Heal thyself doctor.
Perhaps the higher atmospheric pressure and the contained region is the answer. If Gusev has been around subject to aeolian abrasion for a few Gigayears (give or take ;) then the softer stuff (sedimentary) would be preferentialy ablated and removed from basins and deposited as the suspended atmospheric dust precipitates across the planet. Dust deposited in higer altitudes, less likely to have stronger gusts, mostly remains there. Dust redeposited in basins is the material which is accelerated and etches more remaining soft material.
Only an engineer, or no, I know that you know alot about atmospheres and FD. What do YOU think?