Hi Thomas,

We have been doing some work on this topic over the last few years, based on a dataset from the Zechstein Supergroup in the North Sea Basin. We have used borehole and seismic reflection data to try and constrain the present-day, basin-scale distribution, thickness and lithology within this unit, and to try and relate these variations to differences in the structural style associated with the Late Jurassic rift event. We can discuss the details of this particular study 'offline', but we have noticed that relatively anhydrite-rich (i.e. c. 50%) parts of the unit, which seem to be rich in anhydrite due to their depositional location rather than later halite dissolution, seem to be able to flow and form salt structures and minibasins. I guess that things can get pretty impure, but that these 'layered evaporites' will see a large degree of strain partitioning (i.e. flow in the halite and brittle deformation in the anhydrites/carbonates) and will still be able to flow.
Related to your last comment, parts of the evaporites in the Santos Basin, offshore Brazil are relatively rich in non-halite material. See a recent paper by Fiduk and Rowan (2012).
I hope this helps and please email me offline to discuss this further if you wish.

Thanks,

Chris