Epitaxial salt reefs and mushrooms in the southern Dead Sea

Abstract

ABSTRACTSalt usually accumulates in shallow ephemeral brines as simple flat beds. However, in permanent brines deepening in solar evaporation ponds in the southern Dead Sea, salt accumulation is complicated by the growth of isolated vegetable‐like salt structures and networks of polygonal salt walls. These walls divide large salt ponds, which are easily homogenized by wind, into thousands of small compartments that are less easy to mix. Instead, brines in the compartments stratify when a surficial layer about 10 cm thick supersaturates and floats above the cumulate floors on brines that are merely saturated.Salt reef growth in the southern Dead Sea has previously been attributed to mixing of ions common to brines already in the ponds and those pumped in from the northern basin (with or without subsurface brines rising through the pond floors). A new factor is emphasized here that, whatever the origins of the brines, salt reefs remain emergent by epitaxis, the in situ growth of crystalline substrates already in the surficial supersaturated layer.Epitaxis can be invisible in crystal clear brines and prolongs the obvious crystallization season by replacing sedimentation of grains nucleated and grown on the brine surface as the dominant mechanism of deposition. Salt reefs develop botryoidal overhangs that can merge into salt platforms that roof over deep brines. Salt shallows on the reef platforms and around the pond shores are both characterized by the deposition of the thin flat beds with vertical palisade textures well known from other ephemeral brines. Salt reefs are interpreted as tepee structures which have grown by epitaxis as fast as saturated brines have deepened around them. Epitaxis may also account for the rapid deposition of thick beds of pure salt in rifts that open to oceans.Local histories of salt reef compartmentalization can be read from the shapes of reefs that record their relative rates of growth and drowning. Rather than diminish evaporation area, damp emergent salt reefs act as giant transpirative pumps that accelerate salt crystallization and reconfigure the evaporation ponds into areas smaller than thought necessary by chemical engineers. The natural end of reef formation may be when shallow brines on hollow reef platforms become ephemeral on solid salt flats. Former salt reefs are likely to be distinguishable in the undeformed geological record by their botryoidal layers of nonvertical chevron crystals.