Ephemeral saline pan - subaerial and subaqueous

Saline pans in modern continental salt lakes and coastal sabkhas are closely tied to the formation of saline mudflats and dry mudflats (Lowenstein and Hardie, 1985). They define zones on the sump floor where surface brines pond and concentrate long enough to stack into layers and beds composed of aggrading salt crusts (Stage II) sometimes alternating with beds comprising capillary-wicked salt aggregates. Pans typically form in drainage sinks situated in the lower parts of the landscape where sheets of water pond then desiccate.

A pan is typically surrounded by a saline mudflats and is frequently underlain by older bedded or layered salt units. In Lake Magadi for example, the pan is defined as the top of a trona bed some 40 m thick, while in Bristol Dry lake the saline pan facies can be found intercalated with saline mudflat facies throughout the 400 metres of core recovered from the basin centre. Pans range in size from small features a few km across to huge continental salt sinks, as in Salar di Uyuni  where the pan area is more than 9,000 km2.

Textures in the stacked salt layers are a combination of subaqueous aligned features (e.g. chevrons and swallowtails) and exposure overprints (e.g. microkarst pits, subhorizontal erosional bevels and salt-filled polygons) that formed each time pan dried and its surface was exposed (Stage III).

Evolution of a depositional cycle in a saline pan setting alternating subaqueous pan to dessication and subaerial sabkha style (after Lowenstein and Hardie, 1985).

Syndepositional overprints on chevron halite deposited in a saline pan. The textural evolution illustrated on the left shows the formation of vadose microkarst, while that on the right illustrates the effect of bottom freshening on the floor of a perennial brine pool and the formation of a planation surface with no vadose karst development.

Stacked crusts are usually dominated by halite, but other minerals such as gypsum, mirabilite and trona can be important. When the pan dries eolian dust is trapped by capillary adhesion on its moist, rough surface or filters down cracks and fissures in the salt crust. Storm waters cover the pan after the next lake flood and the upper part of the salt crust dissolves, while flocculating muds settle onto the solution surface and sink into intercrystalline karst cavities (Stage I). Ongoing evaporation then creates a new subaqueous salt crust atop the dissolution surface. The knobbly surface seen on some playa salt crusts is the result of continual precipitation and fretting of a subaerial salt layer, fed brine by “capillary wicking.”