Just wondering if the lack of carbonates is evidence that what we're looking at is some sort of underground process rather than deposition in a lake or other surface water?

If the water was on the surface, that implies a significantly denser atmosphere than today, which would probably have been composed of CO2. That would be quite a bit of CO2 on top of the water -- even at today's 6mbar, Mars has a higher partial pressure of CO2 than does the Earth's atmopshere (it's something like 0.3mbar here I think). So with all that water and CO2, shouldn't it have formed carbonates as well as sulfates?

I have no idea what you should expect from water that is rich in sulfates and what, probably mafic metal ions (i.e., probably more iron and magnesium than anything else, right?) that you then dissolve CO2 into, but I'd think it would be something different than if you have the same solution without the CO2.

Though I suppose if we are looking at a thick evaporitic sequence, maybe my hypothetical carbonates are somewhere else in the series.