Gusev's Subsurface Chemical Layers Options

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I remain mystified by the layers of bright material(s) that Spirit discovered just under the surface via the dragging wheel (which may go down in history as one of the most serendipitous breaks in the history of science!) The mystery is why do these layers exist so discretely, just barely below the surface?

The lunar regolith seems well mixed from continuous impacts of all sizes. On Mars, winds & dust-devils in addition to relatively frequent mid-sized meteor impacts presumably provide even more transport, at least within the upper few cm of the soil. So why are the layers there at all?

One reason I'm interested in this is that more than thirty years ago I built a Mars jar for a science fair project. My soil mix consisted of various iron oxides and sulfides/sulfates (magnesium sulfate ws one of them), all in fine powder form. The sides of the tank were transparent, and I noticed after a few weeks that the white MgSO4 formed discrete layers under the surface. Thought this was some sort of screw-up in the mixing on my part at the time, so it was odd indeed to see Spirit find something so similar.

Anyhow. Any thoughts on how these layers in Gusev came to be, and how they apparently manange to persist?