Spec. Publ. Int. Assoc. Sedimentol. (2007),3E,241-252 A shallow-basin model for 'saline g¡ants' based on ¡sostasy-driven subsidence FRANK J.G. VAN DEN BELT ond POPPE L. DE BOER University of Utrecht" Foculty of Eorth Sciences, PO Box 80021, 3508 TA Utrecht, Ihe Nethedonds (Emoil: figvon denbelt@geo.uu.nl; p deboer@geo.u u.nl ) ABSTRACT The common assumption that 'saline giants' must have formed in deep basins and that their thickness reflects initial basin depth ignores the principle of isostasy. Due to the high density of anhydrite and high precipitation rates for evaporite minerals, isostatic compensation is much more important in evaporite than in non-evaporite settings. The main implication is that evaporite precipitation drives subsidence rather than the other way round, and that thick evaporite deposits require an initial basin depth much less than their fìnal thickness, Once initiated, evaporite pre- cipitation and consequent isostatic subsidence is a self-sustaining process that can result in kilometre-scale evaporite stratigraphy. Rapid isostatic compensation is facilitated by thin, fractured crust in extensional basins, which explains the typical occurrence of saline giants in such settings. It is shown that a shallow-basin origin in combination with rapid isostatic compensation can well explain the extreme thickness of saline giants as well as the commonly associated shallow-water sedimentary facies. Although there is no reason to exclude the possibility of a basin-wide dropdown of a few thousand metres as proposed for some saline giants, a desiccated deep basin is certainly not a requirement. An initially shallow basin that rapidly deepens by isostatic adjustment in response to the precipitation of evaporites eliminates the need for deep-basin desiccation, gigantic water- falls, and repeated opening and closure of a connection to the world ocean, and makes the extreme thickness of saline giants less enigmatic. Keywords Saline giants, isostasy, evaporites, halite, anhydrite, Zechstein, Messinian. INTRODUCTION A number of evaporite successions are character- ized by extraordinary thickness and are therefore commonly referred to as 'saline giants'. They are up to 4 km thick and typically consist of a num- ber of stacked, thinning-upward evaporite cycles (Table 1). For example, the carbonate-evaporite succession from the Permian Zechstein reaches a thickness of 2 km (Taylor, 7998); individual halite bodies are up to 600 m thick (Sannemann et a1.,7978) ancl anhydrite bodies are up to 280 m (Van der Baan, 1990). The major Messinian evaporite succession in the western Mediterranean was estimated to be 2-3 km thick (Hsu et a1.,1973) and is 2 km in the eastern Mediterranean (Tay et a1.,2002). Accord- ing to Krijgsman et al. (1999) these Mediterranean evaporites were deposited in no moÍe than 0.6 Myr. In the absence of recent analogues, developing models for saline giants has proven speculative. In the late 19th century Ochsenius (1877) developed a depositional model basecl on evaporite precipi- tation in a restricted lagoonal environment. Hsü ef al. (7973,7977) felt it could not explain the new data from the Mediterranean, which they interpreted as deposits formed by precipitation from shallow- water salt lakes that occupied the deepest parts of kilometres deep, clesiccated basins (Fig. 1). The model is known as the cleep-basin shallow-water model and is often used in explaining thick halite deposits (e.g. Sonnenfeld, 1.984; Warren, 1999). The formation of Zechstein halite bodies has also been attributecl to deep-basin shallow-water