461 Journal of the Geological Society, London, Vol. 171, 2014, pp. 461–464. http://dx.doi.org/10.1144/jgs2013-062 Published Online First on May 28, 2014 © 2014 The Geological Society of London An intra-basinal mechanism for marine-evaporite cyclicity F. J. G. VAN DEN BELT * & P. L. DE BOER Department of Earth Sciences, University of Utrecht, PO Box 80021, 3508 TA Utrecht, Netherlands *Corresponding author (e-mail: f.j.g.vandenbelt@uu.nl) M arine evaporites such as the Zechstein (Permian, NW Europe) consist of thinning-upward sul- phate–halite–potash cycles whose origin is poorly understood. An intra-basinal mechanism pre- sented here explains well their mineral composition and cycle development. It involves the progressive obstruction of ocean connections by sulphate-platform progradation, causing a chain reaction of outflow reduction and subsequent accelerated sulphate precipitation. Numerical modelling shows this to be a self-accelerating process that ultimately triggers halite and pot- ash precipitation. Isostatic compensation of the salt load explains the formation of accommodation space for subsequent cycles, each about half the thickness of the previous cycle. Giant marine evaporite bodies such as the Zechstein (Permian) of NW Europe and the Mediterranean (Messinian) evaporites have formed throughout the Phanerozoic (Zharkov 1981; Warren 2010). They commonly consist of stacked sulphate–halite–potash cycles, each tens to hundreds of metres thick, with sulphate (gyp- sum or anhydrite) constituting basin-margin platforms and thin basin-central annual-varve sequences. The more soluble halite and potash salts fill the basin topography (Fig. 1). Despite many well-studied examples much is uncertain about their formation and the mechanism driving cyclicity; in addition, there are no present-day analogues to help with interpretations. Evaporite models have been dominated by the idea that precipi- tation takes place in deep, desiccated basins, a concept developed for the Late Miocene Mediterranean (Hsü et al. 1973) and applied more generally since (Nurmi & Friedman 1977; Tucker 1991). However, a persistent two-way ocean connection is more in line with the observation that marine evaporites are enriched in sulphate and depleted in higher-solubility salts (Sonnenfeld 1984; Hardie & Lowenstein 2004). Hence, it is unlikely that much deposition took place during desiccation phases, and more recent models are based on the filled-basin concept (Hardie & Lowenstein 2004; Manzi et al. 2005; Becker & Bechstädt 2006; Krijgsman & Meijer 2008). If indeed these deposits formed in basins with open connections to the ocean, the origin of evaporite cyclicity, attributed to eustatic con- trol (Tucker 1991) or tectonics (Krijgsman et al. 1999), must be reassessed. Tectonic mechanisms, such as the repetitive closure and opening of a barrier (Benson 1972), are hard to reconcile with the cyclic character of evaporites because they require a squeeze-box move- ment. Eustatic control (see Tucker 1991), on the other hand, is in line with the cyclic nature of evaporites and may play an important rapid-communicationSpecial XX X 10.1144/jgs2013-062F. J. G. Van Den Belt & P. L. De BoerMarine-Evaporite Cyclicity 2014 role in triggering and terminating evaporite precipitation by chang- ing the cross-sectional area of the ocean connection. It is difficult, however, to explain the formation of thick basin-filling halite bod- ies under desiccated lowstand conditions, unless barrier-seepage rates are very high. In addition, the presence of thick halite bodies on top of carbonate–sulphate platforms (Fig. 1c) suggests that hal- ite precipitates extensively during late transgression and highstand. It is thought therefore that evaporite precipitation may be triggered by sea-level lowering, but proceeds under relatively high sea level, thus allowing continuous communication with the ocean. A possi- ble driving mechanism is presented below. An alternative mechanism. Considering the abundance of marine evaporite bodies and the similarities between them, the potential of a general mechanism, independent of external factors, was investigated. With gypsum precipitation rates up to 10 m ka -1 (Schreiber & Hsü 1980), sulphate platforms are capable of rapidly prograding over tens of kilometres (Cameron et al. 1992; Kendall 2010), which is reflected in the broad anhydrite rims that typi- cally surround evaporite basins and also occur near their ocean corridors (Fig. 1). This may lead to progressive obstruction of such corridors, causing reduced brine outflow and raised salini- ties, followed by sulphate precipitation and accelerated platform progradation (Van den Belt & De Boer 2007). If not interrupted externally (e.g. by sea-level rise or tectonics), this will lead to the obstruction of brine outflow and halite and potash–salt precipita- tion in the basin centre. The mechanism has the advantage that it progressively obstructs outflow while still allowing ocean-water inflow to continuously replenish evaporated basin water and thus maintain the salt influx. Model description. A simple numerical model was developed to test the mechanism and verify if it results in realistic evaporite- cycle composition and operates on short enough time scales to outpace tectonic and eustatic processes. The model simulates sea- water–brine exchange, salinity evolution and precipitation in cir- cular basins of constant depth, connected to the ocean by a narrow corridor (Fig. 2). The model is initiated at a steady state of incipi- ent sulphate saturation (7.2 g l -1 ). Precipitation is then initiated by implementing a small reduction of the cross-sectional area of the ocean corridor, causing a chain reaction of (1) outflow reduction, (2) salinity rise, (3) sulphate precipitation on the basin floor and around the basin margin with consequent platform progradation, which then causes further outflow reduction, and so on. Sulphate saturation is considered as a state that is reached occa- sionally; for example, when marginal continental basins located in an arid-climate belt are invaded by the sea (e.g. in response to deglaciation). If the cross-section of their ocean corridors is small enough, they may then become saturated for sulphate, halite and finally potash. Basins were modelled starting with sulphate satura- tion and their evolution to halite and potash basins. Main input values are surface area, evaporation rate and corridor width. Seawater composition and saturation concentrations are based on Sonnenfeld (1984). The model is initiated at steady-state sulphate saturation, when Salt in = Salt out and the salinity (σ out ) in the basin equals 0.16 (Lucia 1972). Then Vol in ρ in σ in = Vol out ρ out 0.16 (Lucia 1972); substitution of density and salinity values (ρ in = 1.03 kg l -1 , ρ out = 1.12, σ in = 0.0351) gives Vol out /Vol in = 0.20, where Vol in = Evaporation + Vol out. A constant flow rate through the corridor is assumed, which equals Vol tot /A, where A is the cross- sectional area of the corridor. A reduction of the corridor width is at the expense of corridor outflow. Calcium sulphate precipitation SPECIAL