re: Minor correction:
Sedimentary phosphate is not exactly part of the evaporite sequence
I'd qualify that as "On Earth, sedimentary phosphate is not exactly part of the evaporite sequence", because phosphate is such a biologically important molecule that it is usually scavenged from the oceans and formation waters before it can build up in concentration. Without this biological sequestration, phosphate could well be a feature of an evaporite sequence on Mars.
I'm not worried about mud, or clay. It's the organic detritus I don't want to stick! :-)