I keep hearing about all sorts of observations that seem to indicate Mars once had oceans (the latest was a geological structure that resembles Earth river deltas). But on first sight it seems like old dried up oceans should be easy to notice due to the salt flats they’d leave behind. I’m obviously making an assumption that isn’t true, but I can’t figure out which. Can anyone please point out what I’m missing?

As far as I can tell, my assumptions are:

1) Planets as similar to Earth as Mars is will have similarly high amounts of salt dissolved in their oceans, conditional on having oceans. (Though I don’t know why NaCl in particular is so highly represented in Earth’s oceans, rather than other soluble salts.)

2) Most processes that drain oceans will leave the salt behind, or at least those that are plausible on Mars will.

3) Very large flat areas with a thick cover of salt will be visible at least to orbiters even after some billions of years. This is the one that seems most questionable, but seems sound assuming:

3a) a large NaCl-covered region will be easily detectable with remote spectroscopy, and 3b) even geologically-long term asteroid bombardment will retain, over sea-and-ocean-sized areas of salt flats, concentrations of salt abnormally high, and significantly lower than on areas previously washed away.

Again, 3b. sounds as the most questionable. But Mars doesn’t look like it its surface was completely randomized to a non-expert eye. I mean, I know the first few (dozens?) of meters on the Moon are regolith, which basically means the surface was finely crushed and well-mixed, and I assume Mars would be similar though to a lesser extent. But this process seems to randomize mostly locally, not over the entire surface of the planet, and the fact that Mars has much more diverse forms of relief seems to support that.