Karen wrote earlier: "I'm also intersted to hear your interpretation of
rocks like the Castile formation (alternating CaCO3 and CaSO4 -- Permian,
near Capitan Reef). Looking at the structure of rocks like that, it is hard
for me to believe that they were laid down over a long period of time."
I'm not exactly sure what point you were trying to make, but in fact halite
layers that alternate with carbonates and sulfates are quite common. The
reason is because sea salt is not made up of exclusively halite, but also
carbonates and sulfates as well, though in smaller proportions (halite
generally makes up some 80% of sea salt). Each type of salt has different
solubilities, so they crystallize out at different rates.
Using Jonathon's Lake McLeod scenario, the process would go like this. Sea
water in the basin evaporates faster than it can be replenished, so it
gradually concentrates into brine. The first salt to crystallize out is
calcium carbonate, in a form that also incorporates magnesium called
dolomite. As the sea water concentrates further the next salt to
crystallize out would be calcium sulfate in the form of gypsum. Finally
halite would begin to crystallize out. These would create separate layers
because most of the carbonate would crystallize out before the sulfate, most
of which would then crystallize out before the halite. There would be some
overlap at the edges of each layer, and there might even be some mixing
within layers, but for the most part each layer would be descreet.
A large influx of fresh water could dilute the brine enough to halt
crystallization, and might even reset the crystallization order, allowing
any carbonate and sulfate that is still left to crystallize out and form new
layers over the halite, but such layers are bound to be rather thin. An
influx of sea water, however, would not only dilute the brine, but add fresh
carbonate and sulfate, allowing for thicker new layers to be laid down. In
this way, alternating layers of carbonate, sulfate and halite could build up
according to daily, seasonal and yearly cycles of fresh and sea water
influx. In any event, more than a single year would be needed to create a
deposit as large as the Castle formation by this method.
Kevin L. O'Brien