OK, nice to see some serious discussion on this topic.

let me give you my views on the nature of the evidence and the balance of probabilities as I see it. Of course, I can't claim to be 'right" - I know I'll be wrong on several points either in detail or in an astoundingly huge way, but here goes:-

The simplest explanation of the surface age difference between the highlands and lowlands of Mars has been a large ocean in the Northern plains. There are some subtle difficulties with this, but let's accept it for a while. If there ever was an ocean on Mars, it was here.

In this argument the visibly cratered (in fact saturated) areas had no ocean, while the visibly smooth areas with few large craters had an ocean.

Meridiani is about 1 km too high to be in the "ocean" area. A quick look at any map of Mars (for instance the little topographic one that featured in the LPSC article that Rbert recently referred to), shows that Meridiani is an old and heavily cratered terrrain, with the minor modification that some smooth(ish) sheets of "stuff" seem to have been plastered across it, then later partially stripped off. These sheets are the Haematite/sulphate deposits.

It would be very hard to argue that Mars had an ocean to this depth because many other areas equally elevated, and others even less so, are not smoothed, not surfaced by haematite.

You discuss evaporites from a slightly exotic ocean as the source of the haematite deposits, and draw the parallel with the Messinian salinity crisis in the Mediterranean. This was different in two main features - the salts become thicker and best-developed in the very depths of the Mediterranean basins, and they were also fed for some time from a larger sea, via a narrow seaway.

If you needed to evaporate 1500' of seawater, then you would need another 500m global ocean above and beyond the topographic level of Meridiani, AND there would be better and longer-lived evaporites in the deeper lowland basins. (You can escape this last issue by showing that the northern lowlands are younger than Meridiani, so you could, in principle, cover-up the earlier evaporaties. Nonetheless, I'd be surprised if Meridiani was the only occurrence)

You do argue that all that is needed is a global ocean just higher than Meridiani, and a constricted access point, like the Gulf of Kharabogoz on the Caspian Sea, or the Mediterranean to the whole Atlantic. However, at Meridiani, the deposits are clearly NOT in the topographic lows - they overlap but do not match well at all. No model of former distribution (that I have seen) has ever suggested a symmetrical extent around the basin depocentre or topographic low.

Also, there is no neck of land cutting off an enclosed depocentre via a narrow seaway. Instead, the Meridiani deposits are on a monotonic slope that descends to the north and west, all the way to the polecap. Refer again to that topographic map.

What my model requires is a broad valley. No need for 4-way closure of a topographic low, just a regional scoop or bowl-shape on the side of a hill - exactly what is presented by the combination of old multiring basin and the global dichotomy slope. Fluids percolating through the ground tend to work downhill, and focus in the centre of this not-closed-basin.

How long would it have taken to form the Meridiani deposit?

We see layers ~ 1/3 mm thick, building up to a deposit 300m thick. If those are annual (seasonal) layers, that would take 1 million Mars orbits or 2 million Earth years. This is not a long time in the 4.6 billion-year history of Mars, so I contend it's not a stretch. it doesn't have to be continuous, either. This could be spread over 100 million years, and only be active in the best 1% of climatic optimum, similar to modern-day mars' unstable orbit and climate.

Why no mudcracks? That's a good question. Perhaps it's because there's no mud? - no hydrated sheet silicates. Note that NASA finds clays to be exceedingly scarce on Mars. Other materials don't show such dramatic shrink-swell behaviour when they get wetter and drier. The lack of clays on mars is due to the limited water exposure of the majority of martian rocks. On Earth there's lots of clay, because there is and always has been lots of water. On mars, the lack of clays points squarely to an arid or frozen history for most of Mars, most of the time. (Doesn't rule-out a short-lived ocean, but the geographic distribution of Meridiani doesn't match the deposits of a global ocean, whether long- or short-lived)

What we do see are lenticular voids. These are the sign of former minerals, now vanished. They look like Gypsim crystals, but they could actually be plates of ice - formed as the acid sulphates reacted with rocks, became more dilute, and thus became vulnerable to freezing.

Why no rainprints? Well, that's an easy one - no rain!

So, in summary, my "sabkha" model does not literally describe a typical Terrestrial sabkha, which is often in a littoral (seaside) environment. Instead, it's something different, perhaps almost uniquely Martian - a giant salt spring and evaporite deposit, but without the need for pools of open water. Most of the material, IMHO, forms as efflorescence and crusts at the surface. - Like those wonderful cardboard trees that grow pink cherry blossom when you dip their feet in the special solution.

Nonetheless, I reckon "sabkha" is a good name for it, until we find something better. (Meridianic?)

But thanks for asking genuine questions. These are my genuine answeres. :-)