EPITAXIAL SALT REEFS AND MUSHROOMS IN THE SOUTHERN DEAD-SEA

Salt usually accumulates in shallow ephemeral brines as simple flat be ds. However, in permanent brines deepening in solar evaporation ponds in the southern Dead Sea, salt accumulation is complicated by the grow th of isolated vegetable-like salt structures and networks of polygona l salt walls. These walls divide large salt ponds, which are easily ho mogenized by wind, into thousands of small compartments that are less easy to mix. Instead, brines in the compartments stratify when a surfi cial layer about 10 cm thick supersaturates and floats above the cumul ate floors on brines that are merely saturated. Salt reef growth in th e southern Dead Sea has previously been attributed to mixing of ions c ommon to brines already in the ponds and those pumped in from the nort hern 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 growt h of crystalline substrates already in the surficial supersaturated la yer. Epitaxis can be invisible in crystal clear brines and prolongs th e 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 int o salt platforms that roof over deep brines. Salt shallows on the reef platforms and around the pond shores are both characterized by the de position of the thin flat beds with vertical palisade textures well kn own from other ephemeral brines. Salt reefs are interpreted as tepee s tructures which have grown by epitaxis as fast as saturated brines hav e deepened around them. Epitaxis may also account for the rapid deposi tion of thick beds of pure salt in rifts that open to oceans. Local hi stories of salt reef compartmentalization can be read from the shapes of reefs that record their relative rates of growth and drowning. Rath er than diminish evaporation area, damp emergent salt reefs act as gia nt transpirative pumps that accelerate salt crystallization and reconf igure the evaporation ponds into areas smaller than thought necessary by chemical engineers. The natural end of reef formation may be when s hallow brines on hollow reef platforms become ephemeral on solid salt flats. Former salt reefs are likely to be distinguishable in the undef ormed geological record by their botryoidal layers of nonvertical chev ron crystals.