper Claystone member in the Zechstein Basin and the “Lago Mare’ clays in the Mediterranean. This seems to reflect that towards the end of the process the evaporite basin becomes very shallow and is near sea level, and therefore attracts little ocean water. Poor cycle definition within the top cycle seems primarily the result of “scale of observation”, because the very thin Röt evaporite succession (Triassic, Netherlands) comprises four well-defined cycles of 37 m (54%), 18 m (27%), 8 m (12%), 5 m (7%) and a top cycle of 5 m (8%). It is concluded that the initial depth of marine-evaporite basins equalled 50% of the thickness of the succession and decreased to zero in a stepwise fashion during halite infilling. Van den Belt, F.J.G. & De Boer, P.L. (2014) JGS, 171, 461-464.